The role of shear zones, faults and the associated fractures on the formation and character of bedrock surface depressions in crystalline bedrock (Turku, southwestern Finland)

Ruuska, Eemi; Skyttä, Pietari; Ahlqvist, Kati; Nordbäck, Nicklas; Barron, Pelayo; Nikkilä, Kaisa

doi:10.5194/egusphere-2025-4271

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https://doi.org/10.5194/egusphere-2025-4271

Preprints

12 Sep 2025

| 12 Sep 2025

The role of shear zones, faults and the associated fractures on the formation and character of bedrock surface depressions in crystalline bedrock (Turku, southwestern Finland)

Eemi Ruuska, Pietari Skyttä, Kati Ahlqvist, Nicklas Nordbäck, Pelayo Barron, and Kaisa Nikkilä

Abstract. Surface morphology of the crystalline bedrock is controlled by brittle bedrock structures, and while the contribution of the large-scale structures such as shear zones and faults are acknowledged, the contribution of detailed brittle structures associated to large-scale structures is more often overlooked.

In this study, multi-scale analysis of brittle structures and detailed 3D-modelling provide improved knowledge how the detailed brittle structures contribute on the formation and character of the bedrock surface depressions.

While the shear zones, faults and elongated bedrock depressions show parallel trends and overlap, the outcrop-scale fractures largely determine the dimensions and detailed surface morphology of the bedrock depressions. The surface morphology and orientation of bedrock depressions along shear zones and faults are contrasting particularly due to the strong ductile precursor associated with the shear zones and their lack for faults. However, for both the shear zones and faults, localized brittle deformation controls the generation of outcrop-scale fracture, which further controls the detailed surface morphological signatures along the elongated bedrock depressions, and particularly in their damage zones.

This study provides new insights into the detailed bedrock structures, that contribute to the development of complex bedrock surface morphologies within the bedrock surface depressions which are controlled by the brittle deformation and kinematics along the underlying shear zones and faults.

Received: 01 Sep 2025 – Discussion started: 12 Sep 2025

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Eemi Ruuska, Pietari Skyttä, Kati Ahlqvist, Nicklas Nordbäck, Pelayo Barron, and Kaisa Nikkilä

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-4271', Anonymous Referee #1, 18 Oct 2025
The manuscript of Ruuska et al. uses novel, quantitative methodology for analysing deformed Proterozoic rocks and their glacial cover to better evaluate how these rock sequences may impact civil engineering/infrastructure projects. This work is especially important for northern Scandinavia as glacially eroded bedrock sequences are omnipresent.
The main goals of the manuscripts are well summarised in lines 74 to 77 in the Introduction. However, I would also emphasise the importance of the results for civil engineering/infrastructure purposes.
Here are my main points for improving the manuscript:
It is not always clear to me whether the shear zones are brittle or ductile shear zones. A clarification early on in the manuscript might help to make this clearer and easier to follow for the reader.

There is some issue with terminology, e.g., biotite foliation is not structural terminology and is used in the manuscript for a foliation made up by biotite (and other minerals I suppose).

In the description of the shear zones in line 100ff is becomes clear that the dextral, E-W-striking strike-slip shear zones and the NE-striking (not trending) contractional shear zones acted in concert with each other during NW-SE shortening. The NE-striking shear zones were then reactivated by top-E extensional shear. One wonders whether this event also reactivated the dextral shear zones with sinistral kinematics?

The classification of the foliation into high- and low-strain domains (line 150) is nice. I would use strain criteria to support this classification, i.e., the spacing of the foliation planes or the degree of flattening of minerals like quartz or feldspar in this foliation. Then the high- and low-strain foliations could be linked to the dip angle. The dip angle itself does not carry any information on the amount of strain in the rock.

The figures are generally of high quality and easy to read. For ignorant people like me it would help to highlight that inset in Fig.1A represents Finland.

In Fig.3, F2 cuts SZ3 but seems to be cut off by SZ1. F1 does not appear to deform SZ1 and SZ2. The AFZ cuts SZ1 and SZ2. A bit of clarification would help.
Citation: https://doi.org/10.5194/egusphere-2025-4271-RC1
- AC1: 'Reply on RC1', Eemi Ruuska, 10 Nov 2025
  
  Dear RC1,
  Thank you for the constructive comments on the manuscript. I appreciate the time and effort you have put into suggesting improvements for this work. I will comprehensively improve the manuscript based on your comments.
  With best regards on behalf of all co-authors,
  Eemi Ruuska
  
  Citation: https://doi.org/10.5194/egusphere-2025-4271-AC1
RC2:
'Comment on egusphere-2025-4271', Anonymous Referee #2, 22 Oct 2025

The paper by Ruuska et al. proposes a study on how the network of shear zones, faults, and fractures influences the morphology of a crystalline basement. This topic is of particular interest as the Authors attempt to provide a detailed analysis of the spatial relationships between depressions and various types of tectonic structures. However, the paper requires significant work before it can be considered ready for potential publication. Firstly, the English language used is of mediocre quality, with sentence construction that is often clumsy and unclear. The text lacks fluency and it is frequently difficult to understand the Authors' intended meaning. A thorough revision of the text and how concepts are expressed is necessary. There are also many inadequacies in terminology choices, as highlighted in the attached PDF, which are prevalent throughout the work. The Abstract does not provide information on what was done, how it was done, or what results were obtained, appearing more like as a part of the Introduction.
Regarding the methods, it is not at all clear how field mapping was conducted, how foliations and fractures were classified, and there is no reference to classical methods for structural mapping in metamorphic rocks. The proposed high- and low-strain division seems arbitrary; it is unclear how it was determined and lacks a clear basis. To make this division meaningful, it is necessary to document the types of schistosity, mineral assemblage, and fabric through descriptions and images. Furthermore, there is a complete lack of information on how photogrammetric surveys were conducted, how 3D models (DOMs) were processed, and how fractures were mapped. As a result, the results and discussion sections are not grounded in well-documented data, suggesting a need to improve the presentation of results before revisiting discussions. More field images and more effective yet concise descriptions are needed. The discussions currently are verbose and not engaging; they should aim to get straight to the point. The influence of major shear zones and the F1 on the morphology is evident from the provided images. The impact of minor fractures is more questionable and requires better data presentation and documentation. The conclusions are not compelling for a broad audience of geoscientists and should be revised to highlight a broader significance beyond the local study. In summary, I suggest substantial revisions to make the paper presentable for a second review.
I would provide a constructive criticism aimed at improving the clarity, and impact of the paper, focusing on the clarity of language, methodology, and the presentation of results and discussion. I emphasize the importance of using standard geological terminology, including detailed descriptions of the methods used, and suggests improvements in how the findings are presented to engage a wider audience.

Citation: https://doi.org/10.5194/egusphere-2025-4271-RC2
- AC2: 'Reply on RC2', Eemi Ruuska, 10 Nov 2025
  
  Dear RC2,
  Thank you for the constructive comments and criticism on the manuscript. I appreciate the time and effort you have put into suggesting improvements for this work. I will improve the clarity of the manuscript, particularly in terms of language and methodology, as you have emphasized in your comments. Your detailed feedback will undoubtedly enhance the final outcome of the manuscript.
  With best regards on behalf of all co-authors,
  Eemi Ruuska
  
  Citation: https://doi.org/10.5194/egusphere-2025-4271-AC2
RC3:
'Comment on egusphere-2025-4271', K. Högdahl, 11 Nov 2025

The manuscript is on a very interesting and important topic but it has been submitted prematurely and needs more thorough proof-reading as there are numerous formality errors. This is a bit surprising considering the seniority of the co-authors. It might be an effect of different authors writing different sections of the manuscript, that would explain that the same (?) structure (it is a guessing game whether they are the same structure or not) have different names in different sections of the text. Are the Jaaninoja Fault (or Jaaninoja fault as it is also spelled, L369) the same structure as F2, and Aura River SZ (L368, Fig. 12) the same as SZ2? The first author should make sure to harmonise the manuscript. Examples of incoherence can be given by spelling; LiDAR (L144; L197) is also spelled lidar (L147, L195) and Lidar (L155; L199), to highlight one of them and these spellings are mixed throughout. And the description that LiDAR (were used along with aeromagnetic maps) for lidar lineaments supported by field observations (L147) is rather difficult to understand. So is also sometime the phrasing like "For honouring the effect of the continuous 2D forms ..." at L166. Honouring what? Meaning "to include"?
In addition, there are references to a Fig. 2x at L180, to folds in Fig. 4 (L212) that is not present in this figure, and are also several spaces missing after brackets and between references, like at L82 or to many, like in "honour points -method" L179 (Figure caption 2B) for instance.
Normally, Figs. rather than Fig. is used when referred to more than one figure.
The sentence at L243-245 should be rephrased.
There are also unnecessary repetitions like the two following sentences at L323-326. Here, references to Figs. 2 and 3 should be included to guide the reader (L323). The last part of the paragraph ends with " ... SSE- and NNW-side". SSE- and NNW-side of what? A reference to Fig. 2 is also needed at the end of L344.
Is Jaaninoja Fault/fault the same as F2 in Fig. 3?

And why are "dummy points" used for sub-sea topography and not bathymetry data?
L115. At 1.3-1.2 Ga the central part of the shield was in an extensional setting with emplacement of the large Central Scandinavian Dolerite Group, present also in Finland. At this time interval all lithotectonic units to west of the Sveconorwegian Eastern Segment were located outboard.
How is the damage zone defined?
L366. There are no ductile or larger scale brittle structural precursor for the NNW and NNE R-1 fractures, but they are formed as a response to movements in the deformation zones. What deformation zones?
L370 "The R2-fractures show similar ENE-strike than the foliation .."??

Figures
Figure 1. Group II depressions in figure 1 are NE-SW trending but in the description on L122 they are NW-trending.
Figure 2. A) In the legend there should be dummy points in green squares, but the only green in the map are lines. B) Legend is missing.
Figure 3. Explanations to the sterogram and the grey field in the map are missing.
Figure 6. According to L234 the most distinct R1 structures strikes in NNE and NNW, but in Fig. 6b R1 shows a max in N-S. The streograms need a legend.
Figure 7. Green symbols (I-node) on green background should be changed to a better suited colour.
Figure 10. Add that the Aura River SZ equals to SZ2.

Citation: https://doi.org/10.5194/egusphere-2025-4271-RC3
- AC3: 'Reply on RC3', Eemi Ruuska, 12 Nov 2025
  
  Dear K. Högdahl,
  Thank you for your constructive comments and criticism of the manuscript. Your comments regarding the coherence of the work are completely justified, even though the reasons are somewhat different. As you mentioned in your feedback, the terminology and nomenclature in the manuscript are overlapping and need much more clarity, which certainly affects the readability of the work. I will improve these aspects during the revision of the manuscript so that it will form more cohesive unity.
  Thank you for your time, and best regards on behalf of all co-authors,
  Eemi
  
  Citation: https://doi.org/10.5194/egusphere-2025-4271-AC3

Eemi Ruuska, Pietari Skyttä, Kati Ahlqvist, Nicklas Nordbäck, Pelayo Barron, and Kaisa Nikkilä

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Short summary

Shear zones have strong ductile precursors, which controls their localization and trend whereas the faults are cross-cutting in character with respect to the ductile framework. Roughness of the bedrock surface is characterized by bedrock depressions that are spatially coinciding with shear zones and faults. Lowest elevations are detected within the bedrock depressions along the core-domains of shear zone and fault, while the associated fractures control the bedrock surface in the damage zones.


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