the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Nov 2023
Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs
Abstract. Smøla Island, situated within the Mid-Norwegian passive margin, contains crystalline basement-hosted intricate fracture and fault arrays formed during a polyphase brittle tectonic evolution. Understanding similar fracture patterns within basement structural highs offshore, which is crucial in that they are commonly associated with unconventional hydrocarbon reservoirs, remains challenging owing to the lack of rock exposure and geological and chronological constraints. As Smøla is an onshore analogue for basement highs offshore, any insights can therefore be applied to the offshore domain. In this study, we present a multiscalar multi-technique approach whereby various 2D and 3D field, drilling, geophysical, and K-Ar geochronology datasets are integrated to unravel the brittle evolution of Smøla and the surrounding margin. On the regional-scale, geophysical and DTM data-derived lineament analysis, along with local-scale field mapping, high resolution logging of four diamond drill holes, 3D modelling, and petrographic and microstructural analyses indicate that at least five deformation episodes affected Smøla through time. These deformation episodes are characterised by different geometric and kinematic patterns, and mineral assemblages: I) the earliest, ENE-WSW, WNW-ESE to NW-SE striking epidote-prehnite brittle structures are associated with syn to post-Devonian brittle sinistral transtension during post-Caledonian extension of the margin; II) NE-SW or NW-SE striking sericite-chlorite-calcite shear features formed from two separate brittle-ductile phases, which are related with both sinistral and later dextral strike-slip faulting during the Carboniferous to Late Triassic (~300 Ma to ~200 Ma); III) NE-SW, NNE-SSW striking chloritic-hematite breccias and gouges, linked to crustal extension, normal faulting, basin development (~200 Ma to ~100 Ma); IV) NE-SW to NNW-SSE and WNW-ESE striking hematite-zeolite-calcite veins and hydraulic breccias, likely associated to both late Mesozoic dextral strike-slip faulting, and regional ~E-W crustal extension (younger than ~150 Ma to ~75 Ma); and V) the last recorded episode characterised by NW-SE, WNW-ESE and NNE-SSW striking quartz-calcite veins cross-cutting all previous deformation features, associated with ~E-W to NE-SW crustal extension during the Cretaceous to Paleogene. Accordingly, 3D modelling of selected structures, by deformation episode, reproduce the complex brittle arrays on Smøla through time. The structural modelling also highlights the multiscalar nature of basement deformation features, with modelled down-dip and strike extents of the structures varying by D1–D5. Overall, this ‘toolbox’ approach makes it possible to unravel the complex brittle deformation history of Smøla and the wider Mid-Norwegian margin and can be applied to other basement highs offshore Norway and elsewhere.
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RC1: 'Comment on egusphere-2023-2504', Alessandro Petroccia, 07 Dec 2023
Review of “Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs”
by Matthew S. Hodge et al.
Manuscript egusphere-2023-2504
General comments to the authors
Dear Authors, I appreciate the proposed paper “Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs”. The work shows new interesting and high-quality data using a multidisciplinary and multiscalar approach that could be applied to other basements offshore in Norway and/or elsewhere. This is the first round of revisions of the manuscript which comprises an impressive amount of detailed work, from field to micro-scale, combined with K-Ar geochronology, in order to develop a 3D model of the investigated problem. I think the main ideas and the manuscript are strongly valid and suitable for the quality of this journal. I hope my comments on both the PDF and this file will help (please see the annotated version of the PDF file).
I would like to highlight a few important general suggestions to be taken into account:
- The sentences of the manuscript are often verbose and wordy, making it unclear and difficult to follow. Much of this can be easily assessed by adopting a more stringent writing style. I appreciate the serious amount of work that is present behind this manuscript, but I strongly suggest to simplify the sentences and the way to explain the obtained data.
- Abstract: The abstract is quite long, and reads like an introduction. It does not really contain the quantitative results produced in this story. Please concisely present the main results of the study rather than alluding to them.
- The authors used in the manuscript a lot of abbreviations, both in figures and in the main text. Please explain them better or provide a table with all the abbreviations and their relative meanings.
- Figures are cited in the main text without following a numerical order. Please try to cite them progressively or modify the order of the figures
Sincerely,
Petroccia Alessandro
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AC1: 'Reply on RC1', Matthew Hodge, 26 Feb 2024
Response to review comments for the paper titled: “Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs”
Referee 1) Dr Petroccia Alessandro; Citation: https://doi.org/10.5194/egusphere-2023-2504-RC1
Dear Dr Petroccia,
We wish to extend our sincere gratitude for your thorough review and insightful comments on our manuscript entitled "Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs."
We appreciate the time and effort that you have contributed to our manuscript, and the constructive feedback will be invaluable in improving the quality of the work overall. We have thoroughly considered each of your points below and present our responses and revisions.
General comments (summarised):
- Simplify Sentences: The manuscript's sentences are excessively verbose, making it challenging to follow. Adopting a more succinct writing style would enhance clarity and understanding.
- Abstract Length and Content: The abstract is lengthy and resembles an introduction rather than summarising the study's quantitative results. It is advised to present the main findings concisely rather than providing extensive background information.
- Abbreviations: The excessive use of abbreviations in both the text and figures leads to confusion. Improved explanation of abbreviations or inclusion of a table listing their meanings is recommended.
- Figure Organisation: Figures are cited in the text without following a numerical order, leading to inconsistency. It is suggested to either cite them progressively or rearrange the figure order for better coherence.
General responses and revisions made to manuscript:
- We appreciate the suggestions regarding a more succinct and less verbose text. Revisions to the text have involved simplifying the descriptions and characterisations where possible, to aid clarity and understanding. Furthermore, sentences including more than one key point, have been separated where possible.
- The Abstract has been reduced in length, with a focus on the results and interpretations of the study. To provide some introduction into the study, only a minimal inclusion of both the geological background and motivations of the study has been maintained.
- We acknowledge the use of abbreviations in both the text and figures does lead to some confusion. We have tried our best to ensure all abbreviations are explained in the body of text and the figure captions. We understand that a table of abbreviations explanations would be helpful, but we feel the abbreviations are sufficiently explained now in the revised manuscript that a table would be redundant.
- We agree that the figure numbers, as cited in the text, did not always follow sequentially. We have revised the manuscript now to limit this issue, and to ensure that the figures correspond properly in the text.
Annotated comments within the text:
The referee comments within the manuscript are included below with the page number and line number of the original annotated manuscript. Below each referee comment, we provide our response, and our change(s) made within the new revised manuscript.
Abstract
Page 1, line 10: sentence very long and wordy
Response: We agree that the abstract is too long and wordy. We have now amended the abstract to focus on the results, with minimal information regarding the motivation and geological background of the stay.
Introduction
Page 2, line 33: In this section, a lot of sentences are very long and wordy. Please make them shorter with a direct message
Response: We agree many of the sentences can be shortened and improved. We have rewritten and shortened the sentences through the introduction.
Page 2, line 35: Please explain the concept better
Response: The basement highs offshore Norway are typically buried under younger sedimentary rocks, and below the Norwegian Sea. This makes them inaccessible. We have amended to the text to highlight the difficulty of accessing them in deep oceanic waters and beneath younger sedimentary cover.
Page 2, line 34: Text removed
Response: The text has been rewritten due to the above comment, but we do not agree with this deletion. This sentence is in contrast against/a modifier of the previous sentence, so it needs a “however”.
Page 2, line 51: why inside the quote?
Response: The single quotation marks signify the word “gap” is being used as a term. This is unnecessary, so we have amended the text to remove the singular quotation marks.
Page 2, line 51: provide a definition of this word with relative references
Response: We have clarified the term with a relevant reference and changed the text to “connected natural fractures and faults hosted within basement volumes (intrabasement structures) in an offshore context (Holdsworth et al., 2019)
Page 2, line 52: offshore structures
Response: We agree and have updated the text to the suggested.
Page 2, line 53: wordy and hard to follow. please rephrase
Response, we accept the suggestion, and have rewritten the text to a shorter sentence: “Consequently, characterising sub-seismic to regional-scale structures, and their potentially long-lived tectonic evolution, requires more than just low resolution datasets.”
Page 2, line 56: could you provide any examples?
Response, we accept the suggestion and have amended the text to provide some examples.
Page 3, line 63: is this part of the sentence necessary?
Response: We agree and have removed the text.
Page 3, line 70: this is valid for the overall main text, if not necessary of fundamental, try to avoid words like novel, new... etc
Response: We agree and have rewritten this section of the Introduction.
Page 3, line 70: also this one is valid for the overall text, homogenize the use of "-"
Response: We agree, this portion of text has been removed from the introduction.
Page 3, line 75: why the "-" after geological?
Response: We agree, this portion of text has been removed from the introduction.
Page 3, line 76: strange word
Response: We disagree, as the oriented drill holes do indeed offer a relatively unprecedented 3D view of the Smola geology. This has not been previously available. However, this portion of text has been removed from the introduction.
Page 3, line 80: Text removed
Response: We agree, and the text has been removed.
Geological framework
Page 4, line 104: there is an extra space after the bracket
Response: Well noted, and the text has been removed.
Page 5, line 120, Figure 1: the black continue lines are not explained in the legend
Response: We agree, and we have amended the legend of Figure 1A to reflect the symbology of the black lines (major structures).
The applied toolbox
Page 7, line 172: document or material?
Response: we agree and have amended the text to “material”.
Page 7, line 178: what does it mean?
Response: we agree this is too vague, so we have amended the text to “standard geological field methods”.
Page 7, line 180: total,
Response: we agree and have changed the text to the suggested.
Page 7, line 180: collected data?
Response: Data is indeed collected through measurement and observation. We have not amended the text.
Page 7, line 184: you jump from figure 3 up to 8
Response: We have updated the text to also include the mention of the 3D modelling, which provides an intermediate figure. The K-Ar sample figure is however much further through the manuscript, so a ‘jump’ in figure numbers is unavoidable.
Results
Page 8, line 194: how can you trace it? from the DEM?
Response: The onshore lineaments involved a combination of DTM and aeromagnetic data, while the offshore data exclusively used aeromagnetic data. We have amended the text to make this more clear for the reader.
Page 9, line 206, Figure 2: the dashed line is not explained in the legend. I suggest making the colour with higher transparency, so that the different fracture systems would be more visible (the same transparency of the zoom)
Response: We agree with this suggestion, however we have removed the dashed line, which along with the area labels A – F, make this figure item easier to understand. We have adjusted the transparency of the offshore areas to make it easier to distinguish the on and offshore areas. The overall Figure 2A map has also been enlarged to help make the lineaments more visible.
Page 9, line 206, Figure 2: which contour calculation did you use? kriking?
Response: The geophysical imagery was processed using standard gridding (minimum curvature) and levelling methods of the Geosoft Oasis montaj (Geosoft, 2010). A full description of the geophysical data processing and merging methodology is available in (Nasuti et al., 2015). As these details are out of the scope of this manuscript, we have not amended the text, and refer the reader to the supplementary material.
Page 9, line 206, Figure 2: the differences between 2-3 order is hard to distinguish in the figure
Response: We agree, and we have made the trace lines on Figure 2A thicker for the 1st and 2nd order lineaments, so they can be more differentiated from the 3rd order lineaments.
Page 10, line 223: ?
Response: “Abut” means to terminate against something. To avoid any confusion, we have changed the use of “abut” to “terminate” in the text.
Page 11, line 258: homogenize, use always or Figure or Fig.
Response: We agree, and have decided to use “Figure”, and have removed the use of “Fig.” in the text.
Page 11, line 258: Text removed
Response: text rewritten to “a novel 3D perspective on Smøla's geology”.
Page 11, line 260: Text removed
Response: We agree, and the text has been removed.
Page 12, line 282: epidote or episode? please explain. Clc according to what mineral abbreviation guide?? Please use the conventional way for minerals, like Warr, 2023
Response: Thank you for the suggestion, “epi” should be “Ep”, and “clc” should be “Cal”. We are using the mineral abbreviations after Siivola & Schmid (2007). We have amended the text to ensure all mineral abbreviations are standardised.
Page 13, line 290, Figure 4: explain bk
Response: The abbreviation “bk” is already described in the caption for the figure. The use of “bk” is with “Chl (bk)” for black chlorite. No changes have been made to the text.
Page 15, line 331, Figure 5: what is frg?
Response: This is an abbreviation for “fragment”. We have amended the text of the caption and included an explanation of the abbreviation as “Reworked host rock fragments (frg)”.
Page 15, line 332: As a suggestion, the letter of the figure (A...) are very large and heavy to see. Also, if it is not necessary, please make the photos clearer, with less lines. Can you provide a better photo of FIGURE E? It is not so clear as a ECC
Response: We can accommodate this suggestion. We have decreased the size of the letter labels and decreased the number of lines on the sub-figure (E). The photomicrograph is the best that is available for this microstructure. The microstructure is clearly a ECC texture, with the C` planes inclined to a primary slip plane (white line), reworking an older S fabric.
Page 15, line 337: All abbreviations should be explained the first time they appear in the text
Response: We agree, and the caption for the microstructures figure (now Figure 6) has been updated to explain this abbreviation.
Page 16, line 357: reference
Response: We agree and have inserted the following reference: “(e.g. Passchier & Trouw, 2005)”.
Page 16, line 361: decorated seem to not be a scientific geological word
Response: We disagree, this term is in used to describe mineral coatings on the bounding fracture and vein surfaces (e.g. Scheiber & Viola, 2018; Viola et al., 2016). We have not changed the text.
Page 17, line 382: maybe its better to explain abbrevations before in the manuscript. Also a table with all the used abbreviations could be useful
Response: We appreciate this suggestion. The Figure 6 in the reviewed manuscript has now been moved earlier in the manuscript (now Figure 3), with the mineral abbreviations now being explained before the results of the field and drill hole mineral/deformation feature observations.
Page 19, line 439: jumped from figure 7 to 10. Please check the figure order and how you cite them in the text
Response: We agree that this is an issue. This section of the manuscript is actually misplaced (belongs to the 3D modelling section). The amended manuscript does not have this same figure number jump.
K-Ar geochronology and X-ray diffraction
Page 20, line 472: here you can use wt% since you have already explained the abbreviation in line 470
Response: We agree, however this text has now been removed.
Page 24, line 513, Table 4: Mus Capitalize the first letter of the abbreviations
Response: We agree, however this table has now been removed, with a pie chart figure describing the XRD results for the K-Ar samples. The abbreviation for muscovite, has the first letter capitalised as “Mus”.
We would like to extend our gratitude once again to Dr. Petroccia for his invaluable contributions to the improvement of our manuscript. Your thoughtful suggestions have been carefully considered, and where possible, we have made corresponding revisions to enhance the clarity and robustness of our work. We are confident that these adjustments will strengthen the overall quality of the manuscript.
Yours sincerely,
Matthew S. Hodge1*
Guri Venvik2
Jochen Knies2
Roelant van der Lelij2
Jasmin Schönenberger2
Øystein Nordgulen2
Marco Brønner2
Aziz Nasuti2
Giulio Viola1
Affiliations:
- Department of Biological, Geological and Environmental Sciences, University of Bologna, Italy
- Geological Survey of Norway (NGU), Trondheim, Norway
(*Corresponding author)
References
Geosoft, 2010b, OASIS Montaj v7.2 Mapping and processing system, The core software platform for working with large volume spatial data. Quick start tutorials.:
Holdsworth, R.E., McCaffrey, K.J.W., Dempsey, E., Roberts, N.M.W., Hardman, K., Morton, A., Feely, M., Hunt, J., Conway, A., and Robertson, A., 2019, Natural fracture propping and earthquake-induced oil migration in fractured basement reservoirs: Geology, v. 47, p. 700–704, doi:10.1130/G46280.1.
Nasuti, A., Olesen, O., Baranwal, O., and Dumais, M., 2015, Compilation of aeromagnetic data, in Olesen, O. et al. eds., Coop Phase 2 - Crustal Onshore-Offshore Project. NGU confidential Report , Norges Geologiske Undersøkelse, v. 063, p. 11–24.
Passchier, C.W., and Trouw, R.A.J., 2005, Microtectonics: Berlin, Springer Science & Business Media.
Scheiber, T., and Viola, G., 2018, Complex Bedrock Fracture Patterns: A Multipronged Approach to Resolve Their Evolution in Space and Time: Tectonics, v. 37, p. 1030–1062, doi:10.1002/2017TC004763.
Viola, G., Scheiber, T., Fredin, O., Zwingmann, H., Margreth, A., and Knies, J., 2016, Deconvoluting complex structural histories archived in brittle fault zones: Nature Communications, v. 7, doi:10.1038/ncomms13448.
- AC3: 'Reply on RC1', Matthew Hodge, 26 Feb 2024
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RC2: 'Comment on egusphere-2023-2504', Deta Gasser, 17 Jan 2024
Review of “Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs” by Hodge et al.
General comments
This manuscript presents a multi-tool and multi-scalar analysis of the post-Caledonian, brittle evolution of the Island of Smøla, central Norway. The authors present remote sensing lineament mapping on airborne magnetic and DTM images, field observations from 66 field sites, and detailed logs of four boreholes with a total length of 365 m. They also provide thin section analyses of specific fracture and fault domains, K-Ar analyses from 7 fault rocks, as well as 3D modelling of fault and fracture geometries in two of the four boreholes. They describe the observed brittle deformation history in terms of five deformation phases spanning in age from the Devonian/Carboniferous to the Latest Cretaceous, and they discuss this evolution in terms of previous literature and the general evolution of the rifted Mid-Norwegian margin, with a particular focus on the significance of the Smøla geology for similar offshore basement highs.
The manuscript contains a number of nice, detailed structural and mineralogical observations from the field and bore holes, high-quality K-Ar analyses from seven samples, as well as a convincing subdivision of the observed brittle structures into five distinct types of features with specific mineralogical characteristics and cross-cutting relationships, allowing to present a relative chronology assigned to at least five different deformation phases, and placing them in an absolute time frame. I consider therefore the data set to generally be suited for publication in Solid Earth. However, there are several aspects in the data presentation and discussion which I consider to be deficient, and which have to be addressed in a revision before the manuscript can be considered for final publication. I have inserted detailed comments to each section of the manuscript below, and you also find additional minor comments in the attached annotated pdf. To sum up my major points from the comments below, a revision should take care of the following main aspects:
- more detailed and more convincing presentation of cross-cutting relationships of lineaments, preferentially with high-resolution DTM pictures in addition to the magnetic anomalies
- better introduction and presentation of the structural field measurements (a stereoplot showing both strike orientations and dip values, as well as lineation and kinematic data is a minimum)
- a more systematic presentation and interpretation of the K-Ar results sample for sample
- you have to streamline the text – remove wordy parts and repetitions, shorten the introduction, several paragraphs in the results section are placed under the wrong heading
- An interpretation of the brittle tectonic evolution taking into account the dip-values of your features and the orientation of lineations and kinematic indicators (related to point 2)
Implementing these changes requires major revision of the text and the figures along the lines described below. I hope my comments are taken in the positive spirit they are meant and help to improve the presentation and interpretation of this nice data set from a spectacular example of a basement complex along the passive margin of central Norway.
Sogndal, 16.1.2024, Deta Gasser
Specific comments to each section in the text
Title:
I think the title is not covering exactly what the manuscript contains. The Mid-Norwegian margin is much larger than just Smøla, and you haven’t been directly working on offshore basement highs either. I would therefore adjust the title to something like:
Multiscalar 3D characterization of the brittle tectonic evolution of the island of Smøla, Mid-Norwegian passive margin: an analogue for understanding the brittle architecture of offshore basement highs
1) Introduction:
- The introduction is quite lengthy, and should be shortened considerably. I have highlighted potentially superfluous parts in the annotated pdf (in particular, the last three paragraphs can be combined into a single one without loosing crucial information).
2) Geological framework:
- See minor comments in the annotated pdf
3) The applied toolbox
- I would describe the field mapping techniques prior to the bore holes – it is more logical to go from the large scale (remote sensing), via field mapping/outcrop observations, to borehole data.
- Say something about when and why the boreholes were taken – what is the reason for placing them where they are? Where they acquired during your project, or are these earlier cores? Where are the cores stored?
4) Results
4.1 Lineament mapping
- The description of the different orientation trends is somewhat difficult to follow. Use numbering for the 8 orientation trends (orientation trend 1) is N-S, 2) is NW-SE, 3) is E-W etc.) in the table and the text, which makes it easier to follow. You could also consider using colour coding for the eight orientation trends in the rose plots in Fig 2A and in the small inset maps in Fig. 2B, so that it is easier for the reader to separate the different groups. Start the description of each orientation group with where it can be found/in which domain it is most present – currently this information comes at the end of each orientation description.
- I am not sure I buy your cross-cutting relationship descriptions (Fig. 2B and Table 1). According to my experience it is quite difficult to find good cross-cutting relationships with lineaments, and several of the examples you show in Fig. 2B and/or mention in the text are not very convincing based on the figures you show. How can you be sure that the lineament on one side of another lineament is the same as the one on the other side with some offset? Often it is possible to draw the lineament across, and it is quite subjective where you end a lineament – your Fig. 2B shows examples where I would have drawn a lineament across another but you didn’t, and other examples where I would have stopped a lineament at another one, but you have drawn it across (see the screenshot below as an example). I get even more sceptical if you propose dip-slip offset from 2D lineament maps – if you propose this, you have to at least explain better how you identify dip slip offset of lineaments! So I suggest that you (a) either document proposed offsets more thoroughly with better figures at smaller scales and with DTM behind (maybe both with and without your interpretation shown so that the reader can make an own evaluation), pointing at particular examples where you are convinced this is a good offset, or (b) you rewrite the text, highlighting more the difficulties in actually finding good cross-cutting relationships, partly due to the difficulty in defining where a lineament starts and ends. As the small inset maps in Fig. 2B are now, I don’t find them very useful for understanding the crosscutting relationships you describe in the text. So if you want to focus on lineament cross-cutting relationships, I suggest you prepare an own figure for it with the best single examples of crucial cross-cutting relationships shown at a more detailed scale and with both DTM and magnetic images.
Example of an ambiguous lineament interpretation:
- Why are you not continuing the northern black line across the SW-NE lineament (as indicated with the red line)? The southern black line could just be a lineament parallel to the northern thick black line, they do not have to be the same structure which is offset? (the system for uploading an image in the review did not work, so please contact me to get a copy of the screenshot which I wanted to insert below - it is also not possible to attach two different files to the review, so I had to prioritize the annotated pdf).
4.2. Deformation history
- From 4.1 to 4.2, there is a big jump in the manuscript, where you move from the large-scale lineament mapping directly to the boreholes. I highly recommend that you start section 4.2 with a summary of your results from the field stations first. I do miss a proper documentation of your field results, in particular the structural measurements from the 66 stations. Could you provide a stereonet figure showing all your structural measurements from the 66 field stations, including faults, fractures and mineral lineations? Something similar to Fig. 4 in Hestnes et al. (2022) or Fig. 4 in Scheiber and Viola (2018) – possibly sorted according to the domains defined in Fig. 2B based on the lineament mapping? And maybe also containing information about the related mineralizations from each fracture/fault? Such a figure would give a good impression on the general orientation of the structures – are they subvertical, shallow-dipping, what are the lineations, mostly strike-slip, or mostly dip-slip? You also have to give information on kinematic indicators – how many dextral/sinistral or dip-slip/reverse-slip observations do you really have? You refer to kinematics in the D1-D5 section, but without presenting the evidence for it. This is crucial information for the further understanding of the brittle architecture of the area and maybe deserves an own subchapter, “Structural field measurements”? You should also move Fig 6 ahead, which is related to such a stereonet figure, since it shows all the strike orientations, but not the dip orientations. You could either keep Fig. 6 as an own figure (separating out the field vs. borehole measurements with different colors/grey shadings), or incorporate it into the stereonet figure. You also have to decide whether you want to show the structural data from the field together with the measurements from the drill core (which now are shown in Fig. 3), or separate from it. In any case, you have to give the reader a better structural framework before you describe the detailed mineralogical relationships in the sections D1-D5 (which are very nicely written and illustrated with figures 4, 5 and 7).
- At the end of chapter 4.2.5 there are four paragraphs which are wrongly placed - they deal with correlations of particular features in the boreholes shown in Fig. 10, and should entirely be moved to and incorporated into the text in chapter 4.4 (which should be moved to before the K-Ar chapter, see comments below).
4.3. K-Ar geochronology and X-ray diffraction
In my opinion, this is the weakest chapter in the results section. The description of the different samples is not very systematic, the composition of the samples (Table 4) is discussed after the presentation of the ages, and it is not argued well enough for which dates are geologically meaningful and which ones potentially represent mixed ages. My experience is that K-Ar samples all have their own characteristics, based on the host rock lithology and preserved mineralogy in the sample, which needs a careful “sample-for-sample” approach and a careful interpretation of the age data. I suggest you have a look at how Hestnes et al. (2022) introduced and described each sample based on field relationships, mineralogy (preferably with plots showing the compositional variations, which makes it easier to evaluate the amount of K-bearing phases in each sample/fraction, see Fig. 7 in Hestnes et al. 2022 or Fig. 8 in Scheiber et al. 2019), and first then present and interpret the age results. You might then say a few words about your approach towards interpreting the dates from the different fractions as geologically meaningful or not. You might also consider presenting the 7 samples in a different order: introduce the three samples with D2 interpretation first (2012, 2015, 1029_1) ->they have the oldest finest fractions, followed by the three samples with D3,D4 interpretation (1033, 2008, 120714) which have the youngest finest fractions, and finally present 1024_1, which is missing the finest fraction and which is therefore of less importance. By introducing them that way it will be more straight forward to give an age to the different deformation phases. You should also discuss the meaning of the saprolite(?) ages at ca. 128 Ma for sample 1033 in contrast to possible deformation ages for the other samples, so it is really important to treat each sample separately.
4.4 Basement deformation in 3D
I think this section fits better before the K-Ar section, since it still deals with a description and interpretation of the field relationships. As commented on above, a part of the text which belongs to this section is wrongly placed at the end of chapter 4.2.5 and should be incorporated here. Generally, I struggled with correlating what you write in the text with what is shown in figure 10 – which zones do cross-cut both bore holes, and which do not? Which planes represent D1, D2, D3 osv structures? Where are the dated samples from? What is Fig. 10B showing compared to Fig 10A? Could some of the correlations also be shown in Fig. 3? I think this entire chapter needs some careful reorganization, rewriting and alignment with the figures to be understandable and useful.
- Discussion
5.1. Polyphase evolution of Smøla and the passive margin
- Here, you discuss the temporal evolution of the brittle history of Smøla within a larger framework. You put a lot of emphasis on cross-cutting relationships and orientations of lineaments, instead of using your detailed field observations including dip-measurements and lineations/kinematic indicators. I suggest you carefully revise the interpretation once the field results are included in the data part, putting less weight on the lineament mapping, and more weight on actually observed relative cross-cutting relationships between differently-mineralized fractures and faults which I think are more reliable than purely lineament-based observations.
- You should also take the 3 dimension into account – based on lineament analysis one quickly thinks only in terms of steep, strike-slip related structures, but your stereoplots from the boreholes mainly show quite shallow structures? How do they fit into a mainly strike-slip related interpretation/model?
- Similarly, you should also include a more careful interpretation of they age data in your discussion – once you have decided for each sample which date you interpret as geologically meaningful, a more detailed temporal evolution might appear (the 300, 200, 100 Ma periods are too generalized I think).
5.2. Application to and implications for the Frøya High offshore domain
I think this title promises a bit too much – your results do not have direct implications for the interpretation of the Frøya High other than the Frøya High might have experienced a similar complex brittle evolution. So I would tone this a bit down and possibly shorten it, and remove table 5 (its enough to say that there are three bore holes available but unfortunately not with intact cores). See comments in the annotated pdf as well.
- Conclusions
The conclusions and abstract should be updated once the field results and K-Ar results are presented in a more systematic and detailed way.
Figures:
See several smaller comments on the figures in the annotated pdf. A revised order and number of figures based on the comments above could look something like this:
- Fig 1
- Fig 2
- Detailed figure showing crucial lineament cross-cutting relationships with DTM and magnetics
- Fig 6 with field data and drill hole data separated with colors/grey shades
- New Fig. stereonet figure with field data including fractures and faults, lineations and kinematic indicators, sorted according to domain, field station, mineralization state…
- 3 drill hole logs
- 4 Mineralizations from the field/drill holes
- Fig 5 Mineralizations in thin section (specify whether they are from the field or drill holes)
- Fig 7 relative chronology
- 10 3D model
- 8 K-Ar samples map and field pics
- New Fig. composition of K-Ar samples
- 9 K-Ar ages
- 11 Final model
Resulting in a total of 14 figures
Technical corrections/minor comments
See annotated pdf
-
AC2: 'Reply on RC2', Matthew Hodge, 26 Feb 2024
Response to review comments for the paper titled: “Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs”
Referee 2) Prof. Deta Gasser; Citation: https://doi.org/10.5194/egusphere-2023-2504-RC2
Dear Prof. Gasser,
We wish to extend our sincere gratitude and appreciation for your detailed and thorough review and insightful comments on our manuscript entitled "Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs."
We appreciate the time and effort that you have contributed to our manuscript, and the constructive feedback will be invaluable in improving the quality of the work overall.
We have thoroughly considered each of your suggestions and present our responses and revisions. For context on this response document structure: We have first included the general comments (overall, and then by section as was originally done in the review), with the responses below, and then included the ‘in-text’ comments from the annotated .pdf, with specific responses.
Overall general comments (summarised):
- Enhance the presentation of cross-cutting relationships among lineaments, preferably with high-resolution DTM images alongside magnetic anomalies.
- Improve the introduction and display of structural field measurements, including a stereo plot illustrating both strike orientations and dip values, as well as lineation and kinematic data.
- Provide a more systematic presentation and interpretation of the K-Ar results on a sample-by-sample basis.
- Streamline the text by eliminating wordiness and repetitions, condensing the introduction, and ensuring paragraphs in the results section are appropriately categorised.
- Offer an interpretation of brittle tectonic evolution considering dip values of features and the orientation of lineations and kinematic indicators, building upon the structural field measurements.
Overall general comment responses and revisions made to manuscript:
- We agree that the presentation of the cross-cutting relationships is insufficient to effectively support our claims on relative timing of the lineaments. We have now both enlarged the main geophysical map in Figure 2A and included only two ‘zoom-in’ areas in Figure 2B, with both the DTM and geophysical data showing. The ‘zoom-in’ areas are also at a smaller-scale now to aid in visually seeing the different lineament trends and cross-cutting relationships.
- We concede that this was a missing component on the original manuscript. We have now included stereonet plots for both the field and drill data, showing pole to plane trends of all the measured planes, slip lineations, and kinematics (coloured by regime). The data have been sorted by deformation episode as derived from the mineral assemblages and cross-cutting field relationships, and integrated with our other field, drill, and petrographic results.
- We agree that this is a more robust approach to understanding and using K-Ar dates in complex brittle deformed volumes such as Smøla. We have thus reorganised and rewritten the text, systematically characterising each K-Ar sample. Full sample descriptions have been included now in the supplementary material.
- We acknowledge that the manuscript required significant streamlining and reduction in word count. We have now shortened the introduction, rearranged certain sections (e.g. Basement deformation in 3D), and attempted to cut down the length of the manuscript where possible. However, the inclusion of the requested material (see above) has increased the manuscript length.
- We agree that the original manuscript lacked sufficient reference to both the field and drill structural data. We have now incorporated this data into the interpretations, using the strike/dip, slip lineation orientations, and kinematics for understanding each of the deformation episodes, particularly where some of the episodes may be comprised of more than one sub-episode of deformation. In most cases, the field data have proven to be more useful for this purpose.
General comment responses by section:
For the specific comments to each section, the reader is referred to the original review at the citation link: https://doi.org/10.5194/egusphere-2023-2504-RC2.
Title:
Response: We agree the title is ambitious. We have decided to adjust the title to “Multiscalar 3D-temporal structural characterisation of Smøla Island, Mid-Norwegian passive margin: an analogue for unravelling the tectonic history of offshore basement highs”
Introduction:
Response: See general comment response above, we agree the Introduction required shortening. We have consolidated the last three paragraphs into a single paragraph as suggested.
Geological Framework:
Response: Please see specific responses below to comments in the annotated pdf.
The Applied Toolbox:
Response: We agree, the field mapping-related methods should be discussed before the drill hole-related activities. We have now moved this portion of text first. Moreover, for the drill holes, we have now included information on the rationale behind the drill hole locations, when they were drilled, and where they are now stored.
Results
Lineament Mapping:
Response: Please also see the specific responses below to comments in the annotated pdf. The lineation trends are now associated with numbers L1 to L8, and the lineaments have been colour-coded and labelled on Figure 2B. We have included each of the lineament descriptions with the location information. Within the text we have highlight difficulties in establishing cross-cutting relations owing to both the resolution of the imagery, and the difficulty in defining the start and end points of the lineaments. The descriptions of the cross-cutting relationships of each lineament set have been rewritten to improve clarity and simplify the text as much as possible. We have also simplified Table 1 to only indicate either termination or cross-cutting relationships, with no indication on strike-slip or dip-slip offsets of one lineament set by another.
Deformation History:
Response: We agree that this section needed a proper and thorough description of the field and drill structural data. We have now included summaries of the structural data within the field, drill, and petrographic results. We have focussed primarily on the field data as it incorporates data from wider areas across Smola than the drill holes. Corresponding to the summaries, we have included a stereonet figure for both the field and drill data sorted by deformation episode. Within the summaries and on the figure, we incorporate kinematic information and trends of both slip lineations and measured planar features (shear features, significant deformation zones, and veins). We have also moved the misplaced paragraph to within the “Basement deformation in 3D” section.
K-Ar Geochronology and X-ray Diffraction:
Response: We agree this section needed a more systematic approach in presenting the K-Ar results. We have therefore characterised and summarised the results for each sample individually and sequentially based on age and deformation episode. A full description of each sample has also been included in the supplementary material. For each sample, we firstly describe the field relationships, then the X-ray diffraction (XRD) mineralogy results, and finally the K-Ar age results. The age results are then discussed relative to the grain size fraction, associated mineralogy, and compared against the other samples. Sample SK1033_1 is particularly discussed due to the possible saprolite formation age it has yielded.
Basement Deformation in 3D:
Response: This section has been rearranged to be before the K-Ar sample section. The text has been reorganised, rewritten, shortened, and simplified, with better correlations between the text and the figure (Figure 8). Figure 8 has also been updated, with the zones discussed in the text colour-coded and number labelled the same as the text.
Discussion:
Response: We acknowledge that primarily using lineaments to demonstrate a tectonic evolution is insufficient. Although the field and drillhole structural data was utilised in the study, the lack of visual evidence was an issue. We therefore have now incorporated the structural data in the deformation history discussion and toned down the use of lineaments (we have still correlated the lineaments to the structural data where possible). We have also brought in the shallow structures as seen in the drill holes into the interpretations (including the 3D modelling work). Associating the structures to the absolute age data (K-Ar ages) has now been done taking the individual samples into account, and the specific grain size fractions within the samples. The final section of the discussion regarding Frøya High has been significantly shortened and Table 5 has been removed. This section is now intended to convey the importance of using onshore analogues, such as Smola, in understanding offshore basement highs.
Conclusion:
Response: We have rewritten the conclusions based on the revised manuscript, incorporating the new information regarding the field and drill structural data, and the systematic K-Ar sample descriptions and interpretations. The abstract has been similarly rewritten with a focus on the amended results and interpretations of the study. The abstract has also been streamlined and shortened with less emphasis on the project background and rationale.
Annotated comments within the text:
The referee comments within the manuscript are included below with the page number and line number of the original annotated manuscript. Below each referee comment, we provide our response, and our change(s) made within the new revised manuscript.
Title
Page 1, line 1: See suggestion for alternative title in the separate reviewer file
Response: See general comment response above.
Introduction
Page 2, line 45: This part is not absolutely necessary, could be omitted for shortening. shorter: " their size is commonly below seismic resolution"
Response: We agree. A selection of the text has now been removed, and text changed to suggested.
Page 2, line 60: shorter: "is an ideal analogue for basement highs offshore..."
Response: We agree. We have changed the text to the suggested.
Page 3, line 62: Long sentence, shorter would be: "At Smøla, a wealth of local and regional structures are preserved, which document the tectonic evolution of the margin through time".
Response: We agree the sentence can be shorter. Text changed to “Smøla itself possesses a wealth of preserved local and regional structures, which document the tectonic evolution of the margin through time.”
Page 3, line 66: Brittle
Response: We agree. Text changed to suggested
Page 3, line 67: add Hestnes et al. 2022 (https://www.sciencedirect.com/science/article/pii/S0191814122001134?via%3Dihub) and Tartaglia et al 2020 (https://www.sciencedirect.com/science/article/pii/S0012821X20303642) and Scheiber et al. 2019 to this list (https://www.sciencedirect.com/science/article/pii/S0191814118305029)
Response: We agree. We have now inserted these references into the text.
Page 3, line 70: I am not sure your approach is so new. Most of the previous studies cited above use lineament mapping, extensive field observations including mineralogy and K-Ar fault gouge dating (e.g. Scheiber/Viola, Hestnes and Tartaglia)- the only thing that might be new are the boreholes, but the tools used there (macro- and microstructural observations) are not particularly new. I suggest you combine the last three paragraphs of the introduction into one single paragraph, shortening the context considerably and avoiding repetition (the methods are mentioned twice, both in the second and third paragraph), stating that you present a detailed case study applying known methods to a particular area.
Response: We agree, and we have updated the text to highlight the new perspective offered by the drill holes. A concluding point on the relevance of this work in the offshore realm and the implications on generating deterministic inputs for fracture modelling is also included. This is important as this paper will lead on to the next paper currently in production which relates to this aspect specifically. Revision: Text updated to “…a new 3D perspective from four diamond drill holes on Smøla Island, we present a case study that describes and tests a comprehensive workflow for characterising onshore basement blocks.”
Geological framework
Regional perspective
Page 3, line 90: placed along
Response: Text changed to “set along”, as the island is not necessarily “placed” which implies a positioning of the island after the formation of the passive margin.
Page 4, line 93: The Scandian phase is usually assumed to cover the time span from ca. 430-400 Ma
Response: Text changed to “(430 to 400 Ma)” to reflect the time span.
Page 4, line 99: Text deleted
Response: We agree, text deleted
Page 4, line 102: Devonian
Response: we agree, Text updated
Page 4, line 104: Insert also Grønlie&Roberts 1989 and Grønlie et al. 1994.: Fission-track and K-Ar dating of tectonic activity in a transect across the Møre-Trøndelag Fault Zone, central Norway. Norsk Geologisk Tidsskrift. Vol. 74, pp. 24-34.
Response: we agree, inserted the references
Page 4, line 105: Text deleted
Response: We agree, text deleted.
Page 4, line 107: Fig. 1B
Response: We agree, and we have inserted the figure reference.
Page 4, line 114: which time? The time of break-up just described above? Or the entire Devonian-Paleogene time? Specify/rewrite
Response: We have merged the paragraph with paragraph above, which provides more clarity on the time span. Rewrote to remove the uncertainty regarding the time span.
Page 4, line 114: when does a passive margin actually form? I would say at break-up? Before it is just a continent with rifts? Consider rewriting
Response: We agree, we have rewritten it to remove the uncertainty regarding the formation of the passive margin.
Page 5, line 120, Figure 1: Separate out the supracrustals (limestones, volcanics)?
Response: We agree, and now the limestone and volcanics on Figure 1B have been separated out and given their own symbology.
Page 5, line 120, Figure 1: Use brownish colors for gabbros and metagabbros, blueish colors are usually used for carbonates
Response: We agree, and now on Figure 1B all the lithology types have been recoloured with appropriate colours based on the Geological Survey of Norway colour scheme.
Page 5, line 120, Figure 1: Baltica autochthonous
Response: we agree, and the text in Figure 1A has been changed.
Page 6, line 132: not shown on map? Remove
Response: We agree, the text has been removed.
Page 6, line 135: consider placing the reference at the end of the sentence for better readability
Response: Yes, we agree, we have moved the reference.
Page 6, line 138: Text deleted (comma)
Response: We do not agree. The comma must remain, as it is a list, and for consistence with the reminder of the text, the authors are using the Oxford comma convention.
Geological framework
The geology of Smøla
Page 6, line 143: those are not separated out on the map in Fig. 1b - it would be nice to show them with an own color on the map (now they are combined with the gabbro?) since those are the only Caledonian supracrustal rocks on the island
Response: We agree, and now these units are separated in Fig. 1B, with their own colour symbology.
Page 6, line 145: Check the reference style - the "H." and "Ha." has to be removed I guess
Response: We acknowledge this problem; the reference is now corrected.
Page 6, line 151: which fill in
Response: Yes, we agree. The text changed to the suggested
Page 7, line 160: Does not fit here - can be included in the last paragraph of the introduction
Response: We agree, but we have decided to entirely remove “In comparison to the MTFC and the wider margin, the post-Caledonian structural evolution of Smøla remains poorly investigated and understood.”, as it is not relevant to the purpose of the paper, where we are highlighting a methodology of characterising basement rock volumes.
The applied toolbox
Page 7, line 173: It is more logic to describe the field methods first, and then the drill hole logging.
Response: We agree, particularly as the majority of the logged features in drill core are centimetric in scale.
Page 7, line 174: Say something about the drill cores - when where they taken and why? What was the reason for placing them where they are? Were they conducted during your own study, or are these older cores?
Response: The four diamond drill holes used in this study were drilled in 2019 during a previous NGU project (BASE 2). The drill holes were designed to target major structures, and sample weathered basement rocks. We have updated the text to reflect this.
Page 7, line 178: Move this to the start of the paragraph
Response: We agree, and we have moved it to the start of the paragraph.
Results
Lineament mapping from geophysics and DTM data
Page 8, line 196: Refer to the supplement/data repository where the lineaments are available as geodatabase
Response: We agree, and we have added a reference to the supplementary data repository
Page 8, line 197: I cannot see that different magnetic blocks were discussed above? Consider rewriting
Response: We agree, the text has been rewritten.
Page 8, line 201: use this numbering in the table 1 and in the text below, so orientation trend 1) is N-S, 2) is NW-SE, 3) is E-W etc.
Response: We agree, this has been added into the text and the Table 1 has been updated.
Page 9, line 206, Figure 2: It is very difficult to see the domain boundaries - how where the lineaments assigned to the different domains? Consider using thicker white lines to delineate the domain boundaries
Response: We agree, the boundaries have been made to be thick white lines to improve the clarity.
Page 9, line 206, Figure 2: It is hard to see the outline of the island/coast - could it be an idea to make the submarine areas more transparent (or putting a whitish transparent layer on top of the submarine areas, to that the magnetic pattern appears weaker in color?)
Response: Yes, we agree it is difficult to determine on/offshore areas. The areas covered by the Norwegian Sea have been now represented by a transparent white layer above the geophysics imagery.
Page 9, line 206, Figure 2: I am not so convinced about these cross-cutting relationships. The thick black NW-SE trending line could easily be drawn across the thinner, SW-NE trending line (the magnetic anomaly continues across it), so to me it is not clear that the northern black line is the same as the southern
Response: We somewhat agree with your comment, the insert does not particularly show the entire context of that lineament placement for this particular case. However, there is both a high amplitude anomaly flanking the NE-side of the NW-SE lineament, and a possible remnant magnetised/de-magnetised zone to the SE of lineament. Both features were used to guide the location of the lineament. Additionally, the magnetic fabric to the NE appears to be offset over the NE-SW lineament. Lineament mapping using airborne magnetic data is interpretive, but if the mapping is completed in a consistent method, considering specific features in the magnetic data, this is the best possible interpretation. The authors do concede, owing to the interpretive nature of this work, that it is possible that the lineament could be mapped differently by another. The text has been overall rewritten for this section, and this has been removed.
Page 10, line 221: You do not highlight NNW-SSE lineaments in Fig 2BI, so it is difficult to follow your argumentation here
Response: We agree with the need to improve clarity, as such we have highlighted the lineaments in Figure 2B by colour, and labelled the associated rose diagrams.
Page 10, line 225: how do you constrain dip-slip offset from 2D lineament mapping?
Response: We have removed reference to this in the text, as it does have high uncertainty, and will use the other structural data (field and drill hole) to discuss. But a dip-slip offset on a 2D map is a possible interpretation when a magnetic anomaly changes in width (apparent thickness) over a cross-cutting lineament (suggesting a different portion of the magnetic unit is providing a geophysical response). This is a similar method of interpretation as used in reading geological maps. It is not possible to get this interpretation from DTM imagery.
Page 10, line 226: Move this information to the start of the N-S trend description
Response: We agree, the text has been updated to the suggested.
Page 10, line 228: I do not see any convincingly dextrally offset lineaments on this figure - can you point at a particular lineament that you can trace to the right north of the E-W lineament?
Response: We agree, the text has removed.
Page 10, line 228: dip-slip is difficult to constrain on 2D maps
Response: We agree, the text has removed.
Page 10, line 229: Again, you have to point at particular lineaments which you think are offset, since I do not easily see the offsets you describe in the pattern, which shows many cross-cutting, but not offsetting, lines
Response: We agree that the small ‘zoom-in’ inserts with geophysical imagery are not sufficient for showing the offsets. We have now selected two optimal areas with the most examples of lineament cross-cutting relationships. The ‘zoom-in’ images are also at a smaller scale (more ‘zoomed-in’) with both the DTM and geophysics shown. Moreover, we have decided to simplify the text and discuss cross-cutting rather than offsetting relationships, except in very clear examples.
Page 10, line 232: Move this to the start of the NW-SE trend description
Response: We agree, the text has been moved to the beginning of the description.
Page 10, line 235: The HSF is NE-SW trending. Do you assume sinistral or dextral movement along this when considering Riedel orientation? If you assume sinistral movement, and sigma 1 oriented 45 degrees to the fault (approximately N-S), then the Riedel (R, not R prime) orientation would be NNE-SSW. If you assume dekstral movement, sigma 1 would be approximately E-W, so the Riedel orientation would be WSW-ENE - which situation do you envisage here?
Response: We concede that this is too uncertain, and too interpretive for this part of the manuscript, so we have removed the reference to Reidel shears. The lineaments could be splay off the MTFC as Reidel shears or just other potential structures which have formed during a different tectonic episode. The latter is more likely. As such, the possible lineament offsets will be interrogated/compared against the structural data later in the discussion.
Page 10, line 237: This figure looks more like the E-W lineaments cross-cut all the others?
Response: We agree, the figure has now been changed and the text rewritten. The NW-SE, NNE-SSW, and NNW-SSE lineaments do appear to crosscut the E-W lineaments. In the discussion, we suggest that this may be a later reactivation of these features.
Page 11, line 238: Does that mean that the N-S, NW-SE and E-W lineaments all cross-cut each other at some point and it is not possible to say who is oldest/youngest? Be more specific here
Response: We agree that the text is too ambiguous, so it has been rewritten. Where these lineaments are third-order lineaments, they, they are crosscut by the other lineament trends in a similar way. This would suggest that they may be relatively early lineaments. Second-order E-W lineaments crosscut almost all other features, suggesting they continued to develop until later.
Page 11, line 242: Move this to the start of the description
Response: Yes, we agree; the text moved as suggested.
Page 11, line 246: again, you have to point out specific offset lineaments, on Fig. 2BV I noly see straight lines that crosscut the NE-SW lineaments.
Response: We agree. The text has been updated to emphasis cross-cutting relationships rather than offset, and a new figure insert in Fig 2 has been included to highlight these cross-cutting relationships (see general comment above).
Page 11, line 249: Move this to the start of the orientation description
Response: We agree; the text has been moved as suggested.
Deformation history
Page 11, line 257: Rename this into for example "field and drill hole results", or separate into two chapters, 4.2. Structural field measurements and 4.3. Deformation history based on mineralogical relationships" or something similar, based on how you decide to restructure your text based on my comment in the separate reviewer file
Response: We largely agree with this comment. We have now changed the title to “Field, drill hole, and petrographic results”, as the petrographic work was a significant component of these results.
Page 11, line 258: Here you jump directly from the lineament mapping to the borehole scale, and skip the entire field station part! I highly recommend you start your result description with a summary of what your field stations have revealed - se detailed comment in the separate reviewer file.
Response: We concede that this is a significant gap in the results section. Therefore, we have now included summaries of the field structural results along with the relevant field, drill, and petrographic hole observations. These summaries are included by deformation episode and mineral-assemblage. We have decided to nestle the structural data results within the text to improve flow and limit repetition.
Page 11, line 260: They are not properly presented in the current manuscript, and should come prior to the cm-scale borehole observations!
Response: We have included a detailed figure with stereonets displaying all planer, lineation, and kinematic data. Both the field and drill data has been included.
Page 12, line 273, Figure 3: would it be possible to color code the structural measurements according to D1-D5?
Response: Unfortunately, this is not possible. We have included this information in the previous figure and will keep the figure as is. This figure is intended to indicate the distribution of collected data, and the variability of structural geometries downhole.
Page 12, line 283: this has to be documented by a proper stereonet figure showing plane and lineation measurements
Response: We agree. This has now been included the within the Field, drill hole, and petrographic results section.
Page 14, line 309: This is again easier to state if you can refer to a stereonet figure
Response: We have now included a reference to the stereonet figure.
Page 14, line 323: refer to a stereonet figure
Response: We have now included a reference to the stereonet figure.
Page 14, line 327: Strike through
Response: Agreed, text removed.
Page 14, line 327: which subfigure?
Response: We agree this is missing, text modified to “Figure 5E”
Page 14, line 328: stereonet figure - orientation of slicken lines or other lineations associated with it?
Response: This has now been included the within the Field, drill hole, and petrographic results section, and shown on Figure 3.
Page 15, line 329: has to be documented with lineation measurements and kinematic indicators
Response: We agree, and this has now been included within the Field, drill hole, and petrographic results section. Removed from current position.
Page 15, line 333: Fig 5. Specify the samples and/or field stations/drill cores the pictures are coming from - it can be a good idea to add coordinates behind each field and thin section photograph (see figure caption of Hestnes et al. (2022))
Response: Yes, we agree with this. We have included the coordinates (UTM)/Drill hole and depth for the samples within an additional document within the supplementary material. We have also included a reference to this in the paper.
Page 16, line 347: orientation data
Response: This has now been included in the within the Field, drill hole, and petrographic results section, and shown on Figure 3.
Page 16, line 364: lineations and kinematic indicators
Response: This has now been included in the within the Field, drill hole, and petrographic results section, and shown on Figure 3.
Page 16, line 368: if they are well-consolidated, they are not really gouges longer? If they have a cohesion, they might be ultracataclasites?
Response: Yes, this may be true, but in this case, the gouge is indurated/cemented with hematite/Fe-oxides. So, the level of consolidation is secondary. We have amended the text to make this clearer.
Page 16, line 368: Strike through
Response: Agreed, removed “on average”
Page 17, line 380, Figure 6: It would be great to have such a figure as a stereonet, also with associated mineral lineation data plotted.
Response: This has now been compiled for Figure 3 with stereonets for each deformation episode.
Page 18, line 414: "on average" is strange wording in this context. If you have 10 planes dipping 90 degrees and 10 planes dipping 0 degrees, the average dip becomes 45 degrees? You probably mean that most measurements fall within 30-70 degrees, but that is not an average value - I suggest you remove "on average"
Response: Yes agreed. We have removed this phrase from the text.
Page 16, line 415: kinematic indicators?
Response: This has now been included the within the Field, drill hole, and petrographic results section, and shown on Figure 3
Page 19, line 433: The following four paragraphs (until the subchapter 4.3.) are out of place - they deal with correlations of particular features in the boreholes shown in Fig. 10, and should entirely be moved to and incorporated into the text in chapter 4.4
Response: Yes, we apologise, these paragraphs are misplaced. We have moved and rearranged them with the section “Basement deformation in 3D”.
K-Ar geochronology and X-ray diffraction
Page 20, line 466: Thats the aim - but how can you document/make sure that your fractions only contain authigenic and synkinematic clay?? This has to be argued for for each sample carefully
Response: We have now characterised each of the samples systematically, with commentary on the composition of each of the grain size fractions. A full description and characterisation is now available in the supplementary material.
Page 20, line 470: The composition of the different fractions for each sample has to be introduced better - which fractions do most probably only contain illite or smectite, whereas which fractions do probably contain a mixture of illite/smectite and inherited muscovite, K-fsp etc.? This is crucial for the age interpretation.
Response: As stated above, we have now characterised each of the samples systematically, with comments on both the composition of each of the grain size fraction, and also the potential source of the minerals. This will be used later in the discussion to further argue for the age interpretations.
Page 20, line 476: thats too general - introduce the dates for each fraction from each sample systematically, and discuss their geological significance
Response: Yes, we agree, therefore we have introduced each of the samples’ K-Ar ages, by the grain size fractions. With a focus on the most geologically significant size fractions. This is typically the finest and the coarsest, but where the age spectra curve deviates from an inclined curve, we have added comments on this (i.e. where there is an older/younger age which is not associated with the coarsest and the finest fractions).
Page 21, line 480: this statement needs a reference and/or better explanation
Response: Yes, we agree this needs a reference. We have included the reference of Levy & Woldegabriel (1995), which discusses the loss of radiogenic 40Ar from a zeolite (clinoptilolite).
Page 21, line 481: You have to discuss that systematically for each sample, since the mineralogical composition for each sample and each fraction is different
Response: Agreed. We have introduced each of the samples’ XRD results prior to outlining the K-Ar dating results. This has been done by mineral type and by grain size fraction.
Page 22, line 493: respectively
Response: Agreed. Text removed and rewritten.
Page 23, line 496: This is way too generalized. The three youngest finest fractions actually scatter from ca. 70 to 130 Ma potentially belonging to very different deformation phases (e.g. see the discussion of calcite ages at 140, 90-80 and 70-60 in Hestnes et al.). If you argue properly for each sample and why you think the youngest fraction is geologically meaningful, you then can extract much more information from this data than just the three broad belts at 100, 200 and 300.
Response: We have removed this text, as we agree it is too generalised and simplistic. The different samples are now presented with their relevant K-Ar ages. From the XRD and K-Ar geochronology work we now have the following date groups ~287 – 291 Ma, 196 – 204 Ma, 100 – (128?) Ma, and 75 Ma.
Page 23, line 497, Figure 9: Once you have argued properly and systematically for each sample, you can highlight those dates which you think are geologically meaningful - this might be all the finest fractions (74, 99, 128, 196, 201, 204) and maybe a plateau in the coarsest fractions for sample SK2008 at ca 200, but this depends on the mineralogical interpretation of these fractions (inherited or authigenic?). You should remove the three belts at 100, 200 and 300, which are confusing and not precise enough
Response: We have included the dates we feel are geologically meaningful and updated the figure. The belts are in place to highlight clustering of dates for the finest fraction. The earlier large fraction dates for SK2012 and SK2015 are also highlighted as these two different structures provided two very similar dates. We have now included a belt for the youngest age for sample 120714, as this age provides an upper limit for the D4 mineralisation.
Page 24, line 505: With
Response: We agree, the text has been removed.
Page 24, line 507: Consider replacing this with a figure showing the mineralogical content of each fraction from each sample. This information has to be placed before the age information
Response: The figure has been replaced by a series of pie chart graphs for each sample and by grain size fraction. The figure has been furthermore moved to before the K-Ar dating information.
Page 24, line 514: Should come before the age presentation, should be done systematically sample for sample
Response: Agreed. This portion of the text has been integrated into the individual sample characterisations and moved to be before the K-Ar geochronology information.
Page 25, line 525: this has to be introduced and discussed better: do you think this could be a deep weathering zone, not necessarily related to deformation, but to deep infiltration of surface fluids? So the age of ca. 128 Ma for the two finest fractions could represent Cretaceous deep weathering? How would that fit with deep weathering proposed elsewhere along the margin (if I remember right that was supposed to be in the Jurassic?)
Response: It is always possible that percolating meteoric/ground water has penetrated down and along inclined structures. The overall zone is saprolitic, so the age of the finest grain size fraction may well be authigenic clay growth related to saprolite development. However, weathering along the western portion of the Norwegian passive margin (and particularly along the Strand flat) is typically associated with Jurassic times (e.g. between ~221 Ma to 206 Ma for Bømlo, Western Norway) (e.g. Fredin et al., 2017; Olesen et al., 2023). As SK1033_1 has returned a Cretaceous age, there may well be a hydrothermal component to the age.
Basement deformation in 3D
Response: This section has been moved to before the K-Ar geochronology and XRD section.
Page 27, line 558: the 3D model in the middle of Fig 10A shows more than four green planes - it is not so easy to identify the four planes which occur in both bore holes from this plot - could it be an idea to colour the four planes which occur in both cores with a different colour?
Response: We agree that this figure is not clear, and well-correlated with the associated text. We have updated the figure, with the zones colour-coded by deformation episode. The zones mentioned in text have now also been labelled I, II, III, IV to aid identification from what is written in text.
Page 27, line 560: here you say a D1 zone occurs in both B1 and B2, but in the Figure, the planes are all green, where the legend says these are D2 zones - could it be an idea to color the planes according to whether they are D1, D2 etc. features? Out from your description in the text here (zones I to IV i struggle with clearly identifying them in the figure. Could it be an idea to also mark them with I to IV in Fig. 3?
Response: As stated above, the figure has been updated, with the zones colour-coded for deformation episode. The zones mentioned in text have now been labelled I, II, III, IV to aid identification from what is written in text.
Discussion
Polyphase evolution of Smøla and the passive margin
Page 27, line 579: abbreviation not used earlier? Explain
Response: We acknowledge this comment. The text has been removed from the update manuscript.
Page 27, line 581: Such orientations are very minor in Fig. 6 D1, where D1 seems to be dominated by E-W lineaments, so I am not sure what you are referring to here?
Response: These lineament trends are potential early D1 features owing to the cross-cutting and termination relationships. We have amended the text to reflect this better.
Page 27, line 585: ENE-WSW cannot vary from NNE-SSW to NW-SE?
Response: We agree. Text removed; it was attempting to show the range in strike trends.
Page 27, line 587: how do you get local folding and contraction during sinistral transtension? within a restraining bend?
Response: We acknowledge that this is a controversial issue. Works done by Bøe & Bjerkli (1989) and Fossen (2010) (and references within), suggest that there was a N-S contraction of the Devonian Basin adjacent to the MTFC during Mid to Late-Devonian times (based on folds with E-W trending axial traces, and reverse faults). This contraction may have occurred syn-kinematically with strike-slip movement on the MTFC, or if transtension on the MTFC occurred, either before or after this phase. If before, these contractional features may have rotated as they approached the MTFC trace (Fossen, 2010). A portion of the D1 structural data, however, does fit with this contraction direction.
Page 28, line 594: the youngest fractions in the D2 samples are 200 Ma, which possibly can be interpreted as the formation age of the gouges/shear fabrics - the significance of the coarsest fractions at 300 Ma is much less clear, and if you would like to assign those a geological significance, you have to argue for it properly in the results section (maybe the two oldest fractions in 2012 and 2015 make some sort of meaningful plateau?) ->but I would definitively place D2 around 200 Ma, and not 300 Ma based on the youngest fractions
Response: We have adjusted the text to focus mostly on the ~200 Ma age associated with the finest fractions of the samples SK2012, SK2015, and SK1029_1. However, the clustering of dates at ~300 Ma for the size fractions 2-6 and 6-10 µm for the samples SK2012 and SK2015, does suggest that there is distinct inherited component in the K-Ar ages. Considering the differences in both structure orientations and mineral composition (minor difference) in the coarser fractions of these samples, it would be expected that there would be more age difference, particularly if the potassic feldspar in SK2015 afforded a protolithic component. As there is not, there may be an earlier Carboniferous-Permian deformation episode preserved in these deformation zones. The microstructural evidence also suggests that there are two preserved events in the zones.
Page 28, line 595: Fig 6 shows that E-W is minor for D2, but NW-SE is quite important?
Response: Yes, on the rose plots the E-W trend is not visible enough. These structures correlate to the major E-W lineaments (L3). However, the new stereonet plots in Figures 3, do show the importance of the E-W structures. The text has been updated to reflect this as well.
Page 28, line 596: just speculation/correlation with what others have said before?
Response: Yes, we are correlating the findings with the associated literature to try fit our findings within what others have found. This part of the D2 section has been rewritten, to explain how the structures with sinistral kinematics and the associated K-Ar ages can be compared and correlated with the relevant literature.
Page 29, line 603: In this entire discussion of lineaments and orientations it would be really useful to have the real structural data plots, not only lineaments. From the borehole plots it is obvious that many of the structures have a quite shallow dip, which is of importance if one wants to relate them to the basically steep MTFC faults! You should also discuss this in the light of your own kinematic indicators and lineations from the field, not only somewhat ambiguous offsets from the lineament maps.
Response: We agree it is important to integrate the field and drill hole structural data, and as we have stated before, these datasets are now included in the Discussion section.
However, regarding the shallow structures in the drill holes:
Importantly, the drill hole data probably contains more shallow than steep features owing to line-sampling bias (e.g. Terzaghi, 1965).
Most of the shallowly dipping D2 structures come from drill holes BH3 and BH4. These holes intersect several foliated gneiss and diorite units. Most of the shallowly dipping features correspond to slip along pre-existing foliation planes (with chlorite being present) within particularly micaceous intervals.
The drill holes BH1 and BH2, are mostly in massive monzogranite, and are situated north of a significant E-W structure, which locally shifts to a NW-SE trend.
The geometry of the features in BH1, BH2, therefore may rather stem from local stress perturbations (in terms of orientations of the principal stress axes, and stress magnitude) within the damage zone of a major structure (e.g. Kim et al., 2004). Or, if assuming these features are shallow thrusts, they may have formed due to local shortening (transgressive faulting) owing to the strike rotation of the major E-W structure, forming a restraining bend (assuming sinistral kinematics) (e.g. Cunningham & Mann, 2007). As visually demonstrated in the 3D modelling, the D2 structures dip S towards the steep major E-W structure. They may therefore represent either antithetic or conjugate features forming in an extensional setting (which have later rotated to shallower dips), although confirmation is challenging without kinematics of the modelled structures, and resolving the geometry of the major E-W structures (assumed to be steep to sub-vertical).
We have updated the text to include reference to the field and drill hole structural data, and 3D modelling.
Page 29, line 616: But the 200 Ma ages seem to come from very shallow structures (<30degrees) - how does that fit with P- and R-shear dextral strike-slip models based on lineaments?
Response: We somewhat agree, however only the sample SK2012 comes from a structure dipping at <30 degrees. The other two samples SK2015 (36 degrees), and SK1029_1 (80 degrees), show that a variety of structures were experiencing fluid flow and authigenic activity at ~200 Ma. Nonetheless we have rewritten this section to remove the correlation with P and R-shears which would indeed require steep to sub-vertical structures.
Page 29, line 618: that would imply subvertical structures, but your measurements in the boreholes are all quite shallow - these geometries do not look like a classic strike-slip setting?
Response: We agree that the shallow structures in the drill holes are problematic for a strictly strike-slip interpretation. The shallow structures related to the later D2 deformation are likely to be conjugate or antithetic features off the steeply dipping/sub-vertical structures. These structures would have had to have formed in an extensional setting. We have rewritten the text to take the geometries of the structures into account. Particularly, the 3D modelling.
Page 29, line 622: you mean ridge-push? its either ridge-push or slab-pull - I dont think there was a slab around at that time anywhere in the North Atlantic, but there was not ridge either until the Paleogene, so what do you mean here? Far-field effects from the Alpine slab? Along which margin - there was no margin at that time yet?
Response: Yes, we meant ridge-push. The suggestion that possibly ridge-push forces may have induced dextral strike-slip on the MTFC comes from studies looking at later tectonic cases (such as the Cenozoic for Pascal & Gabrielsen (2001)). We concede that this is too distant in time from the Triassic-Jurassic, so we have removed this from the text.
Page 29, line 625: I would try to have a look at the entire structural data set - what are your dip values saying, what are your mineral lineations saying? Are we in an extensional setting with flat-lying structures developping mostly reactivating older foliations (reactivation phase of Peron-Pindivic etc.)? Are the lineations subhorizontal or down-dip? The lineament maps alone do not tell us the entire story
Response: We agree this is the best approach. We have rewritten the section to incorporate this information within the text.
Page 29, line 628: what do you mean with authigenic here? Which mineral? And I would say that all your three D3 samples provide finest fraction ages between 120 and 74, not between 200 and 100 - so once you have described and interpreted your 7 samples more carefully, you can be more precise here.
Response: The authigenic age we used, related to the growth of clay minerals within the samples. We agree this is not the best use of the term considering the larger size fractions for the samples SK2008 and SK1033_1 may well include protolithic/inherited material which would not be authigenic. We have updated the text to be more specific based on the new systematic descriptions of the D3 samples.
Page 29, line 629: can you see such a transition in your data? be more specific on what your orientation and kinematic data say about the D3 phase - in order to do that, you have to present your stereoplots including lineations/kinematics.
Response: The proportion of the structures which exhibit dip-slip (mostly normal to oblique-normal) kinematics compared to D2 is significant. This indicates to us that there was a change in the overall tectonic regime over Smøla. The ~N-S reverse faults, and dextral features provide some limited evidence (along with the work of Bøe and Bjerkli (1989)) in suggesting that there was some possible dextral strike-slip during the later D3 episode. We have updated the text to present the structural data for D3.
Page 30, line 629: what are your field measurements on D3 structures telling?
Response: Most of the D3 structures are dip-slip features. We have updated the text to state this.
Page 30, line 647: where comes this from? shouldnt it maybe be 128 Ma?
Response: The text was reflecting the largest size fraction age within the relative age of the zeolite veins. This is unnecessary, so we have updated the text to just reflect the age of the smallest size fraction (~75 Ma).
Page 30, line 660: Hestnes worked north of Sognefjorden, which I would not call SW Norway (which is Rogaland etc.)
Response: We agree, the text has been updated to describe it as W Norway rather.
Page 30, line 662: possibly related to the
Response: Agreed, text has been updated to the suggested.
Page 30, line 666: how do you really know which lineaments are associated with the different D phases, since many orientations occur in all phases (e.g. Fig. 6?) I understand that you can assign a fracture/fault observed in the field to one of the phases due to the mineralogy, but a lineament on a map, without groundtruthing, is probably difficult to uniequivocally assign to a D phase?
Response: We have previously used the cross-cutting relationships and strike trends of the different D’s to sort the lineament trends. We agree that this is not a very robust method, and we have now updated the text to include the structural data from the field and drill holes to improve the interpretation and sorting. Realistically, the lineaments are likely to have formed and then been reactivated during multiple deformation episodes. The shallowly dipping structures are also not typically represented by the lineaments. However, the orientation and cross-cutting relationships do convey a structural evolutionary story.
Page 31, line 670: Text removed
Response: We agree, text removed.
Page 31, line 674: here the dip-values of these features are again crucial
Response: We are keeping the text the same, as now the D5 features being “sub-parallel” means both the strike trends and the dips are similar to the bounding structures around the Frøya High.
Application to and implications for the Frøya High offshore domain
Page 31, line 677: I am not sure this title is very suited for the content of this chapter - what you discuss is basically an outlook on how onshore detailed field studies can help understanding offhore basement bodies, but that additional methods (permeability modelling etc) are needed to constrain parameters relevant for hydrocarbon exploration. Maybe a title could be "Smøla as an analogue model for similar offshore basement areas and possible future research" or something similar?
Response: I agree and have adjusted the title accordingly to “Smøla as an analogue for similar offshore basement volumes”. The purpose of this section is to try connecting this work with basement volumes offshore, with the Frøya High being the most applicable given the similarity in both basement geology and structural/tectonic history.
Page 31, line 678: consisting of similar basement rocks as Smøla
Response: The section has been rewritten, with this comment incorporated in the new text.
Page 32, line 706: could your saprolite sample be an example of weathered basement?
Response: This is possible. However, the K-Ar age of the saprolite sample, being significantly different to the results from other studies may not make this sample the best to use for this purpose. Further work focussing on specifically weathered basement rocks on Smola may answer this question.
Conclusions
Page 32, line 714: I am still not convinced that the approach is particuarly important or new from this study, but the detailed and careful field and borehole observations are definitively very valuable, so I would focus more on that aspect
Response: We agree, the new component of this study relates to the four drill holes. We have therefore removed the mention of a “new” approach from the text. Moreover, the conclusions concentrate on the field, drill hole findings, with the K-Ar dating, lineament mapping results also highlighted.
Page 32, line 717: These conclusions should be updated once the field structural data including lineations and kinematics is included, and dip orientations have to be taken into account (the shallow dips pointing at not only strike-slip dominated deformation?)
Response: Yes, we agree, the findings and the discussion of these findings have been significantly amended with additional information regarding the structural data. We have updated the text of the conclusion accordingly.
Page 32, line 725: ?
Response: Yes, we concede this is too speculatory (see response above), so we have removed mention of slab and ridge-push from the text.
Page 32, line 741: Has to be updated after a careful evaluation of each sample/fraction
Response: We agree on this, and have updated the text to include the following age intervals based on the systematic evaluation of each of the samples: ~287 – 291 Ma (early D2), 196 – 204 Ma (late D2, early D3?), 100 – (128?) Ma (D3), and 75 Ma (upper age cut-off for D4).
We would like to extend our gratitude once again to Prof. Gasser for her invaluable contributions to the improvement of our manuscript. The detailed and rigorous suggestions have been carefully considered, and where possible, we have made corresponding revisions to update the work. We are confident that these adjustments will strengthen the overall quality of the manuscript.
Yours sincerely,
Matthew S. Hodge1*
Guri Venvik2
Jochen Knies2
Roelant van der Lelij2
Jasmin Schönenberger2
Øystein Nordgulen2
Marco Brønner2
Aziz Nasuti2
Giulio Viola1
Affiliations:
- Department of Biological, Geological and Environmental Sciences, University of Bologna, Italy
- Geological Survey of Norway (NGU), Trondheim, Norway
(*Corresponding author)
References
Bøe, R., & Bjerkli, K. (1989). Mesozoic sedimentary rocks in Edøyfjorden and Beitstadfjorden, Central Norway: Implications for the structural history of the Møre-Trøndelag Fault Zone. Marine Geology, 87(2), 287–299. https://doi.org/https://doi.org/10.1016/0025-3227(89)90066-2
Cunningham, W. D., & Mann, P. (2007). Tectonics of strike-slip restraining and releasing bends. Geological Society, London, Special Publications, 290(1), 1–12. https://doi.org/10.1144/SP290.1
Fossen, H. (2010). Extensional tectonics in the North Atlantic Caledonides: A regional view. Geological Society Special Publication, 335, 767–793. https://doi.org/10.1144/SP335.31
Fredin, O., Viola, G., Zwingmann, H., Sørlie, R., Brönner, M., Lie, J. E., Grandal, E. M., Müller, A., Margreth, A., Vogt, C., & Knies, J. (2017). The inheritance of a mesozoic landscape in western Scandinavia. Nature Communications, 8. https://doi.org/10.1038/ncomms14879
Kim, Y. S., Peacock, D. C. P., & Sanderson, D. J. (2004). Fault damage zones. Journal of Structural Geology, 26(3), 503–517. https://doi.org/10.1016/J.JSG.2003.08.002
Levy, S., & Woldegabriel, G. (1995). Ion Exchange and Dehydration Effects on Potassium and Argon Contents of Clinoptilolite. MRS Proceedings, 412, 791. https://doi.org/DOI: 10.1557/PROC-412-791
Olesen, O., Rueslåtten, H. G., Schönenberger, J., Smelror, M., van der Lelij, R., Larsen, B. E., Olsen, L., Baranwal, V., Bjørlykke, A., & Brönner, M. (2023). Jurassic heritance of the geomorphology in Mid Norway. Norwegian Journal of Geology, 103.
Pascal, C., & Gabrielsen, R. H. (2001). Numerical modeling of Cenozoic stress patterns in the mid‐Norwegian margin and the northern North Sea. Tectonics, 20(4), 585–599. https://doi.org/10.1029/2001TC900007
Terzaghi, R. D. (1965). Sources of Error in Joint Surveys. Géotechnique, 15(3), 287–304. https://doi.org/10.1680/geot.1965.15.3.287
- AC4: 'Reply on RC2', Matthew Hodge, 26 Feb 2024
- more detailed and more convincing presentation of cross-cutting relationships of lineaments, preferentially with high-resolution DTM pictures in addition to the magnetic anomalies
Status: closed
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RC1: 'Comment on egusphere-2023-2504', Alessandro Petroccia, 07 Dec 2023
Review of “Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs”
by Matthew S. Hodge et al.
Manuscript egusphere-2023-2504
General comments to the authors
Dear Authors, I appreciate the proposed paper “Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs”. The work shows new interesting and high-quality data using a multidisciplinary and multiscalar approach that could be applied to other basements offshore in Norway and/or elsewhere. This is the first round of revisions of the manuscript which comprises an impressive amount of detailed work, from field to micro-scale, combined with K-Ar geochronology, in order to develop a 3D model of the investigated problem. I think the main ideas and the manuscript are strongly valid and suitable for the quality of this journal. I hope my comments on both the PDF and this file will help (please see the annotated version of the PDF file).
I would like to highlight a few important general suggestions to be taken into account:
- The sentences of the manuscript are often verbose and wordy, making it unclear and difficult to follow. Much of this can be easily assessed by adopting a more stringent writing style. I appreciate the serious amount of work that is present behind this manuscript, but I strongly suggest to simplify the sentences and the way to explain the obtained data.
- Abstract: The abstract is quite long, and reads like an introduction. It does not really contain the quantitative results produced in this story. Please concisely present the main results of the study rather than alluding to them.
- The authors used in the manuscript a lot of abbreviations, both in figures and in the main text. Please explain them better or provide a table with all the abbreviations and their relative meanings.
- Figures are cited in the main text without following a numerical order. Please try to cite them progressively or modify the order of the figures
Sincerely,
Petroccia Alessandro
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AC1: 'Reply on RC1', Matthew Hodge, 26 Feb 2024
Response to review comments for the paper titled: “Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs”
Referee 1) Dr Petroccia Alessandro; Citation: https://doi.org/10.5194/egusphere-2023-2504-RC1
Dear Dr Petroccia,
We wish to extend our sincere gratitude for your thorough review and insightful comments on our manuscript entitled "Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs."
We appreciate the time and effort that you have contributed to our manuscript, and the constructive feedback will be invaluable in improving the quality of the work overall. We have thoroughly considered each of your points below and present our responses and revisions.
General comments (summarised):
- Simplify Sentences: The manuscript's sentences are excessively verbose, making it challenging to follow. Adopting a more succinct writing style would enhance clarity and understanding.
- Abstract Length and Content: The abstract is lengthy and resembles an introduction rather than summarising the study's quantitative results. It is advised to present the main findings concisely rather than providing extensive background information.
- Abbreviations: The excessive use of abbreviations in both the text and figures leads to confusion. Improved explanation of abbreviations or inclusion of a table listing their meanings is recommended.
- Figure Organisation: Figures are cited in the text without following a numerical order, leading to inconsistency. It is suggested to either cite them progressively or rearrange the figure order for better coherence.
General responses and revisions made to manuscript:
- We appreciate the suggestions regarding a more succinct and less verbose text. Revisions to the text have involved simplifying the descriptions and characterisations where possible, to aid clarity and understanding. Furthermore, sentences including more than one key point, have been separated where possible.
- The Abstract has been reduced in length, with a focus on the results and interpretations of the study. To provide some introduction into the study, only a minimal inclusion of both the geological background and motivations of the study has been maintained.
- We acknowledge the use of abbreviations in both the text and figures does lead to some confusion. We have tried our best to ensure all abbreviations are explained in the body of text and the figure captions. We understand that a table of abbreviations explanations would be helpful, but we feel the abbreviations are sufficiently explained now in the revised manuscript that a table would be redundant.
- We agree that the figure numbers, as cited in the text, did not always follow sequentially. We have revised the manuscript now to limit this issue, and to ensure that the figures correspond properly in the text.
Annotated comments within the text:
The referee comments within the manuscript are included below with the page number and line number of the original annotated manuscript. Below each referee comment, we provide our response, and our change(s) made within the new revised manuscript.
Abstract
Page 1, line 10: sentence very long and wordy
Response: We agree that the abstract is too long and wordy. We have now amended the abstract to focus on the results, with minimal information regarding the motivation and geological background of the stay.
Introduction
Page 2, line 33: In this section, a lot of sentences are very long and wordy. Please make them shorter with a direct message
Response: We agree many of the sentences can be shortened and improved. We have rewritten and shortened the sentences through the introduction.
Page 2, line 35: Please explain the concept better
Response: The basement highs offshore Norway are typically buried under younger sedimentary rocks, and below the Norwegian Sea. This makes them inaccessible. We have amended to the text to highlight the difficulty of accessing them in deep oceanic waters and beneath younger sedimentary cover.
Page 2, line 34: Text removed
Response: The text has been rewritten due to the above comment, but we do not agree with this deletion. This sentence is in contrast against/a modifier of the previous sentence, so it needs a “however”.
Page 2, line 51: why inside the quote?
Response: The single quotation marks signify the word “gap” is being used as a term. This is unnecessary, so we have amended the text to remove the singular quotation marks.
Page 2, line 51: provide a definition of this word with relative references
Response: We have clarified the term with a relevant reference and changed the text to “connected natural fractures and faults hosted within basement volumes (intrabasement structures) in an offshore context (Holdsworth et al., 2019)
Page 2, line 52: offshore structures
Response: We agree and have updated the text to the suggested.
Page 2, line 53: wordy and hard to follow. please rephrase
Response, we accept the suggestion, and have rewritten the text to a shorter sentence: “Consequently, characterising sub-seismic to regional-scale structures, and their potentially long-lived tectonic evolution, requires more than just low resolution datasets.”
Page 2, line 56: could you provide any examples?
Response, we accept the suggestion and have amended the text to provide some examples.
Page 3, line 63: is this part of the sentence necessary?
Response: We agree and have removed the text.
Page 3, line 70: this is valid for the overall main text, if not necessary of fundamental, try to avoid words like novel, new... etc
Response: We agree and have rewritten this section of the Introduction.
Page 3, line 70: also this one is valid for the overall text, homogenize the use of "-"
Response: We agree, this portion of text has been removed from the introduction.
Page 3, line 75: why the "-" after geological?
Response: We agree, this portion of text has been removed from the introduction.
Page 3, line 76: strange word
Response: We disagree, as the oriented drill holes do indeed offer a relatively unprecedented 3D view of the Smola geology. This has not been previously available. However, this portion of text has been removed from the introduction.
Page 3, line 80: Text removed
Response: We agree, and the text has been removed.
Geological framework
Page 4, line 104: there is an extra space after the bracket
Response: Well noted, and the text has been removed.
Page 5, line 120, Figure 1: the black continue lines are not explained in the legend
Response: We agree, and we have amended the legend of Figure 1A to reflect the symbology of the black lines (major structures).
The applied toolbox
Page 7, line 172: document or material?
Response: we agree and have amended the text to “material”.
Page 7, line 178: what does it mean?
Response: we agree this is too vague, so we have amended the text to “standard geological field methods”.
Page 7, line 180: total,
Response: we agree and have changed the text to the suggested.
Page 7, line 180: collected data?
Response: Data is indeed collected through measurement and observation. We have not amended the text.
Page 7, line 184: you jump from figure 3 up to 8
Response: We have updated the text to also include the mention of the 3D modelling, which provides an intermediate figure. The K-Ar sample figure is however much further through the manuscript, so a ‘jump’ in figure numbers is unavoidable.
Results
Page 8, line 194: how can you trace it? from the DEM?
Response: The onshore lineaments involved a combination of DTM and aeromagnetic data, while the offshore data exclusively used aeromagnetic data. We have amended the text to make this more clear for the reader.
Page 9, line 206, Figure 2: the dashed line is not explained in the legend. I suggest making the colour with higher transparency, so that the different fracture systems would be more visible (the same transparency of the zoom)
Response: We agree with this suggestion, however we have removed the dashed line, which along with the area labels A – F, make this figure item easier to understand. We have adjusted the transparency of the offshore areas to make it easier to distinguish the on and offshore areas. The overall Figure 2A map has also been enlarged to help make the lineaments more visible.
Page 9, line 206, Figure 2: which contour calculation did you use? kriking?
Response: The geophysical imagery was processed using standard gridding (minimum curvature) and levelling methods of the Geosoft Oasis montaj (Geosoft, 2010). A full description of the geophysical data processing and merging methodology is available in (Nasuti et al., 2015). As these details are out of the scope of this manuscript, we have not amended the text, and refer the reader to the supplementary material.
Page 9, line 206, Figure 2: the differences between 2-3 order is hard to distinguish in the figure
Response: We agree, and we have made the trace lines on Figure 2A thicker for the 1st and 2nd order lineaments, so they can be more differentiated from the 3rd order lineaments.
Page 10, line 223: ?
Response: “Abut” means to terminate against something. To avoid any confusion, we have changed the use of “abut” to “terminate” in the text.
Page 11, line 258: homogenize, use always or Figure or Fig.
Response: We agree, and have decided to use “Figure”, and have removed the use of “Fig.” in the text.
Page 11, line 258: Text removed
Response: text rewritten to “a novel 3D perspective on Smøla's geology”.
Page 11, line 260: Text removed
Response: We agree, and the text has been removed.
Page 12, line 282: epidote or episode? please explain. Clc according to what mineral abbreviation guide?? Please use the conventional way for minerals, like Warr, 2023
Response: Thank you for the suggestion, “epi” should be “Ep”, and “clc” should be “Cal”. We are using the mineral abbreviations after Siivola & Schmid (2007). We have amended the text to ensure all mineral abbreviations are standardised.
Page 13, line 290, Figure 4: explain bk
Response: The abbreviation “bk” is already described in the caption for the figure. The use of “bk” is with “Chl (bk)” for black chlorite. No changes have been made to the text.
Page 15, line 331, Figure 5: what is frg?
Response: This is an abbreviation for “fragment”. We have amended the text of the caption and included an explanation of the abbreviation as “Reworked host rock fragments (frg)”.
Page 15, line 332: As a suggestion, the letter of the figure (A...) are very large and heavy to see. Also, if it is not necessary, please make the photos clearer, with less lines. Can you provide a better photo of FIGURE E? It is not so clear as a ECC
Response: We can accommodate this suggestion. We have decreased the size of the letter labels and decreased the number of lines on the sub-figure (E). The photomicrograph is the best that is available for this microstructure. The microstructure is clearly a ECC texture, with the C` planes inclined to a primary slip plane (white line), reworking an older S fabric.
Page 15, line 337: All abbreviations should be explained the first time they appear in the text
Response: We agree, and the caption for the microstructures figure (now Figure 6) has been updated to explain this abbreviation.
Page 16, line 357: reference
Response: We agree and have inserted the following reference: “(e.g. Passchier & Trouw, 2005)”.
Page 16, line 361: decorated seem to not be a scientific geological word
Response: We disagree, this term is in used to describe mineral coatings on the bounding fracture and vein surfaces (e.g. Scheiber & Viola, 2018; Viola et al., 2016). We have not changed the text.
Page 17, line 382: maybe its better to explain abbrevations before in the manuscript. Also a table with all the used abbreviations could be useful
Response: We appreciate this suggestion. The Figure 6 in the reviewed manuscript has now been moved earlier in the manuscript (now Figure 3), with the mineral abbreviations now being explained before the results of the field and drill hole mineral/deformation feature observations.
Page 19, line 439: jumped from figure 7 to 10. Please check the figure order and how you cite them in the text
Response: We agree that this is an issue. This section of the manuscript is actually misplaced (belongs to the 3D modelling section). The amended manuscript does not have this same figure number jump.
K-Ar geochronology and X-ray diffraction
Page 20, line 472: here you can use wt% since you have already explained the abbreviation in line 470
Response: We agree, however this text has now been removed.
Page 24, line 513, Table 4: Mus Capitalize the first letter of the abbreviations
Response: We agree, however this table has now been removed, with a pie chart figure describing the XRD results for the K-Ar samples. The abbreviation for muscovite, has the first letter capitalised as “Mus”.
We would like to extend our gratitude once again to Dr. Petroccia for his invaluable contributions to the improvement of our manuscript. Your thoughtful suggestions have been carefully considered, and where possible, we have made corresponding revisions to enhance the clarity and robustness of our work. We are confident that these adjustments will strengthen the overall quality of the manuscript.
Yours sincerely,
Matthew S. Hodge1*
Guri Venvik2
Jochen Knies2
Roelant van der Lelij2
Jasmin Schönenberger2
Øystein Nordgulen2
Marco Brønner2
Aziz Nasuti2
Giulio Viola1
Affiliations:
- Department of Biological, Geological and Environmental Sciences, University of Bologna, Italy
- Geological Survey of Norway (NGU), Trondheim, Norway
(*Corresponding author)
References
Geosoft, 2010b, OASIS Montaj v7.2 Mapping and processing system, The core software platform for working with large volume spatial data. Quick start tutorials.:
Holdsworth, R.E., McCaffrey, K.J.W., Dempsey, E., Roberts, N.M.W., Hardman, K., Morton, A., Feely, M., Hunt, J., Conway, A., and Robertson, A., 2019, Natural fracture propping and earthquake-induced oil migration in fractured basement reservoirs: Geology, v. 47, p. 700–704, doi:10.1130/G46280.1.
Nasuti, A., Olesen, O., Baranwal, O., and Dumais, M., 2015, Compilation of aeromagnetic data, in Olesen, O. et al. eds., Coop Phase 2 - Crustal Onshore-Offshore Project. NGU confidential Report , Norges Geologiske Undersøkelse, v. 063, p. 11–24.
Passchier, C.W., and Trouw, R.A.J., 2005, Microtectonics: Berlin, Springer Science & Business Media.
Scheiber, T., and Viola, G., 2018, Complex Bedrock Fracture Patterns: A Multipronged Approach to Resolve Their Evolution in Space and Time: Tectonics, v. 37, p. 1030–1062, doi:10.1002/2017TC004763.
Viola, G., Scheiber, T., Fredin, O., Zwingmann, H., Margreth, A., and Knies, J., 2016, Deconvoluting complex structural histories archived in brittle fault zones: Nature Communications, v. 7, doi:10.1038/ncomms13448.
- AC3: 'Reply on RC1', Matthew Hodge, 26 Feb 2024
-
RC2: 'Comment on egusphere-2023-2504', Deta Gasser, 17 Jan 2024
Review of “Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs” by Hodge et al.
General comments
This manuscript presents a multi-tool and multi-scalar analysis of the post-Caledonian, brittle evolution of the Island of Smøla, central Norway. The authors present remote sensing lineament mapping on airborne magnetic and DTM images, field observations from 66 field sites, and detailed logs of four boreholes with a total length of 365 m. They also provide thin section analyses of specific fracture and fault domains, K-Ar analyses from 7 fault rocks, as well as 3D modelling of fault and fracture geometries in two of the four boreholes. They describe the observed brittle deformation history in terms of five deformation phases spanning in age from the Devonian/Carboniferous to the Latest Cretaceous, and they discuss this evolution in terms of previous literature and the general evolution of the rifted Mid-Norwegian margin, with a particular focus on the significance of the Smøla geology for similar offshore basement highs.
The manuscript contains a number of nice, detailed structural and mineralogical observations from the field and bore holes, high-quality K-Ar analyses from seven samples, as well as a convincing subdivision of the observed brittle structures into five distinct types of features with specific mineralogical characteristics and cross-cutting relationships, allowing to present a relative chronology assigned to at least five different deformation phases, and placing them in an absolute time frame. I consider therefore the data set to generally be suited for publication in Solid Earth. However, there are several aspects in the data presentation and discussion which I consider to be deficient, and which have to be addressed in a revision before the manuscript can be considered for final publication. I have inserted detailed comments to each section of the manuscript below, and you also find additional minor comments in the attached annotated pdf. To sum up my major points from the comments below, a revision should take care of the following main aspects:
- more detailed and more convincing presentation of cross-cutting relationships of lineaments, preferentially with high-resolution DTM pictures in addition to the magnetic anomalies
- better introduction and presentation of the structural field measurements (a stereoplot showing both strike orientations and dip values, as well as lineation and kinematic data is a minimum)
- a more systematic presentation and interpretation of the K-Ar results sample for sample
- you have to streamline the text – remove wordy parts and repetitions, shorten the introduction, several paragraphs in the results section are placed under the wrong heading
- An interpretation of the brittle tectonic evolution taking into account the dip-values of your features and the orientation of lineations and kinematic indicators (related to point 2)
Implementing these changes requires major revision of the text and the figures along the lines described below. I hope my comments are taken in the positive spirit they are meant and help to improve the presentation and interpretation of this nice data set from a spectacular example of a basement complex along the passive margin of central Norway.
Sogndal, 16.1.2024, Deta Gasser
Specific comments to each section in the text
Title:
I think the title is not covering exactly what the manuscript contains. The Mid-Norwegian margin is much larger than just Smøla, and you haven’t been directly working on offshore basement highs either. I would therefore adjust the title to something like:
Multiscalar 3D characterization of the brittle tectonic evolution of the island of Smøla, Mid-Norwegian passive margin: an analogue for understanding the brittle architecture of offshore basement highs
1) Introduction:
- The introduction is quite lengthy, and should be shortened considerably. I have highlighted potentially superfluous parts in the annotated pdf (in particular, the last three paragraphs can be combined into a single one without loosing crucial information).
2) Geological framework:
- See minor comments in the annotated pdf
3) The applied toolbox
- I would describe the field mapping techniques prior to the bore holes – it is more logical to go from the large scale (remote sensing), via field mapping/outcrop observations, to borehole data.
- Say something about when and why the boreholes were taken – what is the reason for placing them where they are? Where they acquired during your project, or are these earlier cores? Where are the cores stored?
4) Results
4.1 Lineament mapping
- The description of the different orientation trends is somewhat difficult to follow. Use numbering for the 8 orientation trends (orientation trend 1) is N-S, 2) is NW-SE, 3) is E-W etc.) in the table and the text, which makes it easier to follow. You could also consider using colour coding for the eight orientation trends in the rose plots in Fig 2A and in the small inset maps in Fig. 2B, so that it is easier for the reader to separate the different groups. Start the description of each orientation group with where it can be found/in which domain it is most present – currently this information comes at the end of each orientation description.
- I am not sure I buy your cross-cutting relationship descriptions (Fig. 2B and Table 1). According to my experience it is quite difficult to find good cross-cutting relationships with lineaments, and several of the examples you show in Fig. 2B and/or mention in the text are not very convincing based on the figures you show. How can you be sure that the lineament on one side of another lineament is the same as the one on the other side with some offset? Often it is possible to draw the lineament across, and it is quite subjective where you end a lineament – your Fig. 2B shows examples where I would have drawn a lineament across another but you didn’t, and other examples where I would have stopped a lineament at another one, but you have drawn it across (see the screenshot below as an example). I get even more sceptical if you propose dip-slip offset from 2D lineament maps – if you propose this, you have to at least explain better how you identify dip slip offset of lineaments! So I suggest that you (a) either document proposed offsets more thoroughly with better figures at smaller scales and with DTM behind (maybe both with and without your interpretation shown so that the reader can make an own evaluation), pointing at particular examples where you are convinced this is a good offset, or (b) you rewrite the text, highlighting more the difficulties in actually finding good cross-cutting relationships, partly due to the difficulty in defining where a lineament starts and ends. As the small inset maps in Fig. 2B are now, I don’t find them very useful for understanding the crosscutting relationships you describe in the text. So if you want to focus on lineament cross-cutting relationships, I suggest you prepare an own figure for it with the best single examples of crucial cross-cutting relationships shown at a more detailed scale and with both DTM and magnetic images.
Example of an ambiguous lineament interpretation:
- Why are you not continuing the northern black line across the SW-NE lineament (as indicated with the red line)? The southern black line could just be a lineament parallel to the northern thick black line, they do not have to be the same structure which is offset? (the system for uploading an image in the review did not work, so please contact me to get a copy of the screenshot which I wanted to insert below - it is also not possible to attach two different files to the review, so I had to prioritize the annotated pdf).
4.2. Deformation history
- From 4.1 to 4.2, there is a big jump in the manuscript, where you move from the large-scale lineament mapping directly to the boreholes. I highly recommend that you start section 4.2 with a summary of your results from the field stations first. I do miss a proper documentation of your field results, in particular the structural measurements from the 66 stations. Could you provide a stereonet figure showing all your structural measurements from the 66 field stations, including faults, fractures and mineral lineations? Something similar to Fig. 4 in Hestnes et al. (2022) or Fig. 4 in Scheiber and Viola (2018) – possibly sorted according to the domains defined in Fig. 2B based on the lineament mapping? And maybe also containing information about the related mineralizations from each fracture/fault? Such a figure would give a good impression on the general orientation of the structures – are they subvertical, shallow-dipping, what are the lineations, mostly strike-slip, or mostly dip-slip? You also have to give information on kinematic indicators – how many dextral/sinistral or dip-slip/reverse-slip observations do you really have? You refer to kinematics in the D1-D5 section, but without presenting the evidence for it. This is crucial information for the further understanding of the brittle architecture of the area and maybe deserves an own subchapter, “Structural field measurements”? You should also move Fig 6 ahead, which is related to such a stereonet figure, since it shows all the strike orientations, but not the dip orientations. You could either keep Fig. 6 as an own figure (separating out the field vs. borehole measurements with different colors/grey shadings), or incorporate it into the stereonet figure. You also have to decide whether you want to show the structural data from the field together with the measurements from the drill core (which now are shown in Fig. 3), or separate from it. In any case, you have to give the reader a better structural framework before you describe the detailed mineralogical relationships in the sections D1-D5 (which are very nicely written and illustrated with figures 4, 5 and 7).
- At the end of chapter 4.2.5 there are four paragraphs which are wrongly placed - they deal with correlations of particular features in the boreholes shown in Fig. 10, and should entirely be moved to and incorporated into the text in chapter 4.4 (which should be moved to before the K-Ar chapter, see comments below).
4.3. K-Ar geochronology and X-ray diffraction
In my opinion, this is the weakest chapter in the results section. The description of the different samples is not very systematic, the composition of the samples (Table 4) is discussed after the presentation of the ages, and it is not argued well enough for which dates are geologically meaningful and which ones potentially represent mixed ages. My experience is that K-Ar samples all have their own characteristics, based on the host rock lithology and preserved mineralogy in the sample, which needs a careful “sample-for-sample” approach and a careful interpretation of the age data. I suggest you have a look at how Hestnes et al. (2022) introduced and described each sample based on field relationships, mineralogy (preferably with plots showing the compositional variations, which makes it easier to evaluate the amount of K-bearing phases in each sample/fraction, see Fig. 7 in Hestnes et al. 2022 or Fig. 8 in Scheiber et al. 2019), and first then present and interpret the age results. You might then say a few words about your approach towards interpreting the dates from the different fractions as geologically meaningful or not. You might also consider presenting the 7 samples in a different order: introduce the three samples with D2 interpretation first (2012, 2015, 1029_1) ->they have the oldest finest fractions, followed by the three samples with D3,D4 interpretation (1033, 2008, 120714) which have the youngest finest fractions, and finally present 1024_1, which is missing the finest fraction and which is therefore of less importance. By introducing them that way it will be more straight forward to give an age to the different deformation phases. You should also discuss the meaning of the saprolite(?) ages at ca. 128 Ma for sample 1033 in contrast to possible deformation ages for the other samples, so it is really important to treat each sample separately.
4.4 Basement deformation in 3D
I think this section fits better before the K-Ar section, since it still deals with a description and interpretation of the field relationships. As commented on above, a part of the text which belongs to this section is wrongly placed at the end of chapter 4.2.5 and should be incorporated here. Generally, I struggled with correlating what you write in the text with what is shown in figure 10 – which zones do cross-cut both bore holes, and which do not? Which planes represent D1, D2, D3 osv structures? Where are the dated samples from? What is Fig. 10B showing compared to Fig 10A? Could some of the correlations also be shown in Fig. 3? I think this entire chapter needs some careful reorganization, rewriting and alignment with the figures to be understandable and useful.
- Discussion
5.1. Polyphase evolution of Smøla and the passive margin
- Here, you discuss the temporal evolution of the brittle history of Smøla within a larger framework. You put a lot of emphasis on cross-cutting relationships and orientations of lineaments, instead of using your detailed field observations including dip-measurements and lineations/kinematic indicators. I suggest you carefully revise the interpretation once the field results are included in the data part, putting less weight on the lineament mapping, and more weight on actually observed relative cross-cutting relationships between differently-mineralized fractures and faults which I think are more reliable than purely lineament-based observations.
- You should also take the 3 dimension into account – based on lineament analysis one quickly thinks only in terms of steep, strike-slip related structures, but your stereoplots from the boreholes mainly show quite shallow structures? How do they fit into a mainly strike-slip related interpretation/model?
- Similarly, you should also include a more careful interpretation of they age data in your discussion – once you have decided for each sample which date you interpret as geologically meaningful, a more detailed temporal evolution might appear (the 300, 200, 100 Ma periods are too generalized I think).
5.2. Application to and implications for the Frøya High offshore domain
I think this title promises a bit too much – your results do not have direct implications for the interpretation of the Frøya High other than the Frøya High might have experienced a similar complex brittle evolution. So I would tone this a bit down and possibly shorten it, and remove table 5 (its enough to say that there are three bore holes available but unfortunately not with intact cores). See comments in the annotated pdf as well.
- Conclusions
The conclusions and abstract should be updated once the field results and K-Ar results are presented in a more systematic and detailed way.
Figures:
See several smaller comments on the figures in the annotated pdf. A revised order and number of figures based on the comments above could look something like this:
- Fig 1
- Fig 2
- Detailed figure showing crucial lineament cross-cutting relationships with DTM and magnetics
- Fig 6 with field data and drill hole data separated with colors/grey shades
- New Fig. stereonet figure with field data including fractures and faults, lineations and kinematic indicators, sorted according to domain, field station, mineralization state…
- 3 drill hole logs
- 4 Mineralizations from the field/drill holes
- Fig 5 Mineralizations in thin section (specify whether they are from the field or drill holes)
- Fig 7 relative chronology
- 10 3D model
- 8 K-Ar samples map and field pics
- New Fig. composition of K-Ar samples
- 9 K-Ar ages
- 11 Final model
Resulting in a total of 14 figures
Technical corrections/minor comments
See annotated pdf
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AC2: 'Reply on RC2', Matthew Hodge, 26 Feb 2024
Response to review comments for the paper titled: “Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs”
Referee 2) Prof. Deta Gasser; Citation: https://doi.org/10.5194/egusphere-2023-2504-RC2
Dear Prof. Gasser,
We wish to extend our sincere gratitude and appreciation for your detailed and thorough review and insightful comments on our manuscript entitled "Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs."
We appreciate the time and effort that you have contributed to our manuscript, and the constructive feedback will be invaluable in improving the quality of the work overall.
We have thoroughly considered each of your suggestions and present our responses and revisions. For context on this response document structure: We have first included the general comments (overall, and then by section as was originally done in the review), with the responses below, and then included the ‘in-text’ comments from the annotated .pdf, with specific responses.
Overall general comments (summarised):
- Enhance the presentation of cross-cutting relationships among lineaments, preferably with high-resolution DTM images alongside magnetic anomalies.
- Improve the introduction and display of structural field measurements, including a stereo plot illustrating both strike orientations and dip values, as well as lineation and kinematic data.
- Provide a more systematic presentation and interpretation of the K-Ar results on a sample-by-sample basis.
- Streamline the text by eliminating wordiness and repetitions, condensing the introduction, and ensuring paragraphs in the results section are appropriately categorised.
- Offer an interpretation of brittle tectonic evolution considering dip values of features and the orientation of lineations and kinematic indicators, building upon the structural field measurements.
Overall general comment responses and revisions made to manuscript:
- We agree that the presentation of the cross-cutting relationships is insufficient to effectively support our claims on relative timing of the lineaments. We have now both enlarged the main geophysical map in Figure 2A and included only two ‘zoom-in’ areas in Figure 2B, with both the DTM and geophysical data showing. The ‘zoom-in’ areas are also at a smaller-scale now to aid in visually seeing the different lineament trends and cross-cutting relationships.
- We concede that this was a missing component on the original manuscript. We have now included stereonet plots for both the field and drill data, showing pole to plane trends of all the measured planes, slip lineations, and kinematics (coloured by regime). The data have been sorted by deformation episode as derived from the mineral assemblages and cross-cutting field relationships, and integrated with our other field, drill, and petrographic results.
- We agree that this is a more robust approach to understanding and using K-Ar dates in complex brittle deformed volumes such as Smøla. We have thus reorganised and rewritten the text, systematically characterising each K-Ar sample. Full sample descriptions have been included now in the supplementary material.
- We acknowledge that the manuscript required significant streamlining and reduction in word count. We have now shortened the introduction, rearranged certain sections (e.g. Basement deformation in 3D), and attempted to cut down the length of the manuscript where possible. However, the inclusion of the requested material (see above) has increased the manuscript length.
- We agree that the original manuscript lacked sufficient reference to both the field and drill structural data. We have now incorporated this data into the interpretations, using the strike/dip, slip lineation orientations, and kinematics for understanding each of the deformation episodes, particularly where some of the episodes may be comprised of more than one sub-episode of deformation. In most cases, the field data have proven to be more useful for this purpose.
General comment responses by section:
For the specific comments to each section, the reader is referred to the original review at the citation link: https://doi.org/10.5194/egusphere-2023-2504-RC2.
Title:
Response: We agree the title is ambitious. We have decided to adjust the title to “Multiscalar 3D-temporal structural characterisation of Smøla Island, Mid-Norwegian passive margin: an analogue for unravelling the tectonic history of offshore basement highs”
Introduction:
Response: See general comment response above, we agree the Introduction required shortening. We have consolidated the last three paragraphs into a single paragraph as suggested.
Geological Framework:
Response: Please see specific responses below to comments in the annotated pdf.
The Applied Toolbox:
Response: We agree, the field mapping-related methods should be discussed before the drill hole-related activities. We have now moved this portion of text first. Moreover, for the drill holes, we have now included information on the rationale behind the drill hole locations, when they were drilled, and where they are now stored.
Results
Lineament Mapping:
Response: Please also see the specific responses below to comments in the annotated pdf. The lineation trends are now associated with numbers L1 to L8, and the lineaments have been colour-coded and labelled on Figure 2B. We have included each of the lineament descriptions with the location information. Within the text we have highlight difficulties in establishing cross-cutting relations owing to both the resolution of the imagery, and the difficulty in defining the start and end points of the lineaments. The descriptions of the cross-cutting relationships of each lineament set have been rewritten to improve clarity and simplify the text as much as possible. We have also simplified Table 1 to only indicate either termination or cross-cutting relationships, with no indication on strike-slip or dip-slip offsets of one lineament set by another.
Deformation History:
Response: We agree that this section needed a proper and thorough description of the field and drill structural data. We have now included summaries of the structural data within the field, drill, and petrographic results. We have focussed primarily on the field data as it incorporates data from wider areas across Smola than the drill holes. Corresponding to the summaries, we have included a stereonet figure for both the field and drill data sorted by deformation episode. Within the summaries and on the figure, we incorporate kinematic information and trends of both slip lineations and measured planar features (shear features, significant deformation zones, and veins). We have also moved the misplaced paragraph to within the “Basement deformation in 3D” section.
K-Ar Geochronology and X-ray Diffraction:
Response: We agree this section needed a more systematic approach in presenting the K-Ar results. We have therefore characterised and summarised the results for each sample individually and sequentially based on age and deformation episode. A full description of each sample has also been included in the supplementary material. For each sample, we firstly describe the field relationships, then the X-ray diffraction (XRD) mineralogy results, and finally the K-Ar age results. The age results are then discussed relative to the grain size fraction, associated mineralogy, and compared against the other samples. Sample SK1033_1 is particularly discussed due to the possible saprolite formation age it has yielded.
Basement Deformation in 3D:
Response: This section has been rearranged to be before the K-Ar sample section. The text has been reorganised, rewritten, shortened, and simplified, with better correlations between the text and the figure (Figure 8). Figure 8 has also been updated, with the zones discussed in the text colour-coded and number labelled the same as the text.
Discussion:
Response: We acknowledge that primarily using lineaments to demonstrate a tectonic evolution is insufficient. Although the field and drillhole structural data was utilised in the study, the lack of visual evidence was an issue. We therefore have now incorporated the structural data in the deformation history discussion and toned down the use of lineaments (we have still correlated the lineaments to the structural data where possible). We have also brought in the shallow structures as seen in the drill holes into the interpretations (including the 3D modelling work). Associating the structures to the absolute age data (K-Ar ages) has now been done taking the individual samples into account, and the specific grain size fractions within the samples. The final section of the discussion regarding Frøya High has been significantly shortened and Table 5 has been removed. This section is now intended to convey the importance of using onshore analogues, such as Smola, in understanding offshore basement highs.
Conclusion:
Response: We have rewritten the conclusions based on the revised manuscript, incorporating the new information regarding the field and drill structural data, and the systematic K-Ar sample descriptions and interpretations. The abstract has been similarly rewritten with a focus on the amended results and interpretations of the study. The abstract has also been streamlined and shortened with less emphasis on the project background and rationale.
Annotated comments within the text:
The referee comments within the manuscript are included below with the page number and line number of the original annotated manuscript. Below each referee comment, we provide our response, and our change(s) made within the new revised manuscript.
Title
Page 1, line 1: See suggestion for alternative title in the separate reviewer file
Response: See general comment response above.
Introduction
Page 2, line 45: This part is not absolutely necessary, could be omitted for shortening. shorter: " their size is commonly below seismic resolution"
Response: We agree. A selection of the text has now been removed, and text changed to suggested.
Page 2, line 60: shorter: "is an ideal analogue for basement highs offshore..."
Response: We agree. We have changed the text to the suggested.
Page 3, line 62: Long sentence, shorter would be: "At Smøla, a wealth of local and regional structures are preserved, which document the tectonic evolution of the margin through time".
Response: We agree the sentence can be shorter. Text changed to “Smøla itself possesses a wealth of preserved local and regional structures, which document the tectonic evolution of the margin through time.”
Page 3, line 66: Brittle
Response: We agree. Text changed to suggested
Page 3, line 67: add Hestnes et al. 2022 (https://www.sciencedirect.com/science/article/pii/S0191814122001134?via%3Dihub) and Tartaglia et al 2020 (https://www.sciencedirect.com/science/article/pii/S0012821X20303642) and Scheiber et al. 2019 to this list (https://www.sciencedirect.com/science/article/pii/S0191814118305029)
Response: We agree. We have now inserted these references into the text.
Page 3, line 70: I am not sure your approach is so new. Most of the previous studies cited above use lineament mapping, extensive field observations including mineralogy and K-Ar fault gouge dating (e.g. Scheiber/Viola, Hestnes and Tartaglia)- the only thing that might be new are the boreholes, but the tools used there (macro- and microstructural observations) are not particularly new. I suggest you combine the last three paragraphs of the introduction into one single paragraph, shortening the context considerably and avoiding repetition (the methods are mentioned twice, both in the second and third paragraph), stating that you present a detailed case study applying known methods to a particular area.
Response: We agree, and we have updated the text to highlight the new perspective offered by the drill holes. A concluding point on the relevance of this work in the offshore realm and the implications on generating deterministic inputs for fracture modelling is also included. This is important as this paper will lead on to the next paper currently in production which relates to this aspect specifically. Revision: Text updated to “…a new 3D perspective from four diamond drill holes on Smøla Island, we present a case study that describes and tests a comprehensive workflow for characterising onshore basement blocks.”
Geological framework
Regional perspective
Page 3, line 90: placed along
Response: Text changed to “set along”, as the island is not necessarily “placed” which implies a positioning of the island after the formation of the passive margin.
Page 4, line 93: The Scandian phase is usually assumed to cover the time span from ca. 430-400 Ma
Response: Text changed to “(430 to 400 Ma)” to reflect the time span.
Page 4, line 99: Text deleted
Response: We agree, text deleted
Page 4, line 102: Devonian
Response: we agree, Text updated
Page 4, line 104: Insert also Grønlie&Roberts 1989 and Grønlie et al. 1994.: Fission-track and K-Ar dating of tectonic activity in a transect across the Møre-Trøndelag Fault Zone, central Norway. Norsk Geologisk Tidsskrift. Vol. 74, pp. 24-34.
Response: we agree, inserted the references
Page 4, line 105: Text deleted
Response: We agree, text deleted.
Page 4, line 107: Fig. 1B
Response: We agree, and we have inserted the figure reference.
Page 4, line 114: which time? The time of break-up just described above? Or the entire Devonian-Paleogene time? Specify/rewrite
Response: We have merged the paragraph with paragraph above, which provides more clarity on the time span. Rewrote to remove the uncertainty regarding the time span.
Page 4, line 114: when does a passive margin actually form? I would say at break-up? Before it is just a continent with rifts? Consider rewriting
Response: We agree, we have rewritten it to remove the uncertainty regarding the formation of the passive margin.
Page 5, line 120, Figure 1: Separate out the supracrustals (limestones, volcanics)?
Response: We agree, and now the limestone and volcanics on Figure 1B have been separated out and given their own symbology.
Page 5, line 120, Figure 1: Use brownish colors for gabbros and metagabbros, blueish colors are usually used for carbonates
Response: We agree, and now on Figure 1B all the lithology types have been recoloured with appropriate colours based on the Geological Survey of Norway colour scheme.
Page 5, line 120, Figure 1: Baltica autochthonous
Response: we agree, and the text in Figure 1A has been changed.
Page 6, line 132: not shown on map? Remove
Response: We agree, the text has been removed.
Page 6, line 135: consider placing the reference at the end of the sentence for better readability
Response: Yes, we agree, we have moved the reference.
Page 6, line 138: Text deleted (comma)
Response: We do not agree. The comma must remain, as it is a list, and for consistence with the reminder of the text, the authors are using the Oxford comma convention.
Geological framework
The geology of Smøla
Page 6, line 143: those are not separated out on the map in Fig. 1b - it would be nice to show them with an own color on the map (now they are combined with the gabbro?) since those are the only Caledonian supracrustal rocks on the island
Response: We agree, and now these units are separated in Fig. 1B, with their own colour symbology.
Page 6, line 145: Check the reference style - the "H." and "Ha." has to be removed I guess
Response: We acknowledge this problem; the reference is now corrected.
Page 6, line 151: which fill in
Response: Yes, we agree. The text changed to the suggested
Page 7, line 160: Does not fit here - can be included in the last paragraph of the introduction
Response: We agree, but we have decided to entirely remove “In comparison to the MTFC and the wider margin, the post-Caledonian structural evolution of Smøla remains poorly investigated and understood.”, as it is not relevant to the purpose of the paper, where we are highlighting a methodology of characterising basement rock volumes.
The applied toolbox
Page 7, line 173: It is more logic to describe the field methods first, and then the drill hole logging.
Response: We agree, particularly as the majority of the logged features in drill core are centimetric in scale.
Page 7, line 174: Say something about the drill cores - when where they taken and why? What was the reason for placing them where they are? Were they conducted during your own study, or are these older cores?
Response: The four diamond drill holes used in this study were drilled in 2019 during a previous NGU project (BASE 2). The drill holes were designed to target major structures, and sample weathered basement rocks. We have updated the text to reflect this.
Page 7, line 178: Move this to the start of the paragraph
Response: We agree, and we have moved it to the start of the paragraph.
Results
Lineament mapping from geophysics and DTM data
Page 8, line 196: Refer to the supplement/data repository where the lineaments are available as geodatabase
Response: We agree, and we have added a reference to the supplementary data repository
Page 8, line 197: I cannot see that different magnetic blocks were discussed above? Consider rewriting
Response: We agree, the text has been rewritten.
Page 8, line 201: use this numbering in the table 1 and in the text below, so orientation trend 1) is N-S, 2) is NW-SE, 3) is E-W etc.
Response: We agree, this has been added into the text and the Table 1 has been updated.
Page 9, line 206, Figure 2: It is very difficult to see the domain boundaries - how where the lineaments assigned to the different domains? Consider using thicker white lines to delineate the domain boundaries
Response: We agree, the boundaries have been made to be thick white lines to improve the clarity.
Page 9, line 206, Figure 2: It is hard to see the outline of the island/coast - could it be an idea to make the submarine areas more transparent (or putting a whitish transparent layer on top of the submarine areas, to that the magnetic pattern appears weaker in color?)
Response: Yes, we agree it is difficult to determine on/offshore areas. The areas covered by the Norwegian Sea have been now represented by a transparent white layer above the geophysics imagery.
Page 9, line 206, Figure 2: I am not so convinced about these cross-cutting relationships. The thick black NW-SE trending line could easily be drawn across the thinner, SW-NE trending line (the magnetic anomaly continues across it), so to me it is not clear that the northern black line is the same as the southern
Response: We somewhat agree with your comment, the insert does not particularly show the entire context of that lineament placement for this particular case. However, there is both a high amplitude anomaly flanking the NE-side of the NW-SE lineament, and a possible remnant magnetised/de-magnetised zone to the SE of lineament. Both features were used to guide the location of the lineament. Additionally, the magnetic fabric to the NE appears to be offset over the NE-SW lineament. Lineament mapping using airborne magnetic data is interpretive, but if the mapping is completed in a consistent method, considering specific features in the magnetic data, this is the best possible interpretation. The authors do concede, owing to the interpretive nature of this work, that it is possible that the lineament could be mapped differently by another. The text has been overall rewritten for this section, and this has been removed.
Page 10, line 221: You do not highlight NNW-SSE lineaments in Fig 2BI, so it is difficult to follow your argumentation here
Response: We agree with the need to improve clarity, as such we have highlighted the lineaments in Figure 2B by colour, and labelled the associated rose diagrams.
Page 10, line 225: how do you constrain dip-slip offset from 2D lineament mapping?
Response: We have removed reference to this in the text, as it does have high uncertainty, and will use the other structural data (field and drill hole) to discuss. But a dip-slip offset on a 2D map is a possible interpretation when a magnetic anomaly changes in width (apparent thickness) over a cross-cutting lineament (suggesting a different portion of the magnetic unit is providing a geophysical response). This is a similar method of interpretation as used in reading geological maps. It is not possible to get this interpretation from DTM imagery.
Page 10, line 226: Move this information to the start of the N-S trend description
Response: We agree, the text has been updated to the suggested.
Page 10, line 228: I do not see any convincingly dextrally offset lineaments on this figure - can you point at a particular lineament that you can trace to the right north of the E-W lineament?
Response: We agree, the text has removed.
Page 10, line 228: dip-slip is difficult to constrain on 2D maps
Response: We agree, the text has removed.
Page 10, line 229: Again, you have to point at particular lineaments which you think are offset, since I do not easily see the offsets you describe in the pattern, which shows many cross-cutting, but not offsetting, lines
Response: We agree that the small ‘zoom-in’ inserts with geophysical imagery are not sufficient for showing the offsets. We have now selected two optimal areas with the most examples of lineament cross-cutting relationships. The ‘zoom-in’ images are also at a smaller scale (more ‘zoomed-in’) with both the DTM and geophysics shown. Moreover, we have decided to simplify the text and discuss cross-cutting rather than offsetting relationships, except in very clear examples.
Page 10, line 232: Move this to the start of the NW-SE trend description
Response: We agree, the text has been moved to the beginning of the description.
Page 10, line 235: The HSF is NE-SW trending. Do you assume sinistral or dextral movement along this when considering Riedel orientation? If you assume sinistral movement, and sigma 1 oriented 45 degrees to the fault (approximately N-S), then the Riedel (R, not R prime) orientation would be NNE-SSW. If you assume dekstral movement, sigma 1 would be approximately E-W, so the Riedel orientation would be WSW-ENE - which situation do you envisage here?
Response: We concede that this is too uncertain, and too interpretive for this part of the manuscript, so we have removed the reference to Reidel shears. The lineaments could be splay off the MTFC as Reidel shears or just other potential structures which have formed during a different tectonic episode. The latter is more likely. As such, the possible lineament offsets will be interrogated/compared against the structural data later in the discussion.
Page 10, line 237: This figure looks more like the E-W lineaments cross-cut all the others?
Response: We agree, the figure has now been changed and the text rewritten. The NW-SE, NNE-SSW, and NNW-SSE lineaments do appear to crosscut the E-W lineaments. In the discussion, we suggest that this may be a later reactivation of these features.
Page 11, line 238: Does that mean that the N-S, NW-SE and E-W lineaments all cross-cut each other at some point and it is not possible to say who is oldest/youngest? Be more specific here
Response: We agree that the text is too ambiguous, so it has been rewritten. Where these lineaments are third-order lineaments, they, they are crosscut by the other lineament trends in a similar way. This would suggest that they may be relatively early lineaments. Second-order E-W lineaments crosscut almost all other features, suggesting they continued to develop until later.
Page 11, line 242: Move this to the start of the description
Response: Yes, we agree; the text moved as suggested.
Page 11, line 246: again, you have to point out specific offset lineaments, on Fig. 2BV I noly see straight lines that crosscut the NE-SW lineaments.
Response: We agree. The text has been updated to emphasis cross-cutting relationships rather than offset, and a new figure insert in Fig 2 has been included to highlight these cross-cutting relationships (see general comment above).
Page 11, line 249: Move this to the start of the orientation description
Response: We agree; the text has been moved as suggested.
Deformation history
Page 11, line 257: Rename this into for example "field and drill hole results", or separate into two chapters, 4.2. Structural field measurements and 4.3. Deformation history based on mineralogical relationships" or something similar, based on how you decide to restructure your text based on my comment in the separate reviewer file
Response: We largely agree with this comment. We have now changed the title to “Field, drill hole, and petrographic results”, as the petrographic work was a significant component of these results.
Page 11, line 258: Here you jump directly from the lineament mapping to the borehole scale, and skip the entire field station part! I highly recommend you start your result description with a summary of what your field stations have revealed - se detailed comment in the separate reviewer file.
Response: We concede that this is a significant gap in the results section. Therefore, we have now included summaries of the field structural results along with the relevant field, drill, and petrographic hole observations. These summaries are included by deformation episode and mineral-assemblage. We have decided to nestle the structural data results within the text to improve flow and limit repetition.
Page 11, line 260: They are not properly presented in the current manuscript, and should come prior to the cm-scale borehole observations!
Response: We have included a detailed figure with stereonets displaying all planer, lineation, and kinematic data. Both the field and drill data has been included.
Page 12, line 273, Figure 3: would it be possible to color code the structural measurements according to D1-D5?
Response: Unfortunately, this is not possible. We have included this information in the previous figure and will keep the figure as is. This figure is intended to indicate the distribution of collected data, and the variability of structural geometries downhole.
Page 12, line 283: this has to be documented by a proper stereonet figure showing plane and lineation measurements
Response: We agree. This has now been included the within the Field, drill hole, and petrographic results section.
Page 14, line 309: This is again easier to state if you can refer to a stereonet figure
Response: We have now included a reference to the stereonet figure.
Page 14, line 323: refer to a stereonet figure
Response: We have now included a reference to the stereonet figure.
Page 14, line 327: Strike through
Response: Agreed, text removed.
Page 14, line 327: which subfigure?
Response: We agree this is missing, text modified to “Figure 5E”
Page 14, line 328: stereonet figure - orientation of slicken lines or other lineations associated with it?
Response: This has now been included the within the Field, drill hole, and petrographic results section, and shown on Figure 3.
Page 15, line 329: has to be documented with lineation measurements and kinematic indicators
Response: We agree, and this has now been included within the Field, drill hole, and petrographic results section. Removed from current position.
Page 15, line 333: Fig 5. Specify the samples and/or field stations/drill cores the pictures are coming from - it can be a good idea to add coordinates behind each field and thin section photograph (see figure caption of Hestnes et al. (2022))
Response: Yes, we agree with this. We have included the coordinates (UTM)/Drill hole and depth for the samples within an additional document within the supplementary material. We have also included a reference to this in the paper.
Page 16, line 347: orientation data
Response: This has now been included in the within the Field, drill hole, and petrographic results section, and shown on Figure 3.
Page 16, line 364: lineations and kinematic indicators
Response: This has now been included in the within the Field, drill hole, and petrographic results section, and shown on Figure 3.
Page 16, line 368: if they are well-consolidated, they are not really gouges longer? If they have a cohesion, they might be ultracataclasites?
Response: Yes, this may be true, but in this case, the gouge is indurated/cemented with hematite/Fe-oxides. So, the level of consolidation is secondary. We have amended the text to make this clearer.
Page 16, line 368: Strike through
Response: Agreed, removed “on average”
Page 17, line 380, Figure 6: It would be great to have such a figure as a stereonet, also with associated mineral lineation data plotted.
Response: This has now been compiled for Figure 3 with stereonets for each deformation episode.
Page 18, line 414: "on average" is strange wording in this context. If you have 10 planes dipping 90 degrees and 10 planes dipping 0 degrees, the average dip becomes 45 degrees? You probably mean that most measurements fall within 30-70 degrees, but that is not an average value - I suggest you remove "on average"
Response: Yes agreed. We have removed this phrase from the text.
Page 16, line 415: kinematic indicators?
Response: This has now been included the within the Field, drill hole, and petrographic results section, and shown on Figure 3
Page 19, line 433: The following four paragraphs (until the subchapter 4.3.) are out of place - they deal with correlations of particular features in the boreholes shown in Fig. 10, and should entirely be moved to and incorporated into the text in chapter 4.4
Response: Yes, we apologise, these paragraphs are misplaced. We have moved and rearranged them with the section “Basement deformation in 3D”.
K-Ar geochronology and X-ray diffraction
Page 20, line 466: Thats the aim - but how can you document/make sure that your fractions only contain authigenic and synkinematic clay?? This has to be argued for for each sample carefully
Response: We have now characterised each of the samples systematically, with commentary on the composition of each of the grain size fractions. A full description and characterisation is now available in the supplementary material.
Page 20, line 470: The composition of the different fractions for each sample has to be introduced better - which fractions do most probably only contain illite or smectite, whereas which fractions do probably contain a mixture of illite/smectite and inherited muscovite, K-fsp etc.? This is crucial for the age interpretation.
Response: As stated above, we have now characterised each of the samples systematically, with comments on both the composition of each of the grain size fraction, and also the potential source of the minerals. This will be used later in the discussion to further argue for the age interpretations.
Page 20, line 476: thats too general - introduce the dates for each fraction from each sample systematically, and discuss their geological significance
Response: Yes, we agree, therefore we have introduced each of the samples’ K-Ar ages, by the grain size fractions. With a focus on the most geologically significant size fractions. This is typically the finest and the coarsest, but where the age spectra curve deviates from an inclined curve, we have added comments on this (i.e. where there is an older/younger age which is not associated with the coarsest and the finest fractions).
Page 21, line 480: this statement needs a reference and/or better explanation
Response: Yes, we agree this needs a reference. We have included the reference of Levy & Woldegabriel (1995), which discusses the loss of radiogenic 40Ar from a zeolite (clinoptilolite).
Page 21, line 481: You have to discuss that systematically for each sample, since the mineralogical composition for each sample and each fraction is different
Response: Agreed. We have introduced each of the samples’ XRD results prior to outlining the K-Ar dating results. This has been done by mineral type and by grain size fraction.
Page 22, line 493: respectively
Response: Agreed. Text removed and rewritten.
Page 23, line 496: This is way too generalized. The three youngest finest fractions actually scatter from ca. 70 to 130 Ma potentially belonging to very different deformation phases (e.g. see the discussion of calcite ages at 140, 90-80 and 70-60 in Hestnes et al.). If you argue properly for each sample and why you think the youngest fraction is geologically meaningful, you then can extract much more information from this data than just the three broad belts at 100, 200 and 300.
Response: We have removed this text, as we agree it is too generalised and simplistic. The different samples are now presented with their relevant K-Ar ages. From the XRD and K-Ar geochronology work we now have the following date groups ~287 – 291 Ma, 196 – 204 Ma, 100 – (128?) Ma, and 75 Ma.
Page 23, line 497, Figure 9: Once you have argued properly and systematically for each sample, you can highlight those dates which you think are geologically meaningful - this might be all the finest fractions (74, 99, 128, 196, 201, 204) and maybe a plateau in the coarsest fractions for sample SK2008 at ca 200, but this depends on the mineralogical interpretation of these fractions (inherited or authigenic?). You should remove the three belts at 100, 200 and 300, which are confusing and not precise enough
Response: We have included the dates we feel are geologically meaningful and updated the figure. The belts are in place to highlight clustering of dates for the finest fraction. The earlier large fraction dates for SK2012 and SK2015 are also highlighted as these two different structures provided two very similar dates. We have now included a belt for the youngest age for sample 120714, as this age provides an upper limit for the D4 mineralisation.
Page 24, line 505: With
Response: We agree, the text has been removed.
Page 24, line 507: Consider replacing this with a figure showing the mineralogical content of each fraction from each sample. This information has to be placed before the age information
Response: The figure has been replaced by a series of pie chart graphs for each sample and by grain size fraction. The figure has been furthermore moved to before the K-Ar dating information.
Page 24, line 514: Should come before the age presentation, should be done systematically sample for sample
Response: Agreed. This portion of the text has been integrated into the individual sample characterisations and moved to be before the K-Ar geochronology information.
Page 25, line 525: this has to be introduced and discussed better: do you think this could be a deep weathering zone, not necessarily related to deformation, but to deep infiltration of surface fluids? So the age of ca. 128 Ma for the two finest fractions could represent Cretaceous deep weathering? How would that fit with deep weathering proposed elsewhere along the margin (if I remember right that was supposed to be in the Jurassic?)
Response: It is always possible that percolating meteoric/ground water has penetrated down and along inclined structures. The overall zone is saprolitic, so the age of the finest grain size fraction may well be authigenic clay growth related to saprolite development. However, weathering along the western portion of the Norwegian passive margin (and particularly along the Strand flat) is typically associated with Jurassic times (e.g. between ~221 Ma to 206 Ma for Bømlo, Western Norway) (e.g. Fredin et al., 2017; Olesen et al., 2023). As SK1033_1 has returned a Cretaceous age, there may well be a hydrothermal component to the age.
Basement deformation in 3D
Response: This section has been moved to before the K-Ar geochronology and XRD section.
Page 27, line 558: the 3D model in the middle of Fig 10A shows more than four green planes - it is not so easy to identify the four planes which occur in both bore holes from this plot - could it be an idea to colour the four planes which occur in both cores with a different colour?
Response: We agree that this figure is not clear, and well-correlated with the associated text. We have updated the figure, with the zones colour-coded by deformation episode. The zones mentioned in text have now also been labelled I, II, III, IV to aid identification from what is written in text.
Page 27, line 560: here you say a D1 zone occurs in both B1 and B2, but in the Figure, the planes are all green, where the legend says these are D2 zones - could it be an idea to color the planes according to whether they are D1, D2 etc. features? Out from your description in the text here (zones I to IV i struggle with clearly identifying them in the figure. Could it be an idea to also mark them with I to IV in Fig. 3?
Response: As stated above, the figure has been updated, with the zones colour-coded for deformation episode. The zones mentioned in text have now been labelled I, II, III, IV to aid identification from what is written in text.
Discussion
Polyphase evolution of Smøla and the passive margin
Page 27, line 579: abbreviation not used earlier? Explain
Response: We acknowledge this comment. The text has been removed from the update manuscript.
Page 27, line 581: Such orientations are very minor in Fig. 6 D1, where D1 seems to be dominated by E-W lineaments, so I am not sure what you are referring to here?
Response: These lineament trends are potential early D1 features owing to the cross-cutting and termination relationships. We have amended the text to reflect this better.
Page 27, line 585: ENE-WSW cannot vary from NNE-SSW to NW-SE?
Response: We agree. Text removed; it was attempting to show the range in strike trends.
Page 27, line 587: how do you get local folding and contraction during sinistral transtension? within a restraining bend?
Response: We acknowledge that this is a controversial issue. Works done by Bøe & Bjerkli (1989) and Fossen (2010) (and references within), suggest that there was a N-S contraction of the Devonian Basin adjacent to the MTFC during Mid to Late-Devonian times (based on folds with E-W trending axial traces, and reverse faults). This contraction may have occurred syn-kinematically with strike-slip movement on the MTFC, or if transtension on the MTFC occurred, either before or after this phase. If before, these contractional features may have rotated as they approached the MTFC trace (Fossen, 2010). A portion of the D1 structural data, however, does fit with this contraction direction.
Page 28, line 594: the youngest fractions in the D2 samples are 200 Ma, which possibly can be interpreted as the formation age of the gouges/shear fabrics - the significance of the coarsest fractions at 300 Ma is much less clear, and if you would like to assign those a geological significance, you have to argue for it properly in the results section (maybe the two oldest fractions in 2012 and 2015 make some sort of meaningful plateau?) ->but I would definitively place D2 around 200 Ma, and not 300 Ma based on the youngest fractions
Response: We have adjusted the text to focus mostly on the ~200 Ma age associated with the finest fractions of the samples SK2012, SK2015, and SK1029_1. However, the clustering of dates at ~300 Ma for the size fractions 2-6 and 6-10 µm for the samples SK2012 and SK2015, does suggest that there is distinct inherited component in the K-Ar ages. Considering the differences in both structure orientations and mineral composition (minor difference) in the coarser fractions of these samples, it would be expected that there would be more age difference, particularly if the potassic feldspar in SK2015 afforded a protolithic component. As there is not, there may be an earlier Carboniferous-Permian deformation episode preserved in these deformation zones. The microstructural evidence also suggests that there are two preserved events in the zones.
Page 28, line 595: Fig 6 shows that E-W is minor for D2, but NW-SE is quite important?
Response: Yes, on the rose plots the E-W trend is not visible enough. These structures correlate to the major E-W lineaments (L3). However, the new stereonet plots in Figures 3, do show the importance of the E-W structures. The text has been updated to reflect this as well.
Page 28, line 596: just speculation/correlation with what others have said before?
Response: Yes, we are correlating the findings with the associated literature to try fit our findings within what others have found. This part of the D2 section has been rewritten, to explain how the structures with sinistral kinematics and the associated K-Ar ages can be compared and correlated with the relevant literature.
Page 29, line 603: In this entire discussion of lineaments and orientations it would be really useful to have the real structural data plots, not only lineaments. From the borehole plots it is obvious that many of the structures have a quite shallow dip, which is of importance if one wants to relate them to the basically steep MTFC faults! You should also discuss this in the light of your own kinematic indicators and lineations from the field, not only somewhat ambiguous offsets from the lineament maps.
Response: We agree it is important to integrate the field and drill hole structural data, and as we have stated before, these datasets are now included in the Discussion section.
However, regarding the shallow structures in the drill holes:
Importantly, the drill hole data probably contains more shallow than steep features owing to line-sampling bias (e.g. Terzaghi, 1965).
Most of the shallowly dipping D2 structures come from drill holes BH3 and BH4. These holes intersect several foliated gneiss and diorite units. Most of the shallowly dipping features correspond to slip along pre-existing foliation planes (with chlorite being present) within particularly micaceous intervals.
The drill holes BH1 and BH2, are mostly in massive monzogranite, and are situated north of a significant E-W structure, which locally shifts to a NW-SE trend.
The geometry of the features in BH1, BH2, therefore may rather stem from local stress perturbations (in terms of orientations of the principal stress axes, and stress magnitude) within the damage zone of a major structure (e.g. Kim et al., 2004). Or, if assuming these features are shallow thrusts, they may have formed due to local shortening (transgressive faulting) owing to the strike rotation of the major E-W structure, forming a restraining bend (assuming sinistral kinematics) (e.g. Cunningham & Mann, 2007). As visually demonstrated in the 3D modelling, the D2 structures dip S towards the steep major E-W structure. They may therefore represent either antithetic or conjugate features forming in an extensional setting (which have later rotated to shallower dips), although confirmation is challenging without kinematics of the modelled structures, and resolving the geometry of the major E-W structures (assumed to be steep to sub-vertical).
We have updated the text to include reference to the field and drill hole structural data, and 3D modelling.
Page 29, line 616: But the 200 Ma ages seem to come from very shallow structures (<30degrees) - how does that fit with P- and R-shear dextral strike-slip models based on lineaments?
Response: We somewhat agree, however only the sample SK2012 comes from a structure dipping at <30 degrees. The other two samples SK2015 (36 degrees), and SK1029_1 (80 degrees), show that a variety of structures were experiencing fluid flow and authigenic activity at ~200 Ma. Nonetheless we have rewritten this section to remove the correlation with P and R-shears which would indeed require steep to sub-vertical structures.
Page 29, line 618: that would imply subvertical structures, but your measurements in the boreholes are all quite shallow - these geometries do not look like a classic strike-slip setting?
Response: We agree that the shallow structures in the drill holes are problematic for a strictly strike-slip interpretation. The shallow structures related to the later D2 deformation are likely to be conjugate or antithetic features off the steeply dipping/sub-vertical structures. These structures would have had to have formed in an extensional setting. We have rewritten the text to take the geometries of the structures into account. Particularly, the 3D modelling.
Page 29, line 622: you mean ridge-push? its either ridge-push or slab-pull - I dont think there was a slab around at that time anywhere in the North Atlantic, but there was not ridge either until the Paleogene, so what do you mean here? Far-field effects from the Alpine slab? Along which margin - there was no margin at that time yet?
Response: Yes, we meant ridge-push. The suggestion that possibly ridge-push forces may have induced dextral strike-slip on the MTFC comes from studies looking at later tectonic cases (such as the Cenozoic for Pascal & Gabrielsen (2001)). We concede that this is too distant in time from the Triassic-Jurassic, so we have removed this from the text.
Page 29, line 625: I would try to have a look at the entire structural data set - what are your dip values saying, what are your mineral lineations saying? Are we in an extensional setting with flat-lying structures developping mostly reactivating older foliations (reactivation phase of Peron-Pindivic etc.)? Are the lineations subhorizontal or down-dip? The lineament maps alone do not tell us the entire story
Response: We agree this is the best approach. We have rewritten the section to incorporate this information within the text.
Page 29, line 628: what do you mean with authigenic here? Which mineral? And I would say that all your three D3 samples provide finest fraction ages between 120 and 74, not between 200 and 100 - so once you have described and interpreted your 7 samples more carefully, you can be more precise here.
Response: The authigenic age we used, related to the growth of clay minerals within the samples. We agree this is not the best use of the term considering the larger size fractions for the samples SK2008 and SK1033_1 may well include protolithic/inherited material which would not be authigenic. We have updated the text to be more specific based on the new systematic descriptions of the D3 samples.
Page 29, line 629: can you see such a transition in your data? be more specific on what your orientation and kinematic data say about the D3 phase - in order to do that, you have to present your stereoplots including lineations/kinematics.
Response: The proportion of the structures which exhibit dip-slip (mostly normal to oblique-normal) kinematics compared to D2 is significant. This indicates to us that there was a change in the overall tectonic regime over Smøla. The ~N-S reverse faults, and dextral features provide some limited evidence (along with the work of Bøe and Bjerkli (1989)) in suggesting that there was some possible dextral strike-slip during the later D3 episode. We have updated the text to present the structural data for D3.
Page 30, line 629: what are your field measurements on D3 structures telling?
Response: Most of the D3 structures are dip-slip features. We have updated the text to state this.
Page 30, line 647: where comes this from? shouldnt it maybe be 128 Ma?
Response: The text was reflecting the largest size fraction age within the relative age of the zeolite veins. This is unnecessary, so we have updated the text to just reflect the age of the smallest size fraction (~75 Ma).
Page 30, line 660: Hestnes worked north of Sognefjorden, which I would not call SW Norway (which is Rogaland etc.)
Response: We agree, the text has been updated to describe it as W Norway rather.
Page 30, line 662: possibly related to the
Response: Agreed, text has been updated to the suggested.
Page 30, line 666: how do you really know which lineaments are associated with the different D phases, since many orientations occur in all phases (e.g. Fig. 6?) I understand that you can assign a fracture/fault observed in the field to one of the phases due to the mineralogy, but a lineament on a map, without groundtruthing, is probably difficult to uniequivocally assign to a D phase?
Response: We have previously used the cross-cutting relationships and strike trends of the different D’s to sort the lineament trends. We agree that this is not a very robust method, and we have now updated the text to include the structural data from the field and drill holes to improve the interpretation and sorting. Realistically, the lineaments are likely to have formed and then been reactivated during multiple deformation episodes. The shallowly dipping structures are also not typically represented by the lineaments. However, the orientation and cross-cutting relationships do convey a structural evolutionary story.
Page 31, line 670: Text removed
Response: We agree, text removed.
Page 31, line 674: here the dip-values of these features are again crucial
Response: We are keeping the text the same, as now the D5 features being “sub-parallel” means both the strike trends and the dips are similar to the bounding structures around the Frøya High.
Application to and implications for the Frøya High offshore domain
Page 31, line 677: I am not sure this title is very suited for the content of this chapter - what you discuss is basically an outlook on how onshore detailed field studies can help understanding offhore basement bodies, but that additional methods (permeability modelling etc) are needed to constrain parameters relevant for hydrocarbon exploration. Maybe a title could be "Smøla as an analogue model for similar offshore basement areas and possible future research" or something similar?
Response: I agree and have adjusted the title accordingly to “Smøla as an analogue for similar offshore basement volumes”. The purpose of this section is to try connecting this work with basement volumes offshore, with the Frøya High being the most applicable given the similarity in both basement geology and structural/tectonic history.
Page 31, line 678: consisting of similar basement rocks as Smøla
Response: The section has been rewritten, with this comment incorporated in the new text.
Page 32, line 706: could your saprolite sample be an example of weathered basement?
Response: This is possible. However, the K-Ar age of the saprolite sample, being significantly different to the results from other studies may not make this sample the best to use for this purpose. Further work focussing on specifically weathered basement rocks on Smola may answer this question.
Conclusions
Page 32, line 714: I am still not convinced that the approach is particuarly important or new from this study, but the detailed and careful field and borehole observations are definitively very valuable, so I would focus more on that aspect
Response: We agree, the new component of this study relates to the four drill holes. We have therefore removed the mention of a “new” approach from the text. Moreover, the conclusions concentrate on the field, drill hole findings, with the K-Ar dating, lineament mapping results also highlighted.
Page 32, line 717: These conclusions should be updated once the field structural data including lineations and kinematics is included, and dip orientations have to be taken into account (the shallow dips pointing at not only strike-slip dominated deformation?)
Response: Yes, we agree, the findings and the discussion of these findings have been significantly amended with additional information regarding the structural data. We have updated the text of the conclusion accordingly.
Page 32, line 725: ?
Response: Yes, we concede this is too speculatory (see response above), so we have removed mention of slab and ridge-push from the text.
Page 32, line 741: Has to be updated after a careful evaluation of each sample/fraction
Response: We agree on this, and have updated the text to include the following age intervals based on the systematic evaluation of each of the samples: ~287 – 291 Ma (early D2), 196 – 204 Ma (late D2, early D3?), 100 – (128?) Ma (D3), and 75 Ma (upper age cut-off for D4).
We would like to extend our gratitude once again to Prof. Gasser for her invaluable contributions to the improvement of our manuscript. The detailed and rigorous suggestions have been carefully considered, and where possible, we have made corresponding revisions to update the work. We are confident that these adjustments will strengthen the overall quality of the manuscript.
Yours sincerely,
Matthew S. Hodge1*
Guri Venvik2
Jochen Knies2
Roelant van der Lelij2
Jasmin Schönenberger2
Øystein Nordgulen2
Marco Brønner2
Aziz Nasuti2
Giulio Viola1
Affiliations:
- Department of Biological, Geological and Environmental Sciences, University of Bologna, Italy
- Geological Survey of Norway (NGU), Trondheim, Norway
(*Corresponding author)
References
Bøe, R., & Bjerkli, K. (1989). Mesozoic sedimentary rocks in Edøyfjorden and Beitstadfjorden, Central Norway: Implications for the structural history of the Møre-Trøndelag Fault Zone. Marine Geology, 87(2), 287–299. https://doi.org/https://doi.org/10.1016/0025-3227(89)90066-2
Cunningham, W. D., & Mann, P. (2007). Tectonics of strike-slip restraining and releasing bends. Geological Society, London, Special Publications, 290(1), 1–12. https://doi.org/10.1144/SP290.1
Fossen, H. (2010). Extensional tectonics in the North Atlantic Caledonides: A regional view. Geological Society Special Publication, 335, 767–793. https://doi.org/10.1144/SP335.31
Fredin, O., Viola, G., Zwingmann, H., Sørlie, R., Brönner, M., Lie, J. E., Grandal, E. M., Müller, A., Margreth, A., Vogt, C., & Knies, J. (2017). The inheritance of a mesozoic landscape in western Scandinavia. Nature Communications, 8. https://doi.org/10.1038/ncomms14879
Kim, Y. S., Peacock, D. C. P., & Sanderson, D. J. (2004). Fault damage zones. Journal of Structural Geology, 26(3), 503–517. https://doi.org/10.1016/J.JSG.2003.08.002
Levy, S., & Woldegabriel, G. (1995). Ion Exchange and Dehydration Effects on Potassium and Argon Contents of Clinoptilolite. MRS Proceedings, 412, 791. https://doi.org/DOI: 10.1557/PROC-412-791
Olesen, O., Rueslåtten, H. G., Schönenberger, J., Smelror, M., van der Lelij, R., Larsen, B. E., Olsen, L., Baranwal, V., Bjørlykke, A., & Brönner, M. (2023). Jurassic heritance of the geomorphology in Mid Norway. Norwegian Journal of Geology, 103.
Pascal, C., & Gabrielsen, R. H. (2001). Numerical modeling of Cenozoic stress patterns in the mid‐Norwegian margin and the northern North Sea. Tectonics, 20(4), 585–599. https://doi.org/10.1029/2001TC900007
Terzaghi, R. D. (1965). Sources of Error in Joint Surveys. Géotechnique, 15(3), 287–304. https://doi.org/10.1680/geot.1965.15.3.287
- AC4: 'Reply on RC2', Matthew Hodge, 26 Feb 2024
- more detailed and more convincing presentation of cross-cutting relationships of lineaments, preferentially with high-resolution DTM pictures in addition to the magnetic anomalies
Data sets
Datasets for "Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway" Authored by Hodge et al. Matthew Hodge https://doi.org/10.17632/2nmr2cz9yy.1
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