the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Apr 2024
Transpressional tectonics during the Variscan-Alpine cycle transition: supporting a multi-rifting model, evidence from the European western Southern Alps
Abstract. We delve into the transition between the Paleozoic Variscan cycle and the Meso-Cenozoic Alpine supercontinent cycle, both of which have played a pivotal role in shaping the central European-Mediterranean plate’s architecture. Our focus is on the European western Southern Alps (Varese Area, N Italy), where we documented the tectonic events occurred during this transition. Two main scenarios have been proposed so far for this transition: i) a single, long-lasting, Permo-Triassic rifting event, culminating in the opening of the Alpine Tethys, or ii) multiple, distinct rifting events, preceding the onset of the Alpine cycle. By means of a tectono-stratigraphic and thermochronological approach, we recognized a first early Permian rifting stage associated with magmatic activity, followed during the early-middle Permian by transpressive tectonics and regional-scale erosion that signal the end of the first cycle of crustal rifting. During the Middle Triassic, a second event initiated, which, we propose, marks the onset of the Alpine Tethys opening. This event could represent the stretching phase, which predates the well documented Upper Triassic crustal-thinning phase. Based on our findings, we propose that the Middle Triassic stretching phase represents the first stage of the Alpine Tethys rifting, thereby rejecting the hypothesis of a continuous Permo-Triassic long-lasting phase of extension.
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CC1: 'Comment on egusphere-2024-1135', H. Seebeck, 04 Jun 2024
Egusphere-2024-1135
Scaramuzzo et al.
Transpressional tectonics during the Variscan-Alpine cycle transition: supporting a multi-rifting model, evidence from the European western Southern Alps
This manuscript examines the transition from the Paleozoic Variscan to the Meso-Cenozoic Apline supercontinent cycle through the examination of structural, kinematic and thermochronological data.
The manuscript is well written, clear and concise and I would recommend publication once the comments below have been addressed.
Kind regards
Hannu Seebeck
Major comments:
Given how much weight thickness variations in the early Permian effusive units and overlying Anisian – Ladinian sedimentary strata have in determining fault activity, I am a little surprised at how little information is provided in the methodology about how thickness estimates were derived.
The author states in Line164 That “the geometry and orientation of geological structures were extrapolated in three-dimensions starting from a series of cross-sections that were integrated with constraints from both the surface trace of planar features and direct field measurements.” This is followed by “The lower Permian succession records the activity of the Marzio Fault.” Line225.
By the nature of the description of the effusive units comprising tuffs, ignimbrites and intermediate and felsic lavas, the extrapolation of thicknesses becomes problematic given the highly variable modes of emplacement. Thickness variations of hundreds of metres over a few kilometres could be expected depending on source vent location and the height of pre-existing topography. I would be cautious on making any interpretation of fault activity based on thickness variations of eruptive units, particularly those associated with ignimbrites and pyroclastic flows without a good understanding where these units originated from. For example, if you are close to the boundary of a caldera then thickness variations say nothing about tectonic fault activity.
How many cross-sections were used and where were they located? Only four are shown in the manuscript. I note some inconsistencies with the cross-sections presented and the geologic map. For example the dip of the Anisian-Ladinian units on the northwestern side of the Marzio Fault dip to the northwest at 45° however the section shows a very shallow southeast dip. The thickness of the Effusive unit in section A southeast of the Marizo Fault is difficult to understand as the anticline shown in the map does not appear to have been represented? There appears to be a c. 20° decrease in the dip between the underlying and overlying units despite being shown to have a parallel contact geometry? Where is the MNF Fault shown on the cross-section in Figure 3? Where are the structural stations on Figure 3?
While these are relatively minor points I find these inconsistencies reduce my confidence in the structural model and the interpretations that follow.
Thermochronology, while this is largely outside of my expertise I have a couple of comments about the interpretation of the age data.
Line390 states that the young age of VA07 indicates a protracted thermal history while VA05 and VA06 reflect post-magmatic cooling of the Ganna Granitic Stock. It appears to me from Figure 3 that all three samples are located within a few hundred meters of the contact between the metamorphic units and the intrusion and within a 2.5 km radius of one another.
Also how do you explain that VA08 and VA06 have essentially the same age, VA06 being a product of post-magmatic cooling while VA08 is not? Line403 states that thermal diffusion modelling excludes the presence of the Ganna Granitic Stock below the exposed basement. The author therefore needs to explain why VA08 has an identical age to VA06 but was not part of the thermal event that generated identical ages.
Unless the Ganna Granitic Stock has a protracted emplacement period spanning at least 50 Myrs, I find it difficult to reconcile how these samples have had a significantly different thermal history. Figure 1 and 10 shows intrusions in the same age range further to the west but this is not really discussed? Could the sample locations have come from different depths with VA07 recording more exhumation? What other reasons could there be for the differences? Where on Figure 2 are your samples from, at least show the study area on this figure.
These seeming inconsistencies in the interpretation of the thermochronology lead me to question either the validity of the thermal diffusion modelling or the suggestion that there aren’t granites beneath the northern block. If I look at Figure 1 I can see Early Permian granite to the east of the study area that would be consistent with intrusions underlying the northern block.
Further to this, I do not feel the author has demonstrated structural control on the emplacement of the Ganna Granitic Stock with the observations presented here. Faults will tend to localise in regions where there are strength contrasts so the author would need to either demonstrate that the fault pre-dated the intrusions or that there are structural or cooling fabrics consistent with fault motion during the time of emplacement. Otherwise faulting may have just exploited strength contrasts associated with the intrusion. This also relates to whether intrusion has occurred beneath the surface in the northern block. If you cannot rule out that VA08 was associated with the thermal event associated with intrusion then fault control for the emplacement of the intrusion is less likely.
Minor comments
Line22: “architectural framework” – what does this mean? Vague term, do you mean structural framework?
Line23: “Nonetheless” doesn’t seem appropriate here. I would reverse this sentence by stating that “Due to the large hiatus in the geological record…the transition between the two-cycles is open to different interpretations.”
Line56: Southalpine one word?
Line58: phase not phases?
Line86: Please make the description of the Early Permian volcanic units consistent throughout the text and figures. Called Volcanic units in Figure 1, Effusive units in Figure 3 and 4 but not described as such in the text (or least not explicitly).
Line118: Como Lake is mentioned in the text a couple of times but not shown on a map? Lake Como or Como Lake? Consistency through text.
Line134: Lake Maggiore not shown on map.
Line170: “key horizons that were assumed as horizontal…” This is a large assumption in a volcanic environment. Provide justification for this assumption.
Line178: Add reference to support correspondence between stress and strain – in my opinion this is not straight forward particularly in transpressive or trantensional settings.
Line200: Your samples are 10x older than the standard used, how might this affect the results?
Line227: “lower Permian” as per Line86 comment please make the description of the Early Permian effusive units consistent throughout the text, makes it difficult for the reader when you keep changing the way the same units are referred to.
Line228: Section A is not the best example to use here as the thickness of the effusive units are not constrained on this section, it is only interpolated. Section C is constrained though refer to my earlier comment regarding dip angles, the dip on the effusive units appears c. 20° higher than the overlying unit which implies deformation prior to the deposition of the overlying sedimentary units.
Line235: “Middle Permian Unconformity seals the Martica-Boarezzo Fault”. Truncates may be a better term, seal implies a fluid-flow property.
Line236: How is the Mondonico push-up kinematically compatible with the Martica-Boarezzo Fault? These faults are near at right-angles to one another? Describe how these structures are kinematically related.
Line248: Describing not showcasing
Line253: Are you sure the apparent left-lateral separation is due to normal fault displacement? It could also be the result of an irregular unconformity surface? Or a result of the southward continuation of the Mondonico push-up? Your restorations are dependent on how you have interpreted fault motion in the different blocks.
Line273: Describe the cross-cutting relationships
Line276: Structural stations not shown on Figure 3
Line283-84: Figure order jumps from 5 to 7.
Line285: What do you mean by transpressive fault architecture? Describe and reference it.
Line 294: What do you mean by “entirely developed in the Anisian-Ladinian succession”? Do you have exposure of the top and bottom tip of the fault? A fault of this size would most likely originate at seismogenic depths. I think you mean it is exposed in the Anisian-Ladinian succession?
Line331: “…and ZHe ages.” Needs a reference here
Line335: Why isn’t the alpha ejection accounted for and what are the implications for you ages? Need to discuss this further as VA07 has a problematic ages relative to VA05 and VA06.
Line350: Show where the previous age sample came from on Figure 3. This provides the reader with all the relevant information.
Line355: “volcanoclastic sample” – be specifis and consistent i.e. Early Permian effusive unit
Line365: Ganna Granitic Stock needs an age range and a reference
Line368: remove word far
Line380: 2-3 km seems a very shallow intrusion depth and do not feel that your estimate should be based on your structural model. The closure temperature would suggest emplacement at depths of less than c. 5-6 km
Line402: Spelling structural
Line404: Refer to previous comments regarding the similarity in ages of VA08 and VA06. The thermal diffusion modelling does not rule out intrusion in beneath the northern block only that it would need to be 2-4 km below the sample locations.
Line489: At present I would disagree with “emplacement of a fault-bounded intrusive stock” – at present I do not feel you have adequately described how the and faulting are related in time.
Figure 1: Unit labels are confusing “Un.” Normally abbreviates unconformity and not units as here. For consistency, Triassic unit colours should match those used in cross-sections.
Figure 2: Study are should be shown
Figure 3: Consistency in unit naming – all units given age names except Effusive units. See text comments relating to consistency between unit naming in text and figure. Where are the structural stations? Show previous age determination location mentioned in the text.
Figure 4: Put horizontal scale on sections to show they are true scale. Caption change to “cross-sections in Figure 3”. Where is Figure 4c on Figure 3? It is not obvious where this is as the fault trends do not seem to match Figure 3 very well?
Figure 7: Strikes of dikes put on early-middle Permian for reference?
Citation: https://doi.org/10.5194/egusphere-2024-1135-CC1 -
AC3: 'Reply on CC1', Maria Giuditta Fellin, 07 Oct 2024
COMMENT: Given how much weight thickness variations in the early Permian effusive units and overlying Anisian – Ladinian sedimentary strata have in determining fault activity, I am a little surprised at how little information is provided in the methodology about how thickness estimates were derived.
The author states in Line164 That “the geometry and orientation of geological structures were extrapolated in three-dimensions starting from a series of cross-sections that were integrated with constraints from both the surface trace of planar features and direct field measurements.” This is followed by “The lower Permian succession records the activity of the Marzio Fault.” Line225.
REPLY: In the method section we described how we derived the isopach map from a 3D model. This model is based on 25 cross-sections and 4 stratigraphic sections, whose thickness was measured in the field. We will add more specific information in the method section to emphasize how data for the thickness analysis were collected, and in the Supplementary material we will enclose a move file with the full restoration procedure and the resultant 3D mesh surfaces. Our model and relating thickness-analysis is referred to the Anisian-Ladinian succession only, lines 163-163: “From the thickness-analysis, we derived an isopach 2D map contouring at the top of the Middle Triassic.”
We will edit the text in line 225 as it is indeed confusing.
COMMENT: By the nature of the description of the effusive units comprising tuffs, ignimbrites and intermediate and felsic lavas, the extrapolation of thicknesses becomes problematic given the highly variable modes of emplacement. Thickness variations of hundreds of metres over a few kilometres could be expected depending on source vent location and the height of pre-existing topography. I would be cautious on making any interpretation of fault activity based on thickness variations of eruptive units, particularly those associated with ignimbrites and pyroclastic flows without a good understanding where these units originated from. For example, if you are close to the boundary of a caldera then thickness variations say nothing about tectonic fault activity.
REPLY: We will expand the discussion to account for the possible role of inherited volcanic morphology and to present critically our interpretations of the Valganna volcanic sequence.
While thickness variations have indeed been used to infer fault activity, this has been done especially for the Middle Triassic extensional phase. Our 3D model does not extend to the lower Permian succession.
We acknowledge that thickness variations within the Permian succession may be influenced more by the geometry of volcanic bodies and pre-existing topography than by fault activity. Nevertheless, we maintain that the Marzio Fault played a role, supported by regional observations and the apparent bounding of the Ganna Granitic Stock. In fact, the Marzio Fault has been interpreted as active during the Late Carboniferous (Casati, 1978), particularly in relation to the distribution of the Carboniferous strata, such as the Manno Conglomerate.
We propose that renewed activity along the Ganna Fault during the Permian may have controlled the emplacement of the Ganna Granitic Stock. We will elaborate on these points in the introduction by reviewing previous studies and in the discussion, where we will also explore alternative interpretations.
COMMENT: How many cross-sections were used and where were they located? Only four are shown in the manuscript.
REPLY: Our 3D geological model is based on 25 cross-sections. We will add a map with the traces of the cross-sections in the method sections and the meshes of the surfaces used for the restoration in the Supplementary Materials.
COMMENT: I note some inconsistencies with the cross-sections presented and the geologic map.
REPLY: We will fix in the sections A and B the dip angle of the base of the volcanic sequence and some additional minor issues. We underline that since we have not restored the volcanic series this does not affect the thickness analysis.
COMMENT: For example the dip of the Anisian-Ladinian units on the northwestern side of the Marzio Fault dip to the northwest at 45° however the section shows a very shallow southeast dip.
REPLY: The succession cropping out on the northwestern side of the Marzio Fault is involved into a series of detached folds (see sections in figure 4). The northwest dipping panels refer to the northern limb of the anticlines (see for example the northern limb of the Mt. Pian Nave fold) and to the southern limb of the synclines.
COMMENT: The thickness of the Effusive unit in section A southeast of the Marzio Fault is difficult to understand as the anticline shown in the map does not appear to have been represented?
REPLY: The southern block of the Marzio Fault is fully involved into a north-verging anticline with a periclinal closure to the west (Figure 3). This anticline, also referred as Arbostora or Maroggia-Brinzio Anticline, has been described also in previous studies (Kälin & Trümpy; 1977). This structure has been interpreted as a deeply rooted fault-propagation fold and we analyzed it in a previous work (Scaramuzzo et al., 2022). To the east, the fault-related fold is presently eroded and the offset rapidly increases eastward giving place to a break-through fault-propagation fold, whose front limb is eroded. Close to the fault trace, only the backlimb of the fold is fully preserved. We will add the projection of the anticline in all the sections.
Kälin O. & Trümpy D.M., 1977: Sedimentation und Paläotektonik in den westlichen Sudalpen: Zur triasisch-jurassischen Geschichte des Monte Nudo-Beckens. Ecl. Geol. Helv. 70/2, 295-350.
COMMENT: There appears to be a c. 20° decrease in the dip between the underlying and overlying units despite being shown to have a parallel contact geometry?
REPLY: The graphical rendering of the sections can be misleading. There is indeed an angular unconformity between the lower Permian and the Triassic. The contact between the Permian and the Triassic series is represented by the Middle Permian Unconformity that is parallel to the bedding of the overlying Middle Triassic.
COMMENT: Where is the MNF Fault shown on the cross-section in Figure 3? Where are the structural stations on Figure 3?
REPLY: We will add the label MNF, the structural stations and the cross-sections in figure 3.
COMMENT: While these are relatively minor points I find these inconsistencies reduce my confidence in the structural model and the interpretations that follow.
REPLY: In the revised manuscript, we will show in greater detail how the 3D model has been built, how many cross-sections have been drawn, and we will discuss at depth the hard constraints to the extrapolated geometries and the weak constraints, given the assumptions for the extrapolation of surfaces. We hope that this will better illustrate the internal consistency of the model.
COMMENT: Thermochronology, while this is largely outside of my expertise I have a couple of comments about the interpretation of the age data.
Line390 states that the young age of VA07 indicates a protracted thermal history while VA05 and VA06 reflect post-magmatic cooling of the Ganna Granitic Stock. It appears to me from Figure 3 that all three samples are located within a few hundred meters of the contact between the metamorphic units and the intrusion and within a 2.5 km radius of one another.
REPLY: All three samples are located near the contact with the intrusion, as discussed earlier in this section (lines 366-367). The cooling ages of samples VA05 and VA06 closely match the emplacement age of the Ganna granitic stock. In contrast, the cooling ages of sample VA07 span a broader range, from 250 to 170 Ma, overlapping the Permian emplacement of the Ganna intrusion and extending into the Middle Jurassic. Our modeling results show that temperatures near a shallowly emplaced intrusion drop rapidly, both spatially and temporally. All three samples (VA05, VA06 and VA07) were close enough to the intrusion to be affected by its thermal effect. Yet, only the cooling ages of VA05 and VA06 reflect post-magmatic cooling, whereas the extended cooling period observed in VA07 is inconsistent with post-magmatic cooling only. The cooling record of sample VA07 reflects its lower structural position relative to samples VA05 and VA06 and to the Ganna Stock.
We will revise this section to clarify our conclusions.
COMMENT: Also how do you explain that VA08 and VA06 have essentially the same age, VA06 being a product of post-magmatic cooling while VA08 is not? Line403 states that thermal diffusion modelling excludes the presence of the Ganna Granitic Stock below the exposed basement. The author therefore needs to explain why VA08 has an identical age to VA06 but was not part of the thermal event that generated identical ages.
REPLY: This topic is addressed in section 5.1, which we will revise to provide a more detailed interpretation. Samples VA08 and VA06 exhibit similar cooling ages, overlapping with the emplacement age of the Ganna granitic stock, due to the combined influences of intrusive emplacement and normal activity of the Marzio fault. The thermal effect of the intrusion in the hanging wall of the Marzio fault caused sample VA06 to record post-magmatic cooling. Meanwhile, the normal faulting activity led to the uplift and exhumation of the footwall, recorded by sample VA08. Consequently, the typical cooling pattern across a normal fault—where younger cooling ages are found in the footwall—has been overprinted by the thermal impact of the granitic intrusion.
COMMENT: Unless the Ganna Granitic Stock has a protracted emplacement period spanning at least 50 Myrs, I find it difficult to reconcile how these samples have had a significantly different thermal history. Figure 1 and 10 shows intrusions in the same age range further to the west but this is not really discussed? Could the sample locations have come from different depths with VA07 recording more exhumation? What other reasons could there be for the differences? Where on Figure 2 are your samples from, at least show the study area on this figure.
REPLY: It is unlikely that the Ganna Granitic stock experienced a protracted emplacement. Sample VA07 is in a position structurally lower than the other samples in the hangingwall of the Marzio Fault – this difference is reflected by the cooling record of this sample. This is clearly explained in the lines 458 to 465. We will expand the discussion in these lines to provide further clarity.
COMMENT: These seeming inconsistencies in the interpretation of the thermochronology lead me to question either the validity of the thermal diffusion modelling or the suggestion that there aren’t granites beneath the northern block.
REPLY: The modeling we present is simple and straightforward, designed to quantify the thermal effects of a shallowly emplaced granitic intrusion. Our interpretation of the thermochronologic data is primarily based on the data itself and its relationship to field-observed structures. While we do not believe there are significant inconsistencies in our interpretation, we acknowledge the need for greater clarity in our discussion of the data and will work to address this.
COMMENT: If I look at Figure 1 I can see Early Permian granite to the east of the study area that would be consistent with intrusions underlying the northern block.
REPLY: Here there is a misinterpretation of the regional structure of the Southern Alps. There are several Permian intrusive and effusive districts in the Southern as shown in the map of Figure 1. Hower there is no physical correlation between these districts and the Ganna Granitic stock, which belongs to the Lugano-Varese district.
COMMENT: Further to this, I do not feel the author has demonstrated structural control on the emplacement of the Ganna Granitic Stock with the observations presented here. Faults will tend to localize in regions where there are strength contrasts so the author would need to either demonstrate that the fault pre-dated the intrusions or that there are structural or cooling fabrics consistent with fault motion during the time of emplacement.
Otherwise faulting may have just exploited strength contrasts associated with the intrusion. This also relates to whether intrusion has occurred beneath the surface in the northern block.
If you cannot rule out that VA08 was associated with the thermal event associated with intrusion then fault control for the emplacement of the intrusion is less likely.
REPLY: The Marzio fault was indeed active already in the late Carboniferous and therefore predate the intrusion (Casati, 1978). During the Ganna Granitic Stock emplacement was reactivated as a normal fault.
We will add this observation in the discussion to highlight that the Marzio Fault predated the granite emplacement.
MINOR COMMENTS
Line22: “architectural framework” – what does this mean? Vague term, do you mean structural framework?
REPLY: We will change it accordingly.
Line23: “Nonetheless” doesn’t seem appropriate here. I would reverse this sentence by stating that “Due to the large hiatus in the geological record…the transition between the two-cycles is open to different interpretations.”
REPLY: We will change it accordingly.
Line56: Southalpine one word?
REPLY: It can be used both ways.
Line58: phase not phases?
REPLY: We will change it accordingly.
Line86: Please make the description of the Early Permian volcanic units consistent throughout the text and figures. Called Volcanic units in Figure 1, Effusive units in Figure 3 and 4 but not described as such in the text (or least not explicitly).
REPLY: We will change it accordingly.
Line118: Como Lake is mentioned in the text a couple of times but not shown on a map? Lake Como or Como Lake? Consistency through text.
REPLY: The Como Lake is shown in figure 1 but it is not labelled. We will add the label.
Line134: Lake Maggiore not shown on map.
REPLY: It is shown in figure 1 but the label is missing. We will add it.
Line170: “key horizons that were assumed as horizontal…” This is a large assumption in a volcanic environment. Provide justification for this assumption.
REPLY: No horizons within the volcanic pile have been considered for restoration. Instead, we used the middle Permian unconformity, that is a planar erosive surface, as a key horizon.
Line178: Add reference to support correspondence between stress and strain – in my opinion this is not straight forward particularly in transpressive or trantensional settings.
REPLY: Yes, we agree. The phrase in this line is confusing. The quantitative inversion of fault slip data provides direct constraints on the orientations and relative magnitudes of the global principal strain rates (e.g., Twiss and Unruh, 1998). Indeed, that is the reason why we used a kinematic approach and strain axes are reported in all the figures and in the text.
We will remove the sentence.
Twiss, R. J., & Unruh, J. R. (1998). Analysis of fault slip inversions: Do they constrain stress or strain rate?. Journal of Geophysical Research: Solid Earth, 103(B6), 12205-12222.
Line200: Your samples are 10x older than the standard used, how might this affect the results?
REPLY: The Fish Canyon Tuff serves as an age standard, but it is not used to calibrate the measurements or the ages, and therefore it does not affect the results. As explained in the methods section, the age standard is processed alongside the samples to ensure the accuracy of the date estimates and to monitor intrasample dispersion. We also provided a summary of the measurement procedures. Helium-4 (⁴He) is measured using an external standard, an aliquot of ⁴He gas from a calibration bottle. Uranium and thorium are measured using internal standard solutions of ²³³U and ²³⁰Th, which are weighed and added to the sample before dissolution. We will add some additional specifications in the method.
Line227: “lower Permian” as per Line86 comment please make the description of the Early Permian effusive units consistent throughout the text, makes it difficult for the reader when you keep changing the way the same units are referred to.
REPLY: We will change it accordingly.
Line228: Section A is not the best example to use here as the thickness of the effusive units are not constrained on this section, it is only interpolated. Section C is constrained though refer to my earlier comment regarding dip angles, the dip on the effusive units appears c. 20° higher than the overlying unit which implies deformation prior to the deposition of the overlying sedimentary units.
REPLY: The thickness variation of the early Permian effusive units was also measured in the stratigraphic sections shown in the stratigraphic block diagram. This will appear clearer as we will add the map showing the location of the measured stratigraphic sections and the traces of the cross-sections.
Line235: “Middle Permian Unconformity seals the Martica-Boarezzo Fault”. Truncates may be a better term, seal implies a fluid-flow property.
REPLY: We will change it accordingly.
Line236: How is the Mondonico push-up kinematically compatible with the Martica-Boarezzo Fault? These faults are near at right-angles to one another? Describe how these structures are kinematically related.
REPLY: The thrusts delimiting the Mondonico push-up are perpendicular to the shortening direction. The Martica Boarezzo Fault act as a strike-slip fault oblique relatively to the shortening direction. Thus, the orientation and kinematic of these faults is consistent with the same strain ellipsoid – the strain ellipsoid that we derived for the Martica Boarezzo Fault is shown in Figure 7.
We will describe this relationship in the result section.
Line285: What do you mean by transpressive fault architecture? Describe and reference it.
REPLY: we will rephrase this sentence. We will write that the inversion of the fault-slip data indicates that this fault was active during a transpressive phase. This is consistent with our observations about the Mondonico push-up and its kinematic relationship with the Martica-Boarezzo fault.
Line248: Describing not showcasing
REPLY: We will change it accordingly.
Line253: Are you sure the apparent left-lateral separation is due to normal fault displacement? It could also be the result of an irregular unconformity surface? Or a result of the southward continuation of the Mondonico push-up? Your restorations are dependent on how you have interpreted fault motion in the different blocks.
REPLY: Although some of the apparent left-lateral separation is related to alpine deformation, we think that our interpretation is solid because of:
- After 3D unfolding and restoration there is a residual offset along the Valganna Fault ;
- The Valganna Fault abruptly delimits to the west the extension of the intra-platform anoxic Middle Triassic facies, i.e., the Besano shales.
- The fault slip data collected along the Valganna Fault, after restoration nearest bedding orientation, clearly shows a normal activity.
In our view, the left lateral separation along the Valganna Fault cannot be related to an irregular unconformity. The middle Permian unconformity at the base of Anisian-Ladinian succession is an erosive surface with very little relief. The almost flat morphology of this erosive surface is inconsistent with a variation in thickness of about 200 meters, which we attribute to the activity of the Valganna Normal Fault.
We also exclude that the apparent left lateral separation is the result of the southern prosecution of the Mondonico push-up because this structure is truncated by the middle Permian unconformity. These observations are outlined in section 4.1.1 and a description of the middle Permian unconformity is in line 96.
Line273: Describe the cross-cutting relationships
REPLY: Yes, we will add the description to the text.
Line276: Structural stations not shown on Figure 3
REPLY: Yes, we will add those to the figures.
Line283-84: Figure order jumps from 5 to 7.
REPLY: We will change it accordingly.
Line 294: What do you mean by “entirely developed in the Anisian-Ladinian succession”? Do you have exposure of the top and bottom tip of the fault? A fault of this size would most likely originate at seismogenic depths. I think you mean it is exposed in the Anisian-Ladinian succession?
REPLY: Here we refer to the outcropping Valganna Fault in the location where the field photos were shot.
The Valganna Fault is a deeply rooted normal fault. Its upper tip is located withing the uppermost part of the Anisian-Ladinian succession: indeed the top of the Ladinian truncates the Valganna Fault. The lowermost outcropping segment of the Valganna Fault cuts through the basement and the Ganna Granitic Stock. A lower tip has not been observed.
Line331: “…and ZHe ages.” Needs a reference here
REPLY: We added appropriate references as, for instance, Reiners and Brandon, 2006.
Line335: Why isn’t the alpha ejection accounted for and what are the implications for you ages? Need to discuss this further as VA07 has a problematic ages relative to VA05 and VA06.
REPLY: We account for the alpha-ejection as explained in the methods section (lines 189-190). However, as stated here, we do not account for the distribution of U and Th. The alpha-ejection correction assumes a homogeneous distribution of U and Th within zircons. Accounting for non-homogeneous distributions would require additional, non-routine measurements. Currently, U and Th are measured from bulk zircon grains after dissolution, aligning with the fact that He is also extracted from the entire grain. Measuring the spatial distribution of U and Th would necessitate laser-ablation ICP-MS profiling of elemental concentrations in the zircon. We will improve this sentence for clarity.
Line350: Show where the previous age sample came from on Figure 3. This provides the reader with all the relevant information.
REPLY: On the map of the Figure 3, we will add the location of the cooling ages from previous studies mentioned in this line.
Line355: “volcanoclastic sample” – be specifis and consistent i.e. Early Permian effusive unit
REPLY: We will change it accordingly.
Line365: Ganna Granitic Stock needs an age range and a reference
REPLY: The age of the Ganna Granitic stock emplacement has been determined by Shalteger and Brack (2006) as stated in lines 84-85 “The emplacement of the Ganna Granitic complex occurred at 281.34 ± 0.48 Ma (Schaltegger and Brack, 2007)”. We can add this reference and age here too.
Line368: remove word far
REPLY: We will change it accordingly.
Line380: 2-3 km seems a very shallow intrusion depth and do not feel that your estimate should be based on your structural model. The closure temperature would suggest emplacement at depths of less than c. 5-6 km
REPLY: We will correct the text and we will refer to the depth of emplacement of the stock as estimated by Bakos et al. 1990.
Line402: Spelling structural
REPLY: We will correct it.
Line404: Refer to previous comments regarding the similarity in ages of VA08 and VA06. The thermal diffusion modelling does not rule out intrusion in beneath the northern block only that it would need to be 2-4 km below the sample locations.
REPLY: Yes, we agree. We will modify the text accordingly.
Line489: At present I would disagree with “emplacement of a fault-bounded intrusive stock” – at present I do not feel you have adequately described how the and faulting are related in time.
REPLY: We partly agree. Indeed, we do not have a smoking gun to prove the absence of the stock north of the Marzio Fault but many clues point to it. We will explicit the degree of reliability of our conclusions in the text.
Figure 1: Unit labels are confusing “Un.” Normally abbreviates unconformity and not units as here. For consistency, Triassic unit colours should match those used in cross-sections.
REPLY: We will change it accordingly.
Figure 2: Study are should be shown
REPLY: We will change it accordingly.
Figure 3: Consistency in unit naming – all units given age names except Effusive units. See text comments relating to consistency between unit naming in text and figure. Where are the structural stations? Show previous age determination location mentioned in the text.
REPLY: We will check for consistency and make necessary amendments. We will add the stations to the map.
Figure 4: Put horizontal scale on sections to show they are true scale. Caption change to “cross-sections in Figure 3”. Where is Figure 4c on Figure 3? It is not obvious where this is as the fault trends do not seem to match Figure 3 very well?
REPLY: We will add a reference frame for locating figure 4c. Consider that the thickness map is created on the restored and unfolded geological model, thus changes in strike of the faults are expected.
Figure 7: Strikes of dikes put on early-middle Permian for reference?
REPLY: Yes, we will add those.
Citation: https://doi.org/10.5194/egusphere-2024-1135-AC3
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AC3: 'Reply on CC1', Maria Giuditta Fellin, 07 Oct 2024
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RC1: 'Comment on egusphere-2024-1135', Anonymous Referee #1, 12 Jul 2024
This manuscript analyzes the polyphase nature of the Permian-Triassic tectonics in a small region of the western Southern Alps. This issue has been the subject of numerous previous studies. However, references to previous work of the last 15-20 years are almost completely missing from sections 1 and 2. The authors presents some new thermochronological data, which however do not provide essential elements for the resolution of the geological problem declared in the Introduction. As a result, the reader may feel that the conclusions presented are not adequately supported by the data shown. In my opinion, the rationale of the article is not adequately clear. A substantially stratigraphic/thermochronological Introduction and Geological Background sections are followed by an almost purely tectonic analysis, leaving the reader disoriented. Also, the title does not reflect the contents of the paper (the term "transpressive tectonics" which begins the title is never used in the text!) and the abstract does not provide a concise and complete summary, and it is largely disconnected from the rest of the manuscript. Because poorly structured, I regret to say that the manuscript is overall unclear and difficult to read, despite the English being basically correct.
I am sorry to conclude that this manuscript would require complete reorganization before it can be considered for publication in a scientific journal.Citation: https://doi.org/10.5194/egusphere-2024-1135-RC1 -
CC2: 'Reply on RC1', Franz Livio, 12 Jul 2024
Thanks so much for your feedback on the manuscript. Once the discussion will be closed we'll submit the revised manuscript taking into consideration your notes on the manuscript structure and point out the rationale of the research.
To better address the points, could you please be more specific indicating some of the missing references you are referring to? Up to our knowledge, we haven't missed a lot of the significant literature on the topic, even if we cannot exclude that some works have been overlooked.
Thanks in advance.
Prof. Franz Livio
Citation: https://doi.org/10.5194/egusphere-2024-1135-CC2 -
AC1: 'Reply on RC1', Maria Giuditta Fellin, 07 Oct 2024
COMMENT: This manuscript analyzes the polyphase nature of the Permian-Triassic tectonics in a small region of the western Southern Alps. This issue has been the subject of numerous previous studies.
However, references to previous work of the last 15-20 years are almost completely missing from sections 1 and 2.
REPLY: To the best of our knowledge, we have thoroughly considered all relevant literature. We have made every effort to incorporate previous work where applicable. On July 12th, we requested the reviewer to provide examples of any missing references, but we have not received a response. As a result, we were unable to include or consider any additional works.
COMMENT: The authors presents some new thermochronological data, which however do not provide essential elements for the resolution of the geological problem declared in the Introduction. As a result, the reader may feel that the conclusions presented are not adequately supported by the data shown. In my opinion, the rationale of the article is not adequately clear.
REPLY: We will carefully revise the text in light of this critique. While we agree that the thermochronologic data alone may not provide definitive constraints on the geological problem we address, they do offer valuable clues. When combined with geological and structural data, these insights contribute to a better understanding of the Variscan-Alpine transition. Additionally, we would like to highlight that the scientific hypotheses under investigation are clearly outlined in lines 36-54, and the specific contributions of this work are summarized in lines 54-60.
COMMENT: A substantially stratigraphic/thermochronological Introduction and Geological Background sections are followed by an almost purely tectonic analysis, leaving the reader disoriented.
REPLY: We respectfully disagree. Constraints come from i) geological mapping of unconformity-bounded stratigraphic sequences: ii) structural analysis including restoration and thickness analysis and iii) thermochronologic data.
COMMENT: Also, the title does not reflect the contents of the paper (the term "transpressive tectonics" which begins the title is never used in the text!)
REPLY: The term traspressive/transpressional has 11 occurrences in the text and we describe transpressive structures such as the Mondonico push-up and the Martica-Boarezzo fault. However, we agree on that the title might not adequately reflect the content. We will change it to: “Evidence for Multi-Rifting in the Variscan-Alpine Cycle Transition: Insights from the European western Southern Alps”
COMMENT:…and the abstract does not provide a concise and complete summary, and it is largely disconnected from the rest of the manuscript.
REPLY: We will rewrite the abstract considering this critique.
Because poorly structured, I regret to say that the manuscript is overall unclear and difficult to read, despite the English being basically correct.
I am sorry to conclude that this manuscript would require complete reorganization before it can be considered for publication in a scientific journal.
Citation: https://doi.org/10.5194/egusphere-2024-1135-AC1
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CC2: 'Reply on RC1', Franz Livio, 12 Jul 2024
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RC2: 'Comment on egusphere-2024-1135', Anonymous Referee #2, 02 Sep 2024
Dear Authors,
I think the manuscript could be interesting for the readers of SE, particularly those working on the Variscan and Alpine tectonics. The paper is well written and organised. However, there are some points that, in my opinion, should be improved.
Firstly, there are some inconsistences between map and sections that must be fixed. Geological boundaries, fold axial traces and faults of tjh map do not correlate with the sections. Geological cross-sections are, moreover, a bit too rough. I suggest to take care of the geological data as well as to improve the quality of design. I also suggest to improve field (and cartographic) evidences of the intersection relationships between faults and intrusion and about the presence or not of faults synchronous with the volcanic activity. Regarding the thermochronology, data are sounding but the interpretation is quite weak. I do not understand how you may exclude the presence of buried intrusion below the northern block. 1D modelling is not enough. This is true for all samples, the distance of which from an intrusion is far to be demonstrated in the 3D space. It is also strange that one of the sample closest to the intrusion shows the youngest age. I lost the explanation for this.
Another point that needs to be addressed is related to the limited consideration of other Alpine or surrounding sectors that shared similar or complementary tectonic evolution during the Mesozoic. I suggest widening the perspective, better accounting information from other Alpine sectors or the well-preserved Variscan section of the Sardinian-Corsica Batholith. Even the info from the middle-lower crust of the Ivrea zone must be implemented. From these areas the Authors may consider significant information to improve the discussion about the best fitting geodynamic models.
For these reasons, I suggest major revision.
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AC2: 'Reply on RC2', Maria Giuditta Fellin, 07 Oct 2024
Comment: Dear Authors,
I think the manuscript could be interesting for the readers of SE, particularly those working on the Variscan and Alpine tectonics. The paper is well written and organised. However, there are some points that, in my opinion, should be improved.
Firstly, there are some inconsistences between map and sections that must be fixed. Geological boundaries, fold axial traces and faults of tjh map do not correlate with the sections. Geological cross-sections are, moreover, a bit too rough. I suggest to take care of the geological data as well as to improve the quality of design. I also suggest to improve field (and cartographic) evidences of the intersection relationships between faults and intrusion and about the presence or not of faults synchronous with the volcanic activity.
REPLY: We will fix all the errors within the sections and the incongruences between the map and the sections.
Comment: Regarding the thermochronology, data are sounding but the interpretation is quite weak. I do not understand how you may exclude the presence of buried intrusion below the northern block. 1D modelling is not enough. This is true for all samples, the distance of which from an intrusion is far to be demonstrated in the 3D space. It is also strange that one of the sample closest to the intrusion shows the youngest age. I lost the explanation for this.
REPLY: The 1D thermal models only helps to quantify the thermal effects of a shallowly emplaced granitic intrusion. While we do not believe there are significant inconsistencies in our interpretation, we acknowledge the need for greater clarity in our discussion of the data and will work to address this. Our interpretation of the thermochronologic data is based on the following observations:
- The cooling ages of samples VA05 and VA06 closely match the emplacement age of the Ganna granitic stock. These samples are located very close to the granitic intrusion, in the hangingwall of the Marzio normal fault that was active at the time of the granite emplacement.
- Sample VA08 exhibits cooling ages overlapping with the emplacement age of the Ganna granitic stock. This sample is located in the footwall of the Marzio normal fault, far from the granite.
- The cooling ages of sample VA07 span a broad range, from 250 to 170 Ma, that encompasses the time of the Ganna granitic intrusion and extends to the Middle Jurassic. This sample is very close to the Ganna granitic stock but in a position structurally lower than both the other samples and the granite. This sample is also proximal to a large syn-rift normal fault immediately to the east of the study area.
The 1D thermal model indicates that the thermal effect of a shallowly emplaced granitic intrusion drops rapidly both in space and time. By combining the observations with the 1D thermal model, we conclude that:
- The cooling ages of VA05 and VA06 may reflect post-magmatic cooling.
- The cooling ages of samples VA07 record both post-magmatic and rift-related cooling.
- The normal activity of the Marzio fault during the Permian led to uplift and exhumation of its footwall, recorded by sample VA08.
- The typical cooling pattern across a normal fault—where younger cooling ages are found in the footwall—has been overprinted by the thermal impact of the granitic intrusion.
Comment: Another point that needs to be addressed is related to the limited consideration of other Alpine or surrounding sectors that shared similar or complementary tectonic evolution during the Mesozoic. I suggest widening the perspective, better accounting information from other Alpine sectors or the well-preserved Variscan section of the Sardinian-Corsica Batholith. Even the info from the middle-lower crust of the Ivrea zone must be implemented. From these areas the Authors may consider significant information to improve the discussion about the best fitting geodynamic models.
REPLY: We will integrate the discussion with the proper literature referred to the Permo-Variscan evolution of the Sardinian-Corsica area.
For these reasons, I suggest major revision.
Citation: https://doi.org/10.5194/egusphere-2024-1135-AC2
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AC2: 'Reply on RC2', Maria Giuditta Fellin, 07 Oct 2024
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