Critical Coulomb Wedge Research Articles

Episodic evolution of intracontinental orogenic deformation: Insight from the southern Bogda fold-and-thrust belt of the Tianshan Mountains, NW China

The Mesozoic-Cenozoic Tianshan orogenic belt, a key area for studying intracontinental orogenic deformation, experienced multiple uplift episodes due to the far-field effects from a series of collision events along the southern margin of Eurasia. Focusing on the fold-and-thwrust belt at the southern margin of the Bogda Mountain, a branch of the eastern Tianshan orogenic belt, we integrated 2D seismic data, borehole and outcrop data to construct a balanced and sequentially restored cross-section across the central part of this belt. The activities and mechanical evolution of the major faults were analyzed by using the discrete-element modeling (DEM) method, offering new insights into the tectonic transformation of the Turpan-Hami Basin influenced by episodic uplifts of the Bogda Mountain in an intracontinental setting. The studied fold-and-thrust belt can be divided into two deformation systems: (i) the piedmont deformation system, comprising a southward basement thrust system and a thin-skinned thrust system on Jurassic detachment, and (ii) the thrust front belt, exhibiting an en-echelon pattern in the basin center and featured by fault-propagation folds along the lower Jurassic coal unit. Stratigraphic and reported thermochronological analyses pinpoint four key contractional deformation periods: Late Jurassic, Late Cretaceous, Paleocene-Pliocene, and Pleistocene-present. The DEM numerical simulation results indicate a critical conical Coulomb wedge model governed by the episodic activities of the southern Bogda fold-and-thrust belts from the Mesozoic to the Cenozoic. Surface processes (syntectonic erosion/deposition) significantly impacted differential activities between the orogenic belt and the basin's thrust structures. We propose that periodic arid climates in the Tianshan Mountains reduced erosion rates, promoting the expansion of the southern Bogda fold-and-thrust belts toward the basin and the widening of the orogenic belt.

Parametric mechanical analysis of thin- versus thick-skinned tectonics applied to the Jura belt

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-skinned) or not (thin-skinned) is an active 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 force balance and for 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 500 simulations (100 for each cross-section), varying the friction angles of the Triassic décollement and of the lower crust between 1° and 10°, 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 systematically 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-sections exhibit only thin-skinned 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-skinned solutions (types 1, 2 and 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-skinned field structures. Overall, our simulations yield tectonic styles compatible with natural seismicity and GPS data for values of the lower-crust friction angle below 3 to 7°. 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, or of higher cohesion values, show that the numerical outcomes are rather robust. They only slightly modify 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 that are necessary to capture the competition between compressive and strike-slip tectonic modes.

Gravity-driven deepwater fold-and-thrust belts as Critical Coulomb Wedges: Model limitations and the role of friction vs. fluid pressure

Gravity-driven Deepwater Fold-and-thrust Belts (GDFB) are the result of gravitational collapse of the sedimentary pile along the continental shelf. These systems can be described in the framework of the Critical Coulomb Wedge (CCW) mechanics to infer their mechanical properties and hydrological conditions. However, not all GDFB can be considered as critical wedges and their mechanical/hydrological parameters should be critically assessed. GDFB driven by gravity spreading and detached onto brittle overpressured shales are the most suitable systems to be modeled via the original CCW theory. In addition, the self-limiting mechanism of gravitational collapse implies that GDFB can reach the critical state only during limited time intervals (in some cases less than ∼10 Myr), in the presence of large sedimentary loads and shortening rates higher than 1.5–2 mm/yr. We present a compilation of GDFB that most likely represent examples of critical tapers, almost exclusively located downdip of large deltas. We then analyze several analytical solutions to the CCW equation, constrained by laboratory and geophysical data, to discuss the role of material friction and fluid pressures in GDFB. Low friction and moderate fluid overpressure localized in the basal detachment are both essential to explain the observed shape of GDFB. Frictional and overpressure discontinuities between detachment and the overlying wedge are likely to be maintained in GDFB.

Widths of imbricate thrust blocks and the strength of the front of accretionary wedges and fold-and-thrust belts

Besides the large-scale wedge shape itself, the most prominent structural feature of accretionary wedges and fold-and-thrust belts is the common pattern of imbricate thrust faults. This study illuminates the fundamental mechanical processes and material properties controlling the width of the crustal blocks bounded by major thrusts using analytical solutions of stress as well as two-dimensional finite-difference models. The numerical models predict that the initial width w0 of a thrust block is set when that block first forms at the very front of the wedge. The width is found to subsequently decreases approximately in proportion to the mean horizontal strain needed for an ideally triangular-shaped Coulomb wedge with a critical taper. Block width is proportional to the thickness H of the incoming, accreting sediment. A key quantity that influences the normalized initial block width w0/H is the distance L forward of the frontal thrust needed for the net horizontal force from shear on the base of the incoming sediment to balance the net force on the frontal thrust. It is within this distance where stress in the incoming sediment is substantially elevated and thus where the new frontal thrust forms. Results show that L/H and, correspondingly, w0/H increase with increasing sediment friction angle ϕ, cohesive strength C0 and pore-fluid pressure ratio λ, and decrease with increasing basal friction angle ϕb and basal dip β. Normalized width is sensitive to ϕ and relatively insensitive to ϕb and λ. Results for submarine and subaerial wedges follow the same scaling law. The scaling law relates the observables, w0/H and β, to the material properties, ϕ, ϕb, λ, and therefore provides a theoretical relation that can be used independent of, or together with critical Coulomb wedge theory (CWT) to constrain these properties.

Controls on Fore‐Arc Deformation and Stress Switching After the Great 2011 Tohoku‐Oki Earthquake From Discrete Numerical Simulations

AbstractThe large magnitude of the 2011 Mw 9.0 Tohoku‐Oki earthquake, which occurred off the east coast of Japan, was not expected or predicted by any previous studies. One surprising observation was the sudden change in stress state; local earthquakes confirmed a compressional stress state before the main shock, whereas an extensional stress state was evident after the main shock. Using discrete element method modeling, this project attempts to reproduce the stress change after the main shock, and explores the conditions that can cause stress switching both onshore and offshore Tohoku. Our simulations demonstrate that rapid fault weakening can produce stress change and predominant normal‐fault earthquake mechanisms in the upper plate of Tohoku‐Oki. Several specific conditions seem to favor such stress switching; the megathrust fault must have been frictionally strong before the main shock, and comparable values of internal (μinternal) and basal friction (μbasal) are necessary to cause the formation of widespread normal faults within the wedge. Furthermore, dynamic extension during elastic unloading appears to play an important role in accommodating stress changes and wedge deformation in the Tohoku area; these cannot be explained solely based on Critical Coulomb Wedge models, but instead require dynamic unloading processes.

Quantifying frictional variations and erosion in the Mexican fold-thrust belt

The impact of frictional crustal heterogeneities and surface processes on the evolution of the ancient thin-skinned Mexican fold-and-thrust belt (MFTB) is investigated and quantified using numerical finite-element simulations. These simulations explore the evolution, total shortening, and structural styles of deforming orogenic wedges. The models include spatial lateral variations in the internal friction, and the topographic erodibility (K), using the power law incision rule. The MFTB exhibits paleogeographical elements manifested as lateral facies changes across two platforms units and two sedimentary basins that show different degrees of deformation. The observed kinematics of the MFTB requires, based on the parameters considered in this study, a rheologically heterogeneous crust, with at least laterally varying internal friction. Yet only limited syntectonic and post-tectonic erosion are necessary to explain the geometries and internal total shortening constrained by field observations and interpretations. Application of the analytical and homogeneous critical Coulomb wedge (CCW) theory to the MFTB closely predicts the external wedge geometry but fails to predict the observed shortening of the individual litho-tectonic units and structural styles.

Mechanical restoration of gravity instabilities in the Brunei margin, N.W. Borneo

In offshore Brunei, the Baram delta displays Pliocene to recent gravitational deformations on top of the overpressured Setap Shales. We use the limit analysis, implemented in SLAMTec, to constrain their kinematics and propose first-order values of compatible pore pressures. Using the critical Coulomb wedge theory, we confirm that extreme overpressure are needed to account for the gravitational deformations currently observed in the delta. SLAMTec simulations quantify the role of the migration of the deltaic system on the deformations and suggest that this overpressure has existed since the initiation of the gravity-driven system. An inverse analysis is performed to identify the fluid overpressure and sedimentation pattern that best reproduces the forward structural model. This inverse approach validates the proposed kinematics, since we can reproduce quantities describing the structural style with overpressure coherent with the present-day observations. It points out the existence of key events explaining the structural interpretations, which leads to propose new kinematic models enriched with compatible pore pressure and sedimentation rates. This mechanical study highlights how mechanical modelling helps design a cross-section with an appropriate kinematics based on mechanical solutions and discuss on the basis of objective mechanical criteria the ranges of pore pressure consistent with a prescribed structural evolution.

Slope stability and exact solutions for cohesive critical Coulomb wedges from Mohr diagrams

This paper deals with questions of the stability and structural characteristics of submarine sedimentary slopes and accreting wedges and with a theoretical treatment, based on the assumption of a Rankine limit equilibrium state of (effective) stress in a rigid-plastic Coulomb material. For plane states of stress in uniformly sloping layers, this allows one to extract stability criteria in terms of generalized earth pressure coefficients, as well as exact cohesive critical taper solutions directly from a Mohr diagram. While an approximate solution of Dahlen et al. (1984), for small taper angles, demands an upward convex surface, cohesive Rankine state solutions require curved detachments, in agreement with curved slip lines, in the shallower part of a cohesive Coulomb-plastic slope. It is further reiterated that in such materials water loading will not affect the stability of normally pressured slopes and that solutions for subaerial (dry) and submarine (normally pressured) non-cohesive critical tapers are identical, with obvious implications for scaled physical (sand box) model experiments.

Mechanical Properties of the Sevier Desert Detachment: An Application of Critical Coulomb Wedge Theory

AbstractLow‐angle normal faults are widely regarded as playing an important role in crustal extension. Among the most influential examples, the Sevier Desert detachment (SDD) has been imaged in seismic reflection profiles beneath the Sevier Desert basin of west‐central Utah. An extensional offset of as much as 47 km is thought to have occurred since the late Oligocene at or near its present dip of 11°. We use the palinspastic geometry of the preextensional Sevier orogen and critical Coulomb wedge (CCW) theory to constrain the friction coefficient, μD of the inferred detachment. It is assumed that the SDD is at least in part a reactivated strand of the Pavant thrust system. Compressive CCW theory suggests μD values of 0.2–0.24 for thrust faults when crustal shortening ceased in the early Paleocene. A decrease in the dip of the SDD from a maximum of 30° to its present‐day inclination of 11° and extensional CCW theory implies that μD decreased from 0.2–0.24 initially to 0.13 today. Salt, for which the normal‐stress dependence of the friction equation disappears with even shallow burial, is widespread in the Sevier Desert basin and presents locally also in Jurassic strata of central Utah. Elevated pore fluid pressure may have played a role in reducing the effective normal stress. Our model can be tested directly by coring across the detachment and by measuring material properties and the pore fluid pressure in the vicinity of the contact.

Orogen-parallel variation in exhumation and its influence on critical taper evolution: The case of the Emilia-Romagna Apennine (Italy)

The Northern Apennine prowedge exposes two adjacent sectors showing a marked along-strike change in erosion intensity, namely the Emilia Apennine to the northwest and the Romagna Apennine to the southeast. This setting has resulted from Pliocene erosion (≤5 Ma) and exhumation, which have affected the whole Romagna sector and mostly the watershed ridge in Emilia. Such an evolution has conceivably influenced the equilibrium of this fold-and-thrust belt, which can be evaluated in terms of critical Coulomb wedge theory. The present state of the thrust wedge has been assessed by crosschecking wedge tapers measured along transverse profiles with fluid pressure values inferred from deep wellbores. The interpretation of available data suggests that both Emilia and Romagna are currently overcritical. This condition is compatible with the presence in both sectors of active NE-dipping normal faults, which would work to decrease the surface slope of the orogenic wedge. However, the presence of Late Miocene-Pliocene passive-roof and out-of-sequence thrusts in Romagna may reveal a past undercritical wedge state ensuing during the regional erosion phase, thereby implying that the current overcritical condition would be a recent feature. The setting of the Emilia Apennine (i.e., strong axial exhumation and limited erosion of the prowedge) suggests instead a long lasting overcritical wedge, which was probably contemporaneous with the Pliocene undercritical wedge in Romagna. The reasons for this evolution are still unclear, although they may be linked to lithosphere-scale processes that have promoted the uplift of Romagna relative to Emilia. The lessons from the Northern Apennine thus suggest that erosion and exhumation have the ability to produce marked along-strike changes in the equilibrium of a fold-and-thrust belt.

Heterogeneous Coulomb wedges: Influence of fluid pressure, porosity, and application to the Hikurangi subduction margin, New Zealand

AbstractDespite the widespread use of critical Coulomb wedge models, the effects of internal heterogeneities have been largely overlooked. Here we consider how spatial variations in pore pressure and porosity within an accretionary prism affect predictions of shear traction along the base of the prism and the internal slip surface geometry. Compared to Coulomb wedge models that use depth‐averaged parameter values, the basal shear traction will be lower if (i) the pore pressure ratio increases monotonically with depth, (ii) the porosity decreases monotonically with depth, or (iii) the internal friction coefficient decreases monotonically with depth. The first two conditions can be considered first‐order descriptions of many natural settings. To illustrate their effects, we consider two specific functional forms: a linear increase in the pore pressure ratio and an exponential decrease in porosity with depth within the prism. Both cases separately can be completely accounted for by simple modifications to the solution for a homogeneous Coulomb wedge. We show that including heterogeneity in the pore pressure ratio and the porosity, together or separately, can lead to large reductions in the basal shear traction, compared to wedges with relevant depth‐averaged parameters. Finally, we apply our results to the Hikurangi margin at the North Island of New Zealand and show that lateral increases in the pore pressure ratio lead to further reductions in basal shear traction.

Deformation pattern during normal faulting: A sequential limit analysis

AbstractWe model in 2‐D the formation and development of half‐graben faults above a low‐angle normal detachment fault. The model, based on a “sequential limit analysis” accounting for mechanical equilibrium and energy dissipation, simulates the incremental deformation of a frictional, cohesive, and fluid‐saturated rock wedge above the detachment. Two modes of deformation, gravitational collapse and tectonic collapse, are revealed which compare well with the results of the critical Coulomb wedge theory. We additionally show that the fault and the axial surface of the half‐graben rotate as topographic subsidence increases. This progressive rotation makes some of the footwall material being sheared and entering into the hanging wall, creating a specific region called foot‐to‐hanging wall (FHW). The model allows introducing additional effects, such as weakening of the faults once they have slipped and sedimentation in their hanging wall. These processes are shown to control the size of the FHW region and the number of fault‐bounded blocks it eventually contains. Fault weakening tends to make fault rotation more discontinuous and this results in the FHW zone containing multiple blocks of intact material separated by faults. By compensating the topographic subsidence of the half‐graben, sedimentation tends to slow the fault rotation and this results in the reduction of the size of the FHW zone and of its number of fault‐bounded blocks. We apply the new approach to reproduce the faults observed along a seismic line in the Southern Jeanne d'Arc Basin, Grand Banks, offshore Newfoundland. There, a single block exists in the hanging wall of the principal fault. The model explains well this situation provided that a slow sedimentation rate in the Lower Jurassic is proposed followed by an increasing rate over time as the main detachment fault was growing.

Application of the critical Coulomb wedge theory to hyper-extended, magma-poor rifted margins

The Critical Coulomb Wedge Theory (CCWT) has been extensively used in compressional tectonics to resolve the shape of orogenic or accretionary prisms, while it is less applied to extensional and gravitational wedges despite the fact that it can be described by the same equation. In particular, the hyper-extended domain at magma-poor rifted margins, forming the oceanward termination of extended continental crust, satisfies the three main requirements of the CCWT: 1) it presents a wedge shape, 2) the rocks forming the wedge are completely brittle (frictional), and 3) the base of the wedge corresponds to a low friction decollement. However hyper-extended margins present a fully frictional behaviour only for a very thin crust; therefore this study is limited to the termination of hyper-extended continental crust which deforms in the latest stage of continental rifting. In this paper we define a method to measure the surface slope and the basal deep of this wedge that we apply to 17 hyper-extended, magma-poor rifted margins in order to compare the results to the values predicted by the CCWT. Because conjugate pairs of hyper-extended, magma-poor rifted margins are commonly asymmetric, due to detachment faulting, the wedges in the upper and lower plate margins corresponding respectively to the hanging wall and footwall of the detachment system are different. While the stress field in the upper plate wedge corresponds to a tectonic extensional wedge, the one in the lower plate matches that of a gravity extensional wedge. Using typical frictional properties of phyllosilicates (e.g. clays and serpentine), the shape of the hyper-extended wedges can be resolved by the CCWT using consistent fluid overpressures. Our results show that all lower plate margins are gravitationally stable and therefore have a close to critical shape whereas the tectonic extensional wedges at upper plate margins are critical, sub or sup critical due to the detachment initial angle and the duration of the tectonic activity. In this paper we discuss the geometry and structural evolution of the most distal parts of hyper-extended continental margins and the formation of extensional allochthons during hyper-extension using the CCWT theory.

Tectonic and gravity extensional collapses in overpressured cohesive and frictional wedges

AbstractTwo modes of extensional collapse in a cohesive and frictional wedge of arbitrary topography, finite extent, and resting on an inclined weak dcollement are examined by analytical means. The first mode consists of the gravitational collapse by the action of a half‐graben, rooting on the dcollement and pushing seaward the frontal part of the wedge. The second mode results from the tectonics extension at the back wall with a similar half‐graben kinematics and the landward sliding of the rear part of the wedge. The predictions of the maximum strength theorem, equivalent to the kinematic approach of limit analysis and based on these two collapse mechanisms, not only match exactly the solutions of the critical Coulomb wedge theory, once properly amended, but generalizes them in several aspects: wedge of finite size, composed of cohesive material and of arbitrary topography. This generalization is advantageous to progress in our understanding of many laboratory experiments and field cases. For example, it is claimed from analytical results validated by experiments that the stability transition for a cohesive, triangular wedge occurs with the activation of the maximum length of the dcollement. It is shown that the details of the topography, for the particular example of the Mejillones peninsula (North Chile) is, however, responsible for the selection of a short length‐scale, dynamic instability corresponding to a frontal gravitational instability. A reasonable amount of cohesion is sufficient for the pressures proposed in the literature to correspond to a stability transition and not with a dynamically unstable state.

The Coulomb critical taper theory applied to gravitational instabilities

AbstractThe critical Coulomb wedge theory has been widely applied to the structural evolution of accretionary wedges and fold and thrust belts, but it also predicts the spreading of a wedge under gravity. This solution may be applied to the formation of gravitational spreading and gliding along passive margins, where elevated pore fluid pressure is common. Following the initial hypotheses of the theory, we provide an alternative expression of the exact solution for a noncohesive wedge, better suited to slope instabilities. Our formulation allows a direct calculation of the fluid pressure required for the system to deform and predicts two kinds of gravitational deformation: shallow slumping and/or deep gravitational spreading rooting on the basal detachment. To verify the predictions of the model, we performed scaled experiments with pore pressure under conditions close to the critical taper hypotheses. The good agreement between our experimental results and the theory confirms the applicability of the critical taper model to the formation of gravitational structures in sedimentary wedges subjected to pore fluid pressure.

Predicting orogenic wedge styles as a function of analogue erosion law and material softening

The evolution of a compressive frictional wedge on a weak, frictional and planar décollement, subjected to frontal accretion, is predicted with a two step method called sequential limit analysis. The first step consists in finding, with the kinematic approach of limit analysis, the length of the active décollement and the dips of the emerging ramp and of the conjugate shear plane composing the emerging thrust fold. The second step leads to a modification of the geometry, first, because of the thrust fold development due to compression and, second, because of erosion. Erosion consists in removing periodically any material above a fictitious line at a selected slope, as done in analogue experiments. This application of sequential limit analysis generalizes the critical Coulomb wedge theory since it follows the internal deformation development. With constant frictional properties, the deformation is mostly diffuse, a succession of thrust folds being activated so that the topographic slope reaches exactly the theoretical, critical value. Frictional weakening on the ramps results in a deformation style composed of thrust sheets and horses. Applying an erosion slope at the critical topographic value leads to exhumation in the frontal, central, or rear region of the wedge depending on the erosion period and the weakening. Erosion at slopes slightly above or below the critical value results in exhumation toward the foreland or the hinterland, respectively, regardless of the erosion period. Exhumation is associated with duplexes, imbricate fans, antiformal stacks, and major backthrusting. Comparisons with sandbox experiments confirm that the thickness, dips, vergence, and exhumation of thrust sheets can be reproduced with friction and erosion parameters within realistic ranges of values.

Low friction along the high slip patch of the 2011 Mw 9.0 Tohoku‐Oki earthquake required from the wedge structure and extensional splay faults

We analyze the mechanical properties needed to account for the large shallow slip during the 2011 Tohoku‐Oki earthquake and the activation of landward normal faulting within the forearc. We show that the morphology and internal structure of the forearc follows closely the prediction of the critical Coulomb wedge in horizontal compression, implying a high internal pore pressure ratio (λ=0.7+0.14/−0.48) and a low effective basal friction (). We then show that the activation of the normal fault requires a lower effective basal friction beneath the outer wedge than beneath the inner wedge (μouter≤0.015), possibly due to transient dynamic weakening associated to the seismic rupture. Forearc normal faults could be considered as evidence for very efficient dynamic weakening along the megathrust and typify megathrust with high tsunamigenic potential.

Deterministic chaos in frictional wedges revealed by convergence analysis

SUMMARYA triangular wedge, composed of a frictional material such as sand, and accreting additional material at its front, is the classical prototype for accretionary wedges and fold‐and‐thrust belts. A simplified method is proposed to capture the internal deformation of this structure resulting from a large number of faulting events during compression. The method combines the application of the kinematic approach of limit analysis to predict the optimum thrust‐fold and a set of geometrical rules to update the geometry accordingly, at each increment of shortening. It is shown that the structure topography remains approximately planar with a slope predicted by the critical Coulomb wedge theory. Failure by faulting occurs anywhere within the wedge at criticality, and its exact position is sensitive to topographic perturbations resulting from the deformation history. The convergence analysis in terms of the shortening increments and of the topography discretization reveals that the timing and the position of a single faulting event cannot be predicted. The convergence is achieved nevertheless in terms of the statistics of the distribution of the faulting events throughout the structure and during the entire deformation history. It is these two convergence properties that are presented to justify the claim that these compressed frictional wedges are imperfection sensitive, chaotic systems. Copyright © 2013 John Wiley & Sons, Ltd.

Wedge geometry, mechanical strength, and interseismic coupling of the Hikurangi subduction thrust, New Zealand

The Hikurangi subduction thrust interface, east coast North Island, New Zealand, exhibits along-strike variations in seismic style: the southern segment is interseismically locked, while the plate boundary thrust slips aseismically in the north. The geometry of the offshore accretionary prism is also heterogeneous, changing from small to large taper angle (fault dip plus surface slope) from south to north. Along-strike variations in accretionary prism geometry generally reflect changes in Coulomb wedge critical taper angle. Such variations are controlled by the relative strengths of the subduction megathrust and the wedge material. In the southern Hikurangi margin, the small taper angle indicates a relatively weak subduction thrust interface where strong interseismic locking is inferred. In the northern margin, high taper angle reflects a strong shallow décollement relative to the south. Thus, based on critical Coulomb wedge theory, the shallow megathrust appears relatively weak in the locked segment, and stronger in weakly coupled regions.

Erosional control of the kinematics of the Aconcagua fold-and-thrust belt from numerical simulations and physical experiments

We investigate the impact of erosion on the geometry and kinematics of the Aconcagua fold-and-thrust belt in central Argentina using analog and numerical models. In these models, the surfaces are eroded according to a rule in which mass removal is limited by the rate of fluvial bedrock incision. This approach unifies principles of frictional failure used in critical Coulomb wedge theory with a quasi-mechanistic erosion rule, which allows us to explicitly relate temporal changes in erosional efficiency in this fold-and-thrust belt to its kinematics. We show that theoretical predictions of the fold-and-thrust belt geometry, as well as the kinematics predicted by both physical and numerical experiments, are both internally consistent and correctly predict the interpreted and measured field geometries. Specifically, the geometric evolution of the Aconcagua fold-and-thrust belt requires relatively high erosion efficiency values ( K ) during the initial stage of deformation, and relatively low K values during the latter stages, consistent with the progressive exposure of different rock types during the different stages of deformation. Model results indicate that the activity of the faults in the hinterland is high when erosion is most efficient during the initial stage of deformation; this activity is facilitated by increased out-of-sequence thrusting. In contrast, the models predict that forward-propagating thrusts dominate the latter stages of deformation when erosion is far less efficient. Comparing the geometry of models subjected to erosion with those in which the surface remains uneroded suggests that erosional mass removal is required to explain the geometry of this fold-and-thrust belt. By implication, the results indicate that the kinematics of this, and perhaps other fold-and-thrust belts, may be intimately tied to the temporal history of erosion of the surface topography of these features.