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(a to c) The sequence of thrusting ending with the onset of the first out of sequence event, for different values of the friction angles f Rf and (d and e) two values of f D . The surface slope a is 4.5° and the décollement dips at b = 3°. The vertical and the horizontal distances are at the same scale.
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The main objective is to determine the three stages of the life of a thrust in an accretionary wedge which are the onset of thrusting along its ramp, the development with the construction of the relief, and the arrest because of the onset of another thrusting event. A simple kinematics is proposed for the geometry of the developing thrust fold base...
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Detailed studies made at the Tordillo and Blanco streams, in the Malargüe fold-thrust belt, allowed us to recognize a broad variety ot tectonic structures affecting both, basement and cover rocks. Using surface structural data together with sub-surface information we made a retrodeformable structural cross-section of the outcropping structures and...
Detailed studies made at the Tordillo and Blanco streams, in the Malargüe fold-thrust belt, allowed us to recognize a broad variety of tectonic structures affecting both, basement and cover rocks. Using surface structural data together with sub-surface information we made a retrodeformable structural cross-section of the outcropping structures and...
Citations
... Previous modeling studies of accretionary wedges have employed physical analog modeling (e.g., Bigi et al., 2010;Ghosh et al., 2020;Gutscher et al., 1996;Koyi, 1995;Lohrmann et al., 2003;Mulugeta & Koyi, 1992;Roy et al., 2020;Saha et al., 2016), work optimization or mechanical approaches to predict the wedge geometry and faulting (Burbidge & Braun, 2002;Cooke & Madden, 2014;Cubas et al., 2008;Del Castello & Cooke, 2007;Gutscher et al., 1998a;Hardy et al., 1998;Leroy & Maillot, 2016;Maillot & Koyi, 2006;Mary et al., 2013;McBeck et al., 2017;Platt, 1986Platt, , 1988Souloumiac et al., 2009Souloumiac et al., , 2010Yagupsky et al., 2014), and numerical simulations (del Castillo et al., 2021;Ito & Moore, 2021;Naylor & Sinclair, 2007;Ruh et al., 2012;Ruh, 2020;Simpson, 2010Simpson, , 2011Stockmal et al., 2007). Early analog modeling studies revealed that the accretion process is episodic rather than steady state (Mulugeta & Koyi, 1992), meaning the wedge does not maintain its original (Mantilla-Pimiento et al., 2009;Rodríguez et al., 2021). ...
... Repetition of episodic frontal accretion allows an accretionary wedge to maintain a critical taper angle (Del Castello & Cooke, 2007;Gutscher et al., 1996;Hoth et al., 2007;Stockmal et al., 2007;Storti et al., 2000). The nature of each accretion cycle has been discussed from the viewpoint of mechanical equilibrium or the work budget (e.g., Cubas et al., 2008;Del Castello & Cooke, 2007;Gutscher et al., 1998a;Herbert et al., 2015;McBeck et al., 2017;Platt, 1988). We now propose another simple conceptual model for an accretion cycle involving Figure S5 in Supporting Information S1). ...
... At the critical point in terms of the force balance or work optimization (cf., Cubas et al., 2008;Del Castello & Cooke, 2007;Gutscher et al., 1998a;McBeck et al., 2017), a new frontal thrust initiates ahead of the deformation front as multiple pre-existing faults in the wedge cease to be active (phase 2 in Figure 10b). This may be related to the frictional resistance of the basal detachment becoming weaker than the internal wedge strength as the internal wedge hardens, which leads to failure of the basal coupling at the plate interface and creep of the wedge on the basal detachment. ...
The architecture (geometry, fault network, and stacking pattern of accreted thrust sheets) of accretionary wedges influences subduction zone processes. However, it remains challenging to constrain the architectural evolution in natural accretionary wedges over geological timescales. In this study, we undertook sandbox analog modeling, with quantitative analysis of the wedge geometry and digital image correlation‐based kinematics, to delineate the wedge growth history with four décollement settings (single or double and continuous or discontinuous). The results show that the wedge is formed by repeated episodic frontal accretion with a constant cycle (i.e., the accretion cycle), and the degree of coupling between the base of the wedge and subducting plate interface appears to depend on the relative strengths of the wedge and detachment. An interbedded décollement layer in the incoming sediment facilitated wedge segmentation and rearrangement of the internal fault network, which weakened the wedge strength. A combination of a detachable high‐friction patch in the basal décollement and a continuous interbedded weak layer enabled underplating of underthrusted sediment beneath the inner wedge, which involved a low‐angle, long‐lived forethrust and multiple cycles of frontal accretion on short‐lived forethrusts at the deformation front. Our findings suggest that décollement configuration is a key factor in controlling the accretion cycle, strain distribution, fault network, and wedge strength on timescales of ∼10⁵ yr in natural accretionary systems. This result should be considered when investigating modern subduction zones.
... Despite its simplicity, LA remains a highly robust approach that can handle complex geometries and loading conditions. Its use in the geological field was strongly highlighted by Maillot and Leroy 36 to assess the development of folds, then extended by Cubas et al. 37 , verified by Souloumiac et al. 38 and later implemented in the SLAMTec software for sequential developments. 27,39 Afterwards, this same concept was adopted by Caër et al. 40 in a 2D parametric study over the Jura region. ...
We develop automated methods for fault detection utilizing static stress and deformation fields at the onset of failure derived from numerical analysis. We calculate combinations and normalization of the distance from the Mohr circle to the Coulomb envelope, and of the second deviatoric strain invariant. A variation of the Cauchy distribution of these fields allows us to focus on the low values indicating rupture, with the help of the scale parameter δ. A threshold is then applied to decide at each spatial node of the mesh whether the material has reached failure or not. We then determine fault lines and planes from these isolated failure zones using image processing techniques, such as the Hough and the Radon transforms, or through a combined approach involving automated sorting of the nodes reaching failure through the k-means clustering technique followed by polynomial fitting to retrieve analytic expressions of the fault curves (in 2D) or fault surfaces (in 3D). The methods are efficient except when the stress field results in diffuse rupture zones that do not localize onto fault surfaces despite tuning δ. We also highlight the advantages of using the combined clustering/poly-fitting approach for 3D models compared to the image processing techniques. These automated fault detection methods should be useful in the interpretation of diverse failure mechanisms obtained through parametric sensitivity analyses requiring hundreds of simulations. The stress and strain fields used were derived from a numerical implementation of limit analysis, but classical finite-difference or finite-element techniques could have been used.
... This study gives a mechanical point of view to the above geological interpretation, making use of the theory of Limit Analysis. This theory oers a natural generalisation of Dahlen's critical Coulomb wedge theory (Dahlen , 1984), to predict the geometry of failure in more complex prototypes including rheological heterogeneities, inherited faults and surface processes (Maillot and Leroy, 2006;Cubas et al., 2008;Souloumiac, 2009;Mary, 2012;Mary et al., 2013a,b). It is a simple approach limited to frictional behaviours and, in its current implementation, limited to the onset of plastic deformation. ...
... Previous uses of this method for kink folding (Maillot and Leroy, 2006) and for frontal accretion (Cubas et al., 2008;Mary, 2012;Mary et al., 2013a,b) describe all details of semi-analytical implementations. Here, we only give the general equations and present briey the numerical implementation. ...
... This observation remains for all the range of possible cohesion values (gure 9). These two remarks suggest the existence of a critical state much in the sense of the CCW theory corresponding to a narrow range in various parameters of the prototypes where several, distinct tectonic solutions may occur (Cubas et al., 2008;Mary et al., 2013a). The stable state corresponds to the tectonic type 1, where deformation goes beyond the Jura front, and the unstable states to cases 2, 3, 4, and 5 (Fig. 3). ...
Field observations and seismic interpretations testify that the front of the Jura fold-and-thrust belt is still submitted to compressive deformation, but whether the basement is deforming (thick-skin) or not (thin-skin) remains an open question. We propose a mechanical point of view using the Kinematic approach of the Limit Analysis theory (KLA). We first draw cross-sections containing a major shallow décollement level in the triassic evaporites, including the Alps up to the topographic maximum and including the whole crust. We submit the cross-sections to a compressive force at their southern end, and the KLA determines the location and geometry of the incipient ruptures by optimisation of the associated compressive force, accounting for mechanical balance and the rock strength (Coulomb criterion). Five cross-sections span the whole Jura from west to East, allowing us to explore the lateral variations. From the analysis of five hundred simulations (one hundred for each cross section), varying the friction angles on the Triassic décollement and of the lower crust between 1 deg and 10 deg, we have identified five types of tectonics at the Jura front depending on the emergence of a basement thrust beyond the Jura front (type 1), at the Jura front (type 2) with simultaneous activation of the shallow décollement (type 3), or south of the Jura front (type 5), with activation of the shallow décollement at the Jura front (type 4). The analysis allows us to draw two conclusions. First, the transitions between the various tectonic styles occur abruptly upon continuous changes in the friction parameters, revealing a threshold behaviour that we interpret as an extension of the concept of wedge criticality in the Critical Coulomb Wedge theory: at criticality, several tectonic types may occur within a narrow, critical, range of parameter values. Second, the critical range evolves sytematically between cross-sections, in such a way that the front of the thick-skinned deformation crosses laterally the Jura belt. The two most western cross-section exhibit only thin skin or no tectonics at the Jura front (types 1, 4 and 5), the central one hosts all five styles, and the two Eastern ones show thick-skin solutions (1, 2, 3), for all values tested. We also show that a thick-skinned tectonic style can be accompanied by a simultaneous activation of the shallow triassic décollement (type 4), complicating the interpretation of apparent thin-skin field structures. Modifications of our cross-sections to explore the effect of a bumpy upper/lower crust interface, or of a major décollement at the upper/lower crust interface show the robustness of our conclusions. They only modify slightly the critical ranges at which the tectonic changes occur. These findings may serve as guides, or first order questions, for more sophisticated mechanical approaches including temperature and rate dependent rheologies and the three dimensions of space.
... The evolution and distribution of long-term internal deformation of forearc wedges has been intensively studied by numerical models in two (Burbidge and Braun, 2002;Strayer et al., 2001;Buiter et al., 2016;Stockmal et al., 2007;Miyakawa et al., 2010;Simpson, 2011;Ruh et al., 2012;Ruh, 2020) and three dimensions (Braun and Yamato, 2010;Ruh et al., 2013;Ruh, 2016) complementing analogue models (see Graveleau et al., 2012, for a review). These models have covered the influence of many parameters such as the geometry (e.g., Dahlen et al., 1984;Davis et al., 1983;Smit et al., 2003;Ruh et al., 2016), basal friction (e.g., Colletta et al., 1991;Nieuwland et al., 2000;Burbidge and Braun, 2002;Ruh et al., 2012;Cubas et al., 2008), surface processes (e.g., Storti and McClay, 1995;Mary et al., 2013;Willett, 1999;Bonnet et al., 2007;Fillon et al., 2013;Simpson, 2006), and the presence of viscous material along the décollement (e.g., Costa and Vendeville, 2002;Smit et al., 2003;Pichot and Nalpas, 2009;Simpson et al., 2010;Ruh et al., 2012;Borderie et al., 2018). Despite the large number of published studies, none of them included the dependence of effective basal friction on temperature due to metamorphic reaction or brittle-ductile transition. ...
The local topographic slope of the accretionary prism is often used together with the critical taper theory to determine the effective friction on subduction megath-rust. In this context, extremely small topographic slopes associated with extremely low effective basal friction (µ ≤ 0.05) can be interpreted either as seismically locked portions of megathrust, which deforms episodically at dynamic slip rates or as a viscously creeping décollement. Existing mechanical models of the long-term evolution of accre-tionary prism, sandbox models, and numerical simulations alike, generally do not account for heat conservation nor for temperature-dependent rheological transitions. Here, we solve for advection-diffusion of heat with imposed constant heat flow at the base of the model domain. This allows the temperature to increase with burial and therefore to capture how the brittle-ductile transition and dehydration reactions within the décollement affect the dynamic of the accretionary prism and its topography. We investigate the effect of basal heat flow, shear heating, thermal blanketing by sediments, and the thickness of the incoming sediments. We find that while reduction of the friction during dewatering reactions results as expected in a flat segment often in the forearc, the brittle-ductile transition results unexpectedly in a local increase of topographic slope by decreasing internal friction. We show that this counterintuitive backproduct of the numerical simulation can be explained by the onset of internal ductile deformation in between the active thrusts. Our models , therefore, imply significant viscous deformation of sediments above a brittle décollement, at geological rates, and we discuss its consequences in terms of interpretation of coupling ratios at subduction megathrust. We also find that, with increasing burial and ductile deformation, the internal brittle deformation tends to be accommodated by backthrusts until the basal temperature becomes sufficient to form a vis-cous channel, parallel to the décollement, which serves as the root to a major splay fault and its backthrust and delimits a region with a small topographic slope. Morphologic resemblances of the brittle-ductile and ductile segments with fore-arc high and forearc basins of accretionary active margins, respectively, allow us to propose an alternative metamorphic origin of the forearc crust in this context.
... In this study, we refer to previous works on the fold and thrust belts, particularly 1) Coulomb critical wedge theory (Chapple, 1978;Davis et al., 1983;Dahlen and Suppe, 1984;Dahlen, 1990;Yin, 1993Yin, , 1994Wang, 2006, 2008;Wang and Hu, 2006;Cubas et al., 2008;Nassif et al., 2019), 2) sandbox physical simulations (Davis et al., 1983;Dahlen and Suppe, 1984;Storti et al., 2000;Bose et al., 2009;Driehaus et al., 2014;Schreurs et al., 2016), and 3) numerical simulations (Makel and Waleters, 1993;Barnichon and Charlier, 1996;Beaumont et al., 2001;Bose et al., 2009;Hardy et al., 2009;Li et al., 2009;Buiter, 2012;Ruh et al., 2012;Dean et al., 2013;Fillon et al., 2013;Ruh et al., 2013Ruh et al., , 2017Buiter et al., 2016;Granado and Ruh, 2019;Zhang et al., 2019). After this, based on regional and local geological and geophysical observations, a series of systematic, reasonably accurate numerical simulations can, in our understanding, provide an adequate conceptual and mathematical framework to understand the dynamics of these structures in the Jiudong Basin since Late Cenozoic. ...
Under regional tectonic shortening in the northern margin of Qinghai–Tibetan Plateau, the fold-and-thrust belts composed of four thrust faults (North Qilian-Shan, North Yumu-Shan, South Heli-Shan, and North Jintanan-Shan) formed from southwest to northeast discontinuously sequentially in the Jiudong Basin area during Late Cenozoic. Meanwhile, the North Qilian-Shan, Yumu-Shan, and Heli-Shan ranges were formed successively, as the Earth's local surface was unequally uplifted. In this study, based on geological and geophysical observations, a simple two-dimensional elastic-plastic numerical finite element method model for a southwest-northeast section in Jiudong Basin is successfully established to simulate the spatiotemporal evolution of the local fold-and-thrust belts. Results show that the computed equivalent plastic strain concentration zones and the four observed thrust faults are consistently correlated in spatial position orientation and time sequence. The simulated upper-surface deformation is congruent with the observed topographic peaks and uplift sequences of the North Qilian-Shan, Yumu-Shan, and Heli-Shan ranges. This study provides a geodynamic basis for understanding the growth mechanism of the northern margin of Qinghai–Tibetan Plateau under tectonic horizontal shortening. Also, we provide a thorough sensitivity analysis for the model parameters of this particular geologic setting. Our sensitivity simulations, considering systematic case variations about the regional geometrical-material parameters, suggest the manifestation of three different possible evolution patterns of fold-and-thrust belts for a wedge above a decollement layer, with wedge plastic deformation migrating from 1) thick to thin end (well-known), 2) thin to thick end, and 3) both ends to middle. Finally, our results suggest that in this region, further growth of mountain ranges is expected to continue in the future.
... In this study, we refer to previous works on the fold and thrust belts, particularly 1) Coulomb critical wedge theory (Chapple, 1978;Davis et al., 1983;Dahlen and Suppe, 1984;Dahlen, 1990;Yin, 1993Yin, , 1994Wang, 2006, 2008;Wang and Hu, 2006;Cubas et al., 2008;Nassif et al., 2019), 2) sandbox physical simulations (Davis et al., 1983;Dahlen and Suppe, 1984;Storti et al., 2000;Bose et al., 2009;Driehaus et al., 2014;Schreurs et al., 2016), and 3) numerical simulations (Makel and Waleters, 1993;Barnichon and Charlier, 1996;Beaumont et al., 2001;Bose et al., 2009;Hardy et al., 2009;Li et al., 2009;Buiter, 2012;Ruh et al., 2012;Dean et al., 2013;Fillon et al., 2013;Ruh et al., 2013Ruh et al., , 2017Buiter et al., 2016;Granado and Ruh, 2019;Zhang et al., 2019). After this, based on regional and local geological and geophysical observations, a series of systematic, reasonably accurate numerical simulations can, in our understanding, provide an adequate conceptual and mathematical framework to understand the dynamics of these structures in the Jiudong Basin since Late Cenozoic. ...
Under regional tectonic shortening in the northern margin of Qinghai-Tibetan Plateau, the fold-and-thrust belts composed of four thrust faults (North Qilian-Shan, North Yumu-Shan, South Heli-Shan, and North Jintanan-Shan) formed from southwest to northeast discontinuously-sequentially in Jiudong Basin area during Late Cenozoic. Meanwhile the North Qilian-Shan, Yumu-Shan and Heli-Shan ranges were formed successively as the Earth's local surface was unequally uplifted. In this study, based on geological and geophysical observations, a simple two-dimensional elastic-plastic numerical FEM model for a southwest-northeast section in Jiudong Basin is successfully established to simulate the spatio-temporal evolution of the local fold-and-thrust belts. Results show that the computed equivalent-plastic-strain concentration zones and the four observed thrust-faults are consistently-correlated in spatial position-orientation and time sequence. The simulated upper-surface deformation is congruent with the observed topographic-peaks and uplift sequences of North Qilian-Shan, Yumu-Shan, and Heli-Shan ranges. This study provides geodynamic basis for understanding the growth mechanism of the northern margin of Qinghai–Tibetan Plateau under tectonic horizontal shortening. Also, we provide a thorough sensitivity analysis for the model parameters of this particular geological setting. Our sensitivity simulations considering systematic case variations about the regional geometrical-material parameters, suggest the manifestation of three different possible evolution patterns of fold-and-thrust belts for a wedge above a decollement layer; with wedge plastic deformation migrating from: (1) thick to thin end (well-known), (2) thin to thick end, and (3) both ends to middle. Finally, our results suggest that in this region further growth of mountain ranges is expected to continue in the future.
... Ruh et al. 2012;Dal Zilio et al. 2020c), the evolution of the wedge follows closely the prediction of the critical taper theory (Dahlen 1984), with localized deformation on discrete faults and shear bands. In particular, the onset, development, and arrest of splay-faults due to the onset of another splay-fault event is in good agreement with the prediction of thrusting sequences deduced from the limit analysis (Cubas et al. 2008). However, our simulations display an increase of the splay fault dip angle, which occurs at the transition between the seismic and aseismic faulting stage. ...
We present a two-dimensional (2-D) thermo-mechanical computational framework for simulating earthquake sequences in a nonlinear visco-elasto-plastic compressible medium. The method is developed for a plane-strain problem and incorporates an invariant formulation of the classical rate- and state-dependent friction law and an adaptive time-stepping, which allows the time step to vary by many orders of magnitude during a simulation. Long-term tectonic convergence is imposed by displacing a boundary at a constant rate, whereas temperature-dependent viscosity is solved using a rapidly-converging Newton-Raphson scheme. The 2-D volume is discretized using finite differences on a fully staggered grid and marker-in-cell techniques. An adaptive free-surface approximation is used to modulate the air viscosity with the time step, which allows stresses to vanish on the free surface during the propagation of fast slipping events. We present a set of increasingly complex models in which we investigate how inertia, radiation damping, thermally activated nonlinear rheology, and off-megathrust splay-fault events affect sequences of seismic and aseismic slip on a simplified subduction megathrust. The new method provides a unique computational framework to analyze earthquake sequences and to connect forearc deformation with the dynamic properties of the megathrust, thus providing a physical link between observations spanning from slow interseismic strain accumulation to localized coseismic slip of individual earthquakes and postseismic viscoelastic relaxation.
... And here again, the static and efficient approach allows for fast computing and hence inversion. Limit analysis has been applied to retrieve the effective basal friction of several accretionary wedges (Cubas et al., 2013a(Cubas et al., ,b, 2008Kuncoro et al., 2015;Pons et al., 2013;Souloumiac et al., 2009) (example Figure 1.4) and has been quantitatively validated from comparison with analogue experiments (Cubas et al., 2013c). However, the limit analysis, as the CTT, is a static approach. ...
... The evolution and distribution of long-term internal deformation of fore-arc wedges has been intensively studied by numerical models in two (Buiter et al., 2016;Burbidge and Braun, 2002;Miyakawa et al., 2010;Ruh, 2020;Ruh et al., 2012;Simpson, 2011;Stockmal et al., 2007;Strayer et al., 2001) and three dimensions (Braun and Yamato, 2010;Ruh, 2016;Ruh et al., 2013) complementing analogue models (see (Graveleau et al., 2012) for a review). These models have covered the influence of many parameters such as the geometry e.g., Davis et al., 1983;Ruh et al., 2016;Smit et al., 2003), basal friction e.g., (Burbidge and Braun, 2002;Colletta et al., 1991;Cubas et al., 2008;Nieuwland et al., 2000;Ruh et al., 2012), surface processes e.g., Fillon et al., 2013;Mary et al., 2013;Simpson, 2006;Storti and McClay, 1995;Willett, 1999), and the presence of viscous material along the décollement e.g., (Borderie et al., 2018;Costa and Vendeville, 2002;Pichot and Nalpas, 2009;Ruh et al., 2012;Simpson et al., 2010;Smit et al., 2003). Despite the large amount of published studies, none of them included the dependence of effective basal friction on temperature due to metamorphic reaction or brittle-ductile transition. ...
... With smaller friction angles on the décollement the spacing of the thrusts is smaller while their spacing increases with the thickness of the sequence (Fig. A.2) (Cubas et al., 2008;Gutscher et al., 1998). ...
Makran has a largely unconstrained seismogenic potential although GPS data indicate the subduction is accumulating some strain to be released during future earthquakes. I first build a structural map along the Iranian part of the Oman Sea which indicates three segments. Then, I retrieve the pore fluid pressure and the frictional properties of the wedge with the critical taper theory and the limit analysis. The results show that along the eastern and western profiles, a transition from very low to extremely low friction is required to activate the large coastal normal fault. To propagate the deformation to the front, an increase of friction along the imbricated zone is necessary.The Makran Models are calibrated the thermal parameters and boundary conditions of the numerical simulations using seafloor and bottom sea reflector depth and few available well data. Considering two décollements, the results show that, -underplating is associated to viscous deformation -dewatering and smectite/illite transition permit to produce three slope segments observed in accretionary prisms but this friction drop is not sufficient for formation of normal faults. - the subduction of a large seamount is accompanied by large normal faults, while their location migrates through time. If the brittle décollements have a seismogenic behavior, the down-dip limit of the seismogenic zone will correspond to the onset of underplating. By assuming that the up-dip limit of seismic asperities correlates with smectite-illite transition, then a seismic asperity may extend from this transition, down to the onset of underplating, and correlate with a relatively flat topography.
... As shown in previous studies (e.g., Cubas et al., 2008;Ruh et al., 2012), the evolution of the wedge follows closely the prediction of the critical taper theory (Dahlen, 1984) with localized deformation on discrete faults and shear bands. The topographic slope of the resulting wedge thus depends on the dip angle and basal friction of the décollement, and on the strength of the wedge. ...
Orogenic wedges commonly display an inner wedge, where crystalline units have been exhumed, and an outer wedge formed by imbricated sedimentary units detached from the basement. Analog experiments have shown that similar structures can emerge naturally in the presence of weak décollements due to the interplay between erosion and deformation. In this study, we further investigate this hypothesis using two‐dimensional, visco‐elasto‐plastic numerical models. Our experiments assume a basal and an intermediate décollement within the wedge. Experiments with a frictional strength of the basal décollement lower or equal to that of the intermediate décollement show a structural evolution of fold‐and‐thrust belts dominated by out‐of‐sequence thrusting. Conversely, when the intermediate décollement is weaker than the basal décollement, distinct outer and inner wedges are formed. This process leads to episodic migration of midcrustal ramps, tectonic underplating, and antiformal stacking facilitated by erosion. Comparison between our models and the Himalayan wedge suggests a low effective friction (∼0.10), which is probably due to dynamic weakening during large (M8+) Himalayan earthquakes. The deeper décollement, along which the lower plate thrusts beneath the High Himalaya, must be a thermally activated ductile shear zone with an apparent friction of ∼0.18. Fold‐and‐thrust belts worldwide exhibit various architectures in which different décollement levels might be activated. Thus, our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise.
... This velocity field does not need to be the exact velocity field and is thus referred to as virtual. It corresponds to a simplified case used to obtain an analytical solution for Q u (Cubas et al., 2008;Maillot & Leroy, 2006;Pons & Leroy, 2012;Yuan et al., 2015). ...
It is well established that slip on a frictionally weak low‐angle normal fault (LANF) can be more favorable than breaking a steep fault in strong crust. Very few studies, however, have considered the specific effect of crust and fault cohesion on LANF viability. We do so using Limit Analysis, a methodology for predicting the optimal orientation of faults with varying strength subjected to a specific set of boundary conditions. Accounting for crustal cohesion in our models reduces the lowest admissible LANF dip and even allows slip on high‐friction LANFs if the contrast between crust and fault cohesion is large. Fault cohesion, however, increases the lowest admissible LANF dip and introduces a locking depth above which LANF slip is not mechanically feasible. This is consistent with observations of steep splay faults rooting onto LANFs in a variety of settings. We further demonstrate that locking depth can help constrain LANF cohesion, friction, and fluid pressure on the Alto Tiberina (Italy) and western Corinth (Greece) LANFs. Specifically, assuming a measured fault friction of 0.2–0.3, we find that the shallow locking depth of the Alto Tiberina fault requires either (1) moderate fluid overpressure (57% of lithostatic) with cohesion of 8–12 MPa or (2) strong overpressure (77% of lithostatic) with cohesion of 13–20 MPa along the fault. By contrast, the larger locking depth characterizing the western Corinth LANF can reflect greater fault cohesion.
