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Importance of basement faulting and salt decoupling for the structural evolution of the Fars Arc, Zagros fold-and-thrust belt: A numerical modeling approach
Abstract. Understanding the tectonic evolution and crustal-scale structure of fold-thrust belts is crucial for exploring geological resources and evaluating seismic hazards. We conducted a series of finite-difference two-dimensional thermo-mechanical numerical models with visco-elasto-plastic/brittle rheology to decipher how the interaction of inherited basement faults and salt décollement levels control the deformation process and structural style of the Fars Arc in the Zagros fold -thrust belt, during tectonic inversion. Results indicate that initial rifting is controlled by the geometry of inherited faults. During the convergence phase, fold-and-thrust belts display folding at two scales: large wavelength folds induced by basement deformation in the form of fault-propagation faults, and small wavelength folds and thrust systems emerge above the salt layer as detachment folds. Reactivated faults can serve as pathways for stress transfer, resulting in the emergence of new faults and thus seismic activity. The tectonic events in orogenic belts like the Zagros do not adhere to a fixed pattern; they are shaped by factors such as the properties of basement rocks and the orientation of faults. Shallow earthquakes predominantly occur along décollement anticlines, while deeper and larger ones are associated with basement faults. Additionally, we observe variations in resistance to deformation based on salt rheology and fault geometry, with listric faults minimizing resistance. The degree of basement involvement in deformation directly influences the model's resistance, with greater involvement facilitating easier deformation. Our results showing the temporal-spatial relationship between thin- and thick-skinned tectonics can work as an analogue for similar orogenic belts worldwide, such as Taiwan, the Pyrenees, the Alps, the Appalachians, and the Kopet Dagh.
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RC1: 'Comment on egusphere-2024-1123', Frédéric Mouthereau, 11 Jul 2024
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Reviews on “Importance of basement faulting and salt decoupling for the structural evolution of the Fars Arc, Zagros fold-and-thrust belt: A numerical modeling approach” submitted to Solid Earth by Gomar et al.
General comments
This study aims to investigate using a numerical modelling approach the coupling between basement-involved shortening and salt-based cover deformation and its impact on the tectonics style of the Zagros fold-thrust belt. One of the novelty of this research is to implement the evolution of the basement/cover shortening in the Zagros from rifting during the NeoTethyan rifting to mountain building, and the exploration of a useful range of parameters including variations of forces and strain rates across the fold-and-thrust belt. The manuscript is well written and organised. However, I don't think that they are yet sufficiently discussing the implications of their results in regard to the few previous studies, for instance, how the topographic slopes they predict conform with observations as the models do not account for flexure ? Moreover, although the references chosen are generally sounding they should refer to the earlier studies on the Fars arc in the introductory part of the study not only in the discussion, so the reader can better understand what has been done on the subject. Finally, I would have like to see how this work can help drawing better balanced cross-section sections. For instance, to account for decoupling in the basement geologists used to draw either a detachment in the middle crust or in the lower crust. According to my reading of the model results, the upper-middle crust appears progressively decoupled from the lower crust so that we see distributed shearing in the lower crust, meaning a significant part of the lower crust is indeed involved in the deformation. One consequence is that geologist should not draw a localised detachment in middle crust but rather a distributed zone of deformation or a detachment deeper in the lower crust to account for the lower crustal material involved. This point is only technical because it has implications in terms of crustal budget during convergence. Find more detailed comments below.
Specific comments
Lines#63-65 : Alternatively looking carefully at the Mountain Front Fault, which is the topographic front of the Fars arc (Asaluyeh anticline), there are evidence that it is related to basement thrusting. First, the topographic offset between for the forelimb and backlimb is best explained by a basement fault. Then, the Fars arc is devoid of salt diapir (they are exposed only across the basement faults) compared to adjacent areas. It is also characterized by deep earthquake that is best explained by heterogeneities in the basement (Mouthereau et al., 2006; Mouthereau et al. (2007). So the arc shape is not simply the result of a fast propagation caused by salt. We showed that the regional topography was the result of basement thrusting because the salt was to weak to reproduce the topography of the Fars arc.
Line#73: Please cite previous works that provided more quantitative estimates in support of basement-involved faulting (Mouthereau et al., 2006; 2007). We also suggested that both thin and thick-skiined deformation occurred synchronously.
Line#95: If you are talking about orogen and collision I think Mouthereau et al. 2012 is a better suited reference here.
Line#114: this is more or less the same team. You should also cited older works conducted in the Fars arc I think like Khadivi et al. , 2009 or the synthesis in Mouthereau et al., 2012.
Line#163: cite Mouthereau et al. 2007; 2012 who provided one of the first cross-section and kinematic analysis for the Fars arc.
Line#165: same here cite Mouthereau et al., 2012.
Lines#236-238: 600°C at 30 km correspond to 20°C/km. Isn'it too low ? Perhaps the fact that the LAB below the Zagros is supposedly thick (see Tunini et al., 2014) might help justify this. Additionally does this fit with the thermal age (Neo-Tethyan) you expect for the margin ?
Line#263: this is little more than the 16-19% we estimated in the centre of the Farc arc. We also estimated a shortening rate of 6.5-8 km/Myr. are these values consistent altogether ?
Line#285: This is exactly what we proposed in Mouthereau et al.(2006, 2007).
Line#295: Perhaps compare with the newtonian salt viscosity we modelled in our 2006 paper.
Line#517-520: This echoes to my previous comments that previous studies of the Fars arc should be better introduced … in the introduction.
Line#568: The analysis of topographic wavelengths is developed in the 2006 paper
Line#592: Yes but this is observed east of the Fars arc along the Bandar-abbas segment and strictly speaking this is not the centre of the Fars arc where I think this study best applies (or as justified below this applies to the Inner Fars). Precisions are needed here.
Line#636: add Mouthereau et al, 2007
Lines#643-644: In our analytical (less quantitative definitively) work (Mouthereau et al., 2006) we concluded that diabase might be too weak to reproduce the topography. Do you have an explanation why we have different conclusions ? Did you have a look to the topographic slope of your models ? The base of your box is horizontal but if you consider flexure the deep decoupling level should be inclined towards the load and your topography might be too low, perhaps below sea level, inconsistent with a mountain range.
Citation: https://doi.org/10.5194/egusphere-2024-1123-RC1 -
RC2: 'Comment on egusphere-2024-1123', Lorenzo Giuseppe Candioti, 26 Jul 2024
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Review on „Importance of basement faulting and salt decoupling for the structural evolution of the Fars Arc, Zagros fold-and-thrust belt: A numerical modeling approach“
I am not an expert in the regional geology of the Zagros fold-and-thrust belt, which is why I can only comment on the modeling aspects of the study.
General comments:
The authors present 2D thermo-mechanical (one-way coupled) numerical models of tectonic extension and subsequent compression applied to the Zagros fold-and-thrust belt. The model features pre-existing basement faults and a salt layer that acts as décollement horizon. Varied model parameters include (1) the thickness and rheology of the salt layer, (2) the geometry of pre-existing basement faults, and (3) the horizontal velocity of the basement during convergence. Generally, the study is well written and logically organized. However, I think the current version of the manuscript needs further editing. In particular, (1) some aspects could be discussed in more detail and (2) the research question and main contribution of the study could be presented in a clearer way.
Introduction: The main insights from previous studies on thin- and thick-skinned tectonics are summarized and introduced well. In addition, the work of Kiss et al. 2020, Spitz et al. 2020, and Humair et al. 2020 could be introduced here as well. They also presented 2D and 3D geodynamic models that highlight the importance of tectonic inheritance for the evolution of fold-and-thrust belts. I would like to read about the observations that all these previous models did not capture. This would help putting this study into perspective.
Geological setting: Not being an expert in the regional geology of the Zagros, it seems to me that a lot of research has been conducted on the study area. What could be more focussed on is what exactly remains poorly understood about the geological evolution of the Fars Arc. Also, the particular geologic problem this study is addressing should be clearly outlined in an additional paragraph, and how the presented models will help gaining new insights.
Results: I think it would help the reader to interpret the results, if the figures were larger. In the current state, I find it challenging to identify all the important details the models seem to predict. It might also be interesting to see stress, temperature, or viscosity fields for at least the reference model. This would make it a lot easier to identify rheological boundaries and structures.
Discussion: Agreements between the presented results and previous studies are discussed well. This suggests that the models are capable of making some realistic predictions for the evolution of the Fars Arc. I think the discussion would benefit from highlighting the advantages of the presented models compared to previous models and how they help gaining new insights into the evolution of fold-and-thrust belts in general. For example, one novel aspect of the models presented here seems to be that rifting is modeled prior to collision. I would like to read some discussion on that particular model feature. Why is it important and what advantages does it bring compared to models that only focus on collision?
Specific comments:
lines 71-76: Seems like more recent studies favor the latter hypothesis. Are both hypotheses still equally supported by all the data collected so far?
lines 80-82: It would help the reader to better understand the general relevance and importance of this study, if it was clearly stated what is poorly understood and why it is crucial to close this knowledge gap, here. Especially, since the previous paragraph outlined a certain degree of agreement in the community on the role of the décollement layer and the basement faults in that region.
line 181: Is the material density assumed to be constant?
line 185: Why are contributions from viscous dissipation and radiogenic heat excluded? Are material conductivity and heat capacity constants? Their values do not seem to be provided.
line 194: typo -> visco-elasto-plastic/brittle
line 196-199: The rotational and advection terms are missing in the objective derivative. How is rigid body rotation included in the model? This is quite crucial for folding simulations (see Schmalholz et al. 2001).
line 200: Where does the prefactor 0.5 come from? Is this an additional weakening factor? I suggest calling this viscosity eta_dis or eta_visc, for clarity. Also, shouldn’t there be a factor that accounts for the conversion of the experimental 1D flow law to a flow law for stress tensor components?
line 206: Is delta_t_e the Maxwell relaxation time? Why is it set to 1000 yrs? Why isn’t the physical time step used here?
lines 207-209: Z should be dimensionless, but it is not. Where does this formulation come from and why is it used?
line 212: There are brackets missing in the equation. Eta_num does not seem to be used in any other equation noted here. Where is it used in the algorithm?
lines 218-219: Sigma_xx and Sigma_xy have not been introduced before, are those components of the total stress tensor? In this formulation it seems that stress tensor components are increased if stresses are below the yield criterion. If this formulation is only valid in case of yielding, maybe using the mathematical notation of cases (curly brace) in the equation would make this formulation clearer.
line 223: This seems non-standard, especially for power-law rheology. Why are calculations not performed on the Eulerian grid?
line 225: Formatting. 10^25 appears as 1025
lines 234 f.: How are basement faults parameterized in the models?
Table 1: What is the underlying assumption for choosing a fluid pressure ratio of 0.4? Values for Cohesion seem to be very low even before softening. What was the motivation to choose such low values and to reduce them further as a function of strain? Why are the sediments parameterized by the same flow law but different densities and friction angles?
lines 243-245: Between the extension and the convergence period in the Zagros there seems to be a 180 Myr period of inactivity which can have an impact, especially on the thermal field and the dynamics in the asthenosphere below, which may in turn have control on the lithospheric deformation. We have shown this in a series of publications (starting with Candioti et al. 2020). As far as I understand, the model switches from extension to convergence instantly. Why is the rifting period included but the passive margin period excluded in the models presented here?
lines 371-375: A figure showing the stress and temperature field of the described models would be helpful to support the line of argumentation here.
line 461-462: This is likely a result of the one-way thermomechanical coupling and the strain weakening. I suspect that this frictional weakening of already weak lithologies promotes immediate material failure under compression. In that case, visco-elastic stresses cannot be build up to significantly high values and then be released when shear zones form. I would also not expect to see this effect in Fig 10e. Instead, this may explain the signal pattern (if vx_b < vx) in Fig. 11: In absences of stress built-up and release, the only signal recorded is crustal thickening. As the belt grows, more and more force is necessary to drive the convergence at constant speed. I suspect that if the lithologies were stronger and shear heating would be considered, stresses would build up to higher values and then drop once a shear zone forms. This might then also be visible in at least Fig. 11 (compare to Fig. 11b in Candioti et al. 2021). How do the values for forces compare to estimates for collision zones in general?
line 491: Missing word „of“
line 497: Missing word „to“
lines 595-596: The diapir in the rifting model is hardly visible. An enlargement or generally larger figures would help identifying the diapirs in the models. A brief discussion about earlier work (e.g., Fernandez & Kaus 2014) would be suitable here.
Figure 12: I have the impression that the models are generally dominated by faulting whereas the reconstruction seems to show more folding dominated deformation. It would be interesting to see a movie that shows the folding and thrusting in one of these models.
line 643-644: The depth of the brittle-ductile transition does not only depend on material parameters and the temperature, but also on strain rate (stress) among other variables. Depending on local conditions, this depth can vary for the same material parameters. It should therefore be generally possible to get similar depths of the brittle-ductile transition for different material parameters at different conditions. Hence, it might not be the best justification for the choice of flow law parameters here.
Figure 13: A description of panel d is missing.
References:
Candioti, L. G., Schmalholz, S. M., & Duretz, T. (2020). Impact of upper mantle convection on lithosphere hyperextension and subsequent horizontally forced subduction initiation. Solid Earth, 11(6), 2327-2357.
Candioti, L. G., Duretz, T., Moulas, E., & Schmalholz, S. M. (2021). Buoyancy versus shear forces in building orogenic wedges. Solid Earth, 12(8), 1749-1775.
Fernandez, N., & Kaus, B. J. (2014). Influence of pre-existing salt diapirs on 3D folding patterns. Tectonophysics, 637, 354-369.
Kiss, D., Duretz, T., & Schmalholz, S. M. (2020). Tectonic inheritance controls nappe detachment, transport and stacking in the Helvetic nappe system, Switzerland: insights from thermomechanical simulations. Solid Earth, 11(2), 287-305.
Humair, F., Bauville, A., Epard, J. L., & Schmalholz, S. M. (2020). Interaction of folding and thrusting during fold-and-thrust-belt evolution: Insights from numerical simulations and application to the Swiss Jura and the Canadian Foothills. Tectonophysics, 789, 228474.
Schmalholz, S. M., Podladchikov, Y. Y., & Schmid, D. W. (2001). A spectral/finite difference method for simulating large deformations of heterogeneous, viscoelastic materials. Geophysical Journal International, 145(1), 199-208.
Spitz, R., Bauville, A., Epard, J. L., Kaus, B. J., Popov, A. A., & Schmalholz, S. M. (2020). Control of 3-D tectonic inheritance on fold-and-thrust belts: insights from 3-D numerical models and application to the Helvetic nappe system. Solid Earth, 11(3), 999-1026.
Citation: https://doi.org/10.5194/egusphere-2024-1123-RC2
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