|After reading the revised version of this manuscript I reaffirm that I appreciate the work and results performed on the dating of the thrusts in the Subalpine Molasse. These provide important and new information on the young collisional history of the Alps, but I remain a somehow skeptical about the lithospheric-scale interpretation of these data.|
What I appreciate:
the results of the AHe dating. The correlation of these ages with those existing further east. The new assessment that they are the expression of a large-scale event and that this event (northward thrusting) is discontinuous in time. The paper is very nicely illustrated, properly structured and clearly written.
I acknowledge that Late Miocene shortening terminates in the area of Salzburg and that shortening further east is older (my mistake in my last review, sorry!), hence I think that seeking and discussing a large scale explanation for this is reasonable indeed.
What I disagree with:
The authors discuss with lots of emphasis a general transition between vertical and horizontal tectonics around 12 Ma. I acknowledge that this revised version clearly defines these terms, however, I consider that the inferred vertical tectonics only affect a small part of the western Central Alps (Aar Massif), and not the rest. If the Central Alps extend to Salzburg as suggested in the ms, only a very small part of the Central Alps was affected by vertical tectonics. While the western Central Alps are deforming by “vertical” tectonics, “horizontal” tectonics are still ongoing (e.g. Ortner et al., 2014), even if the deformation front seems to be stationary. Therefore I see two problems with the inferred transition from vertical to horizontal tectonics: (1) it does not affect the Central Alps outside the Aar Massif. Its extrapolation from the Aar Massif to the entire Central Alps is not justified in my view. (2) This transition is presented as if horizontal shortening along low angle thrusts ceased between 20 and 12 Ma, which is not the case.
I still think that shortening in the Subalpine Molasse decreases progressively eastward, based on the data of fig. 6 (see detailed comments below), that Salzburg does not coincide with a change in slab geometry at depth, and that the discussion is very speculative.
Line 3. “vertical exhumation”: I don’t think this term makes much sense. Vertical uplift? Or just “exhumation” if this is the process referred to.
Line 5: please specify that this is true in the Aar Massif and not generally.
Line 7: The question is what is meant by “This scenario”. It sounds as if it refers to the transition from vertical tectonics to horizontal tectonics. If this is the case I don’t think that all these references take the Mt Blanc into account.
Line 15. Central Alpine slab until 12.5°? This is a rather arbitrary value. It really depends a lot on the literature. A review by Kästle et al. 2020 shows that this value may be anywhere between 10.5 and 12.5°. Therefore the coincidence of this slab configuration change with the area of Salzburg is not obvious to me.
Line 20: based on the tomographic literature this could well be along the Giudicarie Brenner system (see the compiled horizontal sections in Kästle et al. 2020).
Lines 24-25: same comment as above. The coincidence of Salzburg and the lateral termination of the Central Alpine Slab is largely over-interpreted.
Line 30: add a space after “plate”
Line 5: delete “the” before rock.
Line 6. “Uplift”: I guess exhumation is the appropriate term here. Note that Alpine topography (at least at present) is high in the eastern Central Alps, even where no vertical component of displacement took place along the Insubric Line.
Lines 9-10: I don’t understand why the Simplon fault is mentioned here. Do you mean that exhumation of the Lepontine Dome was due to extension along the Simplon Fault? I don’t think that Berger et al. and Todd and Engi stated this.
Line 17. Add “Inferred” before buoyancy-driven
Line 28: The S-Alps did not result by a late stage evolution…, maybe just their youngest thrusts did.
Line 20. “rapidly decreasing”. Rapidly is just a little adverb…, but it has some importance for the later discussion. I don’t see a real difference in the “rate” of decrease of shortening vs distance between the eastern Lake Geneva and Zurich, and between Lake Konstanz and Salzburg.
Lines 21-22. I don’t agree with this statement. The level of exhumation of the European basement decreases eastward just as the amount of post 12 Ma shortening inferred in this study. Fig. 6c shows the increasing eastward depth of the top of the EU basement, hence its decrease of exhumation level.
Line 13. INFERRED buoyancy-driven…
Line 16: replace “when” by “that”
Line 18. Why is this sentence here? I don’t think it is important and it breaks the thread of the discussion.
Line 10. This sentence is copy-pasted from page 11 (and as on page 11, I still disagree…)
Lines 10-13: I cannot follow the logic of this sentence. I agree that Miocene thrusting is a large-scale feature, but simply because I see it on the maps, certainly not because of an inferred continuous down-going slab.
Line 19: INFERRED buoyancy-driven…
Lines 14-15: No, this is only true for the western part of the Central Alps, where the Aar Massif is exposed, not for the eastern part. And if the Central Alps are inferred to continue until Salzburg, then it is only true for one third of the Central Alps.
Lines 20-21: in principle I agree, but something disturbs me about the way the concepts are expressed. The sentence gives the impression that the stationary Alpine Front was not associated to thrusting, hence not associated to “horizontal tectonics”. Whether stationary or not, thrusting was active even before 12 Ma.
Lines 21-22: classical model: why not? Continuous: no, discontinuous.
Line 10. INFERRED buoyancy-driven
Line 24. The way the boundary between Eastern and Central Alps is defined here sounds like if it was the Giudicarie-Brenner system. This is not the way it was suggested in the Introduction.
Line 28: See comments above. It is true, but the shortening value decreases to less than 20 already west of Zurich. So there is nothing special about the 20 km decrease between Lake Constance and Salzburg.
Line 29: but the distance between Lake Constance and Salzburg is close to 200 km… as you write, between 10.5° and Salzburg (12.8°).
Lines 33-34: why such upper crustal processes should mask late Miocene deformation?
“may also reflect”: yes, if expressed in such general terms it is difficult to disagree, but more specifically, why?
Line 1. The precise value of longitude and geographic position of Salzburg is at odd with the very general general and unprecise character of the rest of the sentence.
Lines 2-3: I certainly agree in general. But the statement is too general to be of relevant.
Lines 6-8: This sentence is also too general. So general that it is obvious and not really relevant to the discussion.
Line 11: there is nothing specific in the papers quoted that allows to relate these large events to the end of Miocene time.
Line 14. INFERRED buoyancy-driven
Line 28. See comment on page 4: it could coincide with the Giudicarie Brenner Fault.