the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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
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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
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