Strong Lower Crust Research Articles

Numerical Modeling of Earthquake Cycles Based On Navier‐Stokes Equations With Viscoelastic‐Plasticity Rheology

AbstractVisco‐elastic‐plastic modeling approaches for long‐term tectonic deformation assume that co‐seismic fault displacement can be integrated over 1000s–10,000s years (tens of earthquake cycles) with the appropriate failure law, and that short‐timescale fluctuations in the stress field due to individual earthquakes have no effect on long‐term behavior. Models of the earthquake rupture process generally assume that the tectonic (long‐range) stress field or kinematic boundary conditions are steady over the course of multiple earthquake cycles. This study is aimed to fill the gap between long‐term and short‐term deformations by modeling earthquake cycles with the rate‐and‐state frictional (RSF) relationship in Navier‐Stokes equations. We reproduce benchmarks at the earthquake timescale to demonstrate the effectiveness of our approach. We then discuss how these high‐resolution models degrade if the time‐step cannot capture the rupture process accurately and, from this, infer when it is important to consider coupling of the two timescales and the level of accuracy required. To build upon these benchmarks, we undertake a generic study of a thrust fault in the crust with a prescribed geometry. It is found that lower crustal rheology affects the periodic time of characteristic earthquake cycles and the inter‐seismic, free‐surface deformation rate. In particular, the relaxation of the surface of a cratonic region (with a relatively strong lower crust) has a characteristic double‐peaked uplift profile that persists for thousands of years after a major slip event. This pattern might be diagnostic of active faults in cratonic regions.

Dynamic Evolution of Porosity in Lower‐Crustal Faults During the Earthquake Cycle

AbstractEarthquake‐induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake‐derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post‐ and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25–0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid‐assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain‐size sensitive creep. Porosity reduction on the order of >85% may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake‐induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep.

Impact of crustal rheology and inherited mechanical weaknesses on early continental rifting and initial evolution of double graben structural configurations: Insights from 2D numerical models

During the early stages of continental rift, two main grabens are often formed in the upper crust. This double graben structural pattern is typically short-lived, only briefly accommodating rift-related extension. Although the formation and evolution of this structural pattern could be related to the existence of different types of (pre-rift) inherited crustal weaknesses, and possibly determined by their different number and spatial disposition, the process that causes the early nucleation of these grabens is still not fully understood. We hence carry out a set of numerical experiments to investigate the influence of lower crust mechanical weaknesses (weak-seeds) in the early rift nucleation of grabens, while simultaneously assuming different rheological configurations for the extending continental crust. Our results show that double graben structural patterns are generally favoured by crustal rheological configurations comprising a weak middle layer sandwiched between an upper brittle crust and a strong lower crust. In models in which this middle layer is relatively thick, two upper crustal main grabens nucleate above one single seed. In numerical experiments with a thinner middle layer this double graben pattern is observed to form even in the absence of any seeds, illustrating the critical role of the rheological configuration in early-rift double graben nucleation. We argue that different assumed crustal rheologies determine different modes of accommodating brittle deformation in the upper crust, resulting in different structural-mechanical grains imprinted in this layer, prior to the formation of the main grabens, and driving the subsequent nucleation of different structural configurations (including double grabens). The transient nature of the double graben configuration is also confirmed by our results, which consistently show that rift-related extensional strain is eventually fully concentrated in only one of these early formed structures.

3D Geodynamic Models for HP‐UHP Rock Exhumation in Opposite‐Dip Double Subduction‐Collision Systems

AbstractThe dynamics of high pressure (HP) and ultra‐high pressure (UHP) rock exhumation in opposite‐dip double subduction‐collision systems remain enigmatic. Here, we present 3D thermo‐mechanical numerical models to study the geodynamics of continental‐margin subduction, collision, and crustal exhumation of HP‐UHP metamorphic rocks in oppositely dipping adjacent subduction zones. In our model setups, an initial transform‐fault separates a purely oceanic subduction zone from an adjacent oceanic subduction zone that in time transforms into continental‐margin subduction. The results show that most simulations cause continental‐margin deep subduction within the collisional belts. In contrast to the results of 2D and 3D single subduction‐collision systems, the slab‐slab interactions strongly modify the down‐channel Couette flow and the up‐channel Poiseuille flow in the interplate region when the plate edges are close to each other. This prevents the exhumation of UHP metamorphic rocks to the Earth's surface. However, far from the interplate region, rapid exhumation of UHP rocks occurs in Alpine‐type models with a low Moho temperature, strong lower crust, and fast upper‐plate divergence, whereas the Taiwan‐type models occur for a high Moho temperature, weak lower crust, fast convergence, fast erosion, and are likely to facilitate rapid dome‐like HP exhumation. Based on our results, we suggest that the main characteristics, such as deep crustal subduction and rapid exhumation of HP‐UHP metamorphic rocks during mountain building in Taiwan (∼8 Ma to present) and in the Western Alps (∼49–35 Ma), may be the result of opposite‐dip double subduction‐collision systems.

Revisiting the seismogenic characteristics of stable continental interiors: The case of three Indian events

Kinematic source process models using tele-seismic waveform inversion of three stable continental region (SCR) earthquakes from India (1993 Mw 6.2 Latur, 1997 Mw 5.8 Jabalpur and 2001 Mw 7.6 Bhuj) are presented in this paper to demonstrate the slip evolution and stress drop. These finite-fault models are methodologically unique to constrain the source dimensions, compared with assumptions and quasi-observations around blind faults. Our results show that these events do have a compact zone of singular asperity breakage within the Indian crust. Whereas the Bhuj and Jabalpur events have their rupture majorly restricted within the lower crustal regions. The Latur event ruptured a very shallow crustal asperity. The estimated rupture velocities are in the range of 2.6–3.2 km/s, Bhuj event the slowest and lengthiest. Our results do not favour an updip shallow component of asperity breakage for the Bhuj earthquake, as evident from lack of surface ruptures. Study also indicates that the 1993 Latur event occurred on a fault with high stress concentration which indicates stronger fault asperities or slip within a newer fault. Models on 1997 Jabalpur event imply higher frictional strength and very brittle nature at the lower crustal regions of the Indian slab, indicating a mechanically very strong lower crust. We conclude that an event like Latur can occur anywhere within continental interiors of Indian SCR, or elsewhere globally, and is an underestimated seismic hazard.

Transient High Strain Rate During Localized Viscous Creep in the Dry Lower Continental Crust (Lofoten, Norway)

AbstractUnderstanding the ability of the lower crust to support transient changes in stresses and strain rates during the earthquake cycle requires a detailed investigation of the deformation mechanisms and rheology of deep crustal fault rocks. Here, we show that lower crustal pseudotachylyte‐bearing shear zones are able to accommodate short‐term episodes of high strain rate and high stress deformation by accelerated viscous creep, followed by a reduction in stresses to some ambient deformation condition. Quartz microstructure within pseudotachylyte‐bearing shear zones in otherwise undeformed granulites from Lofoten, Norway, indicates that dynamic recrystallization occurred during viscous creep under rapid strain rates and high stresses of ~10−9 s−1 and ~100 MPa, respectively. Lower stress microstructures (i.e., foam textures) are also recorded in the shear zones, indicating spatial and temporal variations of stress and strain rate during deformation cycles. Both the high and lower stress quartz recrystallization took place under granulite facies conditions of 650°C–750°C and 0.7–0.8 GPa and represented a record of highly localized viscous creep within the lower crust. This implies that lower crustal pseudotachylytes are potentially able to form extremely localized weak zones within strong lower crust, enabling a deep mechanical response to perturbations in stress and strain rate such as those experienced during the seismic cycle, for example, seismogenic loading followed by subsequent postseismic relaxation.

Constant strain accumulation rate between major earthquakes on the North Anatolian Fault

Earthquakes are caused by the release of tectonic strain accumulated between events. Recent advances in satellite geodesy mean we can now measure this interseismic strain accumulation with a high degree of accuracy. But it remains unclear how to interpret short-term geodetic observations, measured over decades, when estimating the seismic hazard of faults accumulating strain over centuries. Here, we show that strain accumulation rates calculated from geodetic measurements around a major transform fault are constant for its entire 250-year interseismic period, except in the ~10 years following an earthquake. The shear strain rate history requires a weak fault zone embedded within a strong lower crust with viscosity greater than ~1020 Pa s. The results support the notion that short-term geodetic observations can directly contribute to long-term seismic hazard assessment and suggest that lower-crustal viscosities derived from postseismic studies are not representative of the lower crust at all spatial and temporal scales.

Determining the key conditions for the formation of metamorphic core complexes by geodynamic modeling and insights into the destruction of North China Craton

Abstract Metamorphic core complex (MCC) is characterized by the exhumation of lower crust over a large-scale detachment fault, providing natural records for tectonic extension. MCCs are widely identified in the North China Craton (NCC), which have been intensively studied on their structural and geological characteristics. Yet, the condition for the formation of MCCs and their link with NCC destruction are still in debate. In this study, we perform numerical simulations to investigate MCC formation under extension, with a focus on the effect of crustal rheologies. Results indicate that three end-member modes of deformation may occur: the metamorphic core complex mode, the detachment fault-uplifting mode and the pure shear mode. Weaker lower crust and stronger upper crust may promote the formation of MCC. In contrast, stronger lower crust (>1.3×1021 Pa s) may prohibit the exhumation of lower crust (detachment fault-uplifting mode), while weaker upper crust (<7.8×1021 Pa s) may fail to develop detachment faults (pure shear mode). Given that cratons typically have a strong crust, we suggest that the lower crust of NCC was weakened prior to extension, which promoted the formation of MCC in a later stage under the back-arc extension.

Vertical deformation associated with normal fault systems evolved over coseismic, postseismic, and multiseismic periods

AbstractVertical deformation of extensional provinces varies significantly and in seemingly contradictory ways. Sparse but robust geodetic, seismic, and geologic observations in the Basin and Range province of the western United States indicate that immediately after an earthquake, vertical change primarily occurs as subsidence of the normal fault hanging wall. A few decades later, a ±100 km wide zone is symmetrically uplifted. The preserved topography of long‐term rifting shows bent and tilted footwall flanks rising high above deep basins. We develop finite element models subjected to extensional and gravitational forces to study time‐varying deformation associated with normal faulting. We replicate observations with a model that has a weak upper mantle overlain by a stronger lower crust and a breakable elastic upper crust. A 60° dipping normal fault cuts through the upper crust and extends through the lower crust to simulate an underlying shear zone. Stretching the model under gravity demonstrates that asymmetric slip via collapse of the hanging wall is a natural consequence of coseismic deformation. Focused flow in the upper mantle imposed by deformation of the lower crust localizes uplift under the footwall; the breakable upper crust is a necessary model feature to replicate footwall bending over the observed width (&lt;10 km), which is predicted to take place within 1–2 decades after each large earthquake. Thus the best‐preserved topographic signature of rifting is expected to occur early in the postseismic period. The relatively stronger lower crust in our models is necessary to replicate broader postseismic uplift that is observed geodetically in subsequent decades.

Lower crustal high-velocity bodies along North Atlantic passive margins, and their link to Caledonian suture zone eclogites and Early Cenozoic magmatism

In this study we use crustal-scale Ocean Bottom Seismic models to infer the presence of two types of lower crustal bodies at North Atlantic passive margins; Type I, primarily interpreted as Early Eocene magmatic intrusions, and Type II, interpreted as Caledonian eclogites. We discuss how these eclogites might be related to the main Caledonian Suture Zone and other tectonic features in a conjugate North Atlantic setting. Based on the first-order approximation that P-wave velocities can be related to rock strength, the narrower continental margin at the southern (Møre) transect may be explained by stronger lower crust there, compared with the northern (Vøring) transect. This difference in strength, possibly resulting in a steeper dip in the subducting Baltica Plate south of the proto-Jan Mayen Lineament, may explain the asymmetry in extensional style observed across this lineament. Our interpretation locates the main suture off mid-Norway close to the Møre Trøndelag Fault Zone on the Møre Margin, along the western boundary of the Trøndelag Platform on the Vøring Margin, and further northwards beneath the Lofoten Ridge. The Lower Crustal Body Type I is about 60% thicker on the Greenland side, for both transects, and its thickness along the northern transect is more than twice that of the southern transect. These differences are consistent with sub-lithospheric interaction between the Icelandic hotspot and the continental rift/oceanic accretion system around the time of continental break-up.

Styles of rifting and fault spacing in numerical models of crustal extension

AbstractExtension of the Earth's crust can result in differing styles of rifting, such as horst‐and‐graben, half‐graben, metamorphic core complexes and areas of distributed crustal thinning. Faulting patterns can range from either being distributed to highly localized. Observations indicate that the factors controlling the extensional deformation, symmetry, and fault spacing include rheological aspects such as the yielding mechanism and strain softening, and physical aspects such as initial heterogeneities and the strength of the lower crust compared to the upper crust. Time‐dependent numerical models of extension are presented, which investigate the influence of the yielding mechanism, lower crust strength, strain weakening, and initial heterogeneity in the crust have on (a) the style of rifting, (b) fault spacing, and (c) integrated strength in the upper crust. Models with an anisotropic yielding mechanism result in more realistic lithospheric strength profiles, slip plane angle distributions, and fault interaction than models with an isotropic yielding mechanism. Heterogeneity type and yielding mechanisms have the largest effect on the resulting symmetry of deformation, whereas the amount of strain weakening has the greatest influence on asymmetry. The likelihood of the metamorphic core complex mode occurring is primarily controlled by lower crust strength. Crustal thinning is encouraged by both low amounts of strain weakening and a strong lower crust. Lower crust viscosity exerts the primary control on whether the resulting deformation is distributed or localized. The degree of strain weakening has the largest influence on the average strength of the upper crust and the slope of the strength profile in the upper crust.

Crustal rheological strength heterogeneities control the formation of continental plateau margins

The margins bordering the Tibetan Plateau show two end-member morphologies in topographic gradient: steep margins and low-gradient margins. To investigate the formation mechanism of convergent intracontinental plateau margins, we conduct 2D numerical experiments to simulate crustal deformation process across plateau margins. Our numerical experiments demonstrate that heterogeneities in crustal rheological strength control significantly the formation of plateau margins when subjected to crustal convergence. A very steep margin is the result of crustal convergence between plateau with weak lower crust and foreland basin with strong lower crust. By contrast, a low-gradient margin could result from crustal convergence between plateau and foreland with less strength contrast. This finding suggests that the diversity in topographic gradient along the Tibetan Plateau borders reflects heterogeneities in crustal rheological strength across the plateau margins. Steep gradient at the margins indicate large crustal rheological strength contrasts between the weak ductile lower crust of the Tibetan Plateau and its strong surrounding foreland basins, like the Sichuan Basin, the Tarim Basin and the Qaidam Basin. Beneath these steep margins the horizontal flow of the Tibetan ductile lower crust is inhibited and forced to extrude to support escarpments. Low-gradient at the margins indicate less crustal strength variations between the plateau and outer forelands, like at the northeastern and southeastern margins, where they might be outlets for the weak ductile Tibetan lower crust to flow away from the plateau.

Rifted continental margins: The case for depth-dependent extension

Even though many basic properties of non-volcanic rifted margins are predicted by uniform extension of the lithosphere, uniform extension fails to explain other important characteristics. Particularly significant discrepancies are observed at: 1) the Iberia–Newfoundland conjugate margins (Type I), where large tracts of continental mantle lithosphere are exposed at the seafloor, and at 2) ultra-wide central South Atlantic margins (Type II) where continental crust spans wide regions below which it appears that lower crust and mantle lithosphere were removed. Neither corresponds to uniform extension in which crust and mantle thin by the same factor. Instead, either the crust or mantle lithosphere has been preferentially removed during extension. We show that the Type I and II styles are respectively reproduced by dynamical numerical lithospheric stretching models (Models I-A/C and II-A/C) that undergo depth-dependent extension. In this notation A and C imply underplating of the rift zone during rifting by asthenosphere and lower cratonic lithosphere, respectively. We also present results for models with a weak upper crust and strong lower crust, Models III-A/C, to show that lower crust can also be removed from beneath the rift zone by horizontal advection with the mantle lithosphere. From the model results we infer that these Type I, II, and III margin styles are controlled by the strength of the mid/lower crust, which determines the amount of decoupling between upper and lower lithosphere during extension and the excision of crust or mantle. We also predict the styles of sedimentary basins that form on these margins as a test of the concepts presented.

Thin viscous middle-crust and evolving fault distribution during continental rifting: Insights from analog modeling experiments

Analog modeling of continental rifting, assuming a crustal scale “jelly sandwich”-like rheology, was carried out to test the mechanical effect of varying the absolute thickness of a weak (viscous) middle crust (silicone layer), interbedded between a brittle upper crust (sand layer) and a strong lower crust. Results consistently show a delocalization of the brittle deformation (i.e. a uniform scattering of the faults) throughout the upper brittle layer. This effect is interpreted to be associated with pressure driven flow in the viscous layer, caused by the tectonic collapse of upper brittle fault blocks into the viscous substratum. A reduction of the overall viscous layer thickness increases its resistance to accommodate internal thickness variations, which promotes delocalization of the fault pattern in the upper brittle layer. Our results contribute to the understanding of the mechanics of the so-called “upper plate paradox”, a large-scale structure often recognized at non-volcanic rifted margins. A thin viscous middle crust provides means of decoupling the deformation affecting upper and lower crust during rifting. On one hand this promotes a uniform scattering of faults throughout the upper brittle crust, on the other hand it allows for a strong localization of the deformation in lower crust and upper mantle expressed by the lithospheric necking in the rift center. • Analog modeling of continental rifting • “Jelly sandwich”-like crustal rheological structure • Middle crust thinning and delocalization of brittle fault pattern • Tectonic configuration of non-volcanic rifted margins • “Upper plate paradox” and decoupling effect of viscous middle crust

Fluid and deformation induced metamorphic processes around Moho beneath continent collision zones: Examples from the exposed root zone of the Caledonian mountain belt, W-Norway

Abstract Exposed High Pressure (HP) and Ultra High Pressure (UHP) metamorphic terrains have been studied in order to assess the metamorphic processes and their role in changing petrophysical properties near Moho depth in continental root zones. The investigation points to a critical role of fluid and deformation in metamorphic transformation in the deep crust and upper mantle. This applies to a) formation of granulite facies areas, b) transformation of granulites to eclogites, c) retrogression of eclogite facies rocks to amphibolite and green schist facies rocks and d) the spinel lherzolite to garnet lherzolite transition. Dry rocks both feldspar bearing and ultramafic remain with their Pre-HP and UHP structures and anhydrous mineralogy preserved while reactions occur where fluid has been introduced along deformation zones. A mixture of metamorphic facies formed at widely variable times on metre to km scale will be present throughout the crust and upper mantle. Pseudotachylytes (frictional melts or ultracomminuted material) are observed in both ultramafic and feldspar bearing lithologies spatially associated with HP and UHP rocks suggesting that rock properties at Moho depth allow earthquakes. Seismicity enhances the metamorphic and metasomatic transitions through fragmentation and by opening the rock for fluid influx. Ductile eclogite facies shear zones nucleate along the brittle structures. These observations point to Moho as a rock processing zone with the following facets: 1. A metastable dry and strong lower crust and upper mantle. 2. Earthquakes and tremors result in fluid flow and HP metamorphism. 3. A pronounced weakening of the hydrated and transformed rocks allows flow of material and the development of new fabrics (LPO) in the transformed rocks. Deep tremor and earthquakes at Moho depth may record ongoing metamorphic transitions.

Fault segmentation, deep rift earthquakes and crustal rheology: Insights from the 2009 Karonga sequence and seismicity in the Rukwa–Malawi rift zone

The Rukwa–Malawi rift zone has a record of seismic events down to depths in excess of 30 km, deep for a zone of active continental extension. This deep seismicity, as well as the presence of long (~ 100 km) border faults, has previously been explained by the long-term bulk rheology of intact, old, cold, anhydrous strong crust in east Africa, or the presence of mafic material in the lower crust. The Karonga sequence of 2009 showed a style of faulting different from continuous slip along long border faults, and is interpreted as segmented failure of hanging wall faults. Coulomb stress transfer in this sequence is calculated, and found to be consistent with segmented slip on a fault system synthetic to a nearby border fault and restricted to depths < 12 km. The inferred thermal structure of the Malawi rift indicates that slip at depths in excess of 30 km occurs at temperatures greater than the 350–450 °C commonly inferred at the base of the seismogenic zone. Crustal strength calculations indicate that long border faults and deep seismicity require the presence of a weak zone of localized deformation with increased strain rate (or fluid pressure), within a strong lower crust. A hypothesis is proposed where shallow, segmented frictional failure occurs in regions of relatively strong, intact crust (e.g. the Karonga sequence), whereas long border faults and deep earthquakes are representative of zones of weakness within strong crust. This hypothesis, if correct, implies that seismogenic thickness can vary within thick elastic lithosphere, such that localized weak zones of the crust enable nucleation of larger seismic events, whereas strong, intact crust favors smaller, segmented events and a shallower seismogenic zone.

Seismic structure and rheology of the crust under mainland China

The crust and upper mantle in mainland China were relatively densely probed with wide-angle seismic profiling since 1958, and the data have provided constraints on the amalgamation and lithosphere deformation of the continent. Based on the collection and digitization of crustal P-wave velocity models along related wide-angle seismic profiles, we construct several crustal transects across major tectonic units in mainland China. In our study, we analyzed the seismic activity, and seismic energy releases during 1970 and 2010 along them. We present seismogenic layer distribution and calculate the yield stress envelopes of the lithosphere along the transects, yielding a better understanding of the lithosphere rheology strength beneath mainland China. Our results demonstrate that the crustal thicknesses of different tectonic provinces are distinctively different in mainland China. The average crustal thickness is greater than 65km beneath the Tibetan Plateau, about 35km beneath South China, and about 36–38km beneath North China and Northeastern China. For the basins, the thickness is ~55km beneath Qaidam, ~50km beneath Tarim, ~40km beneath Sichuan and ~35km beneath Songliao. Our study also shows that the average seismic P-wave velocity is usually slower than the global average, equivalent with a more felsic composition of crust beneath the four tectonic blocks of mainland China resulting from the complex process of lithospheric evolution during Triassic and Cenozoic continent–continent and Mesozoic ocean–continent collisions. We identify characteristically different patterns of seismic activity distribution in different tectonic blocks, with bi-, or even tri-peak distribution of seismic concentration in South Tibet, which may suggest that crustal architecture and composition exert important control role in lithosphere deformation. The calculated yield stress envelopes of lithosphere in mainland China can be divided into three groups. The results indicate that the lithosphere rheology structure can be described by jelly sandwich model in eastern China, and crème brulee models with weak and strong lower crust corresponding to lithosphere beneath the western China and Kunlun orogenic belts, respectively. The spatial distribution of lithospheric rheology structure may provide important constraints on understanding of intra- or inter-plate deformation mechanism, and more studies are needed to further understand the tectonic process(es) accompanying different lithosphere rheology structures.

Crustal and lithospheric thinning beneath the seismogenic Kachchh rift zone, Gujarat (India): Its implications toward the generation of the 2001 Bhuj earthquake sequence

A high-resolution passive seismic experiment in the Kachchh rift zone of the western India has produced an excellent dataset of several thousands teleseismic events. From this network, 500 good teleseismic events recorded at 14 mobile broadband sites are used to estimate receiver functions (for the 30–310° back-azimuth ranges), which show a positive phase at 4.5–6.1 s delay time and a strong negative phase at 8.0–11.0 s. These phases have been modeled by a velocity increase at Moho (i.e. 34–43 km) and a velocity decrease at 62–92 km depth. The estimation of crustal and lithospheric thicknesses using the inversion of stacked radial receiver functions led to the delineation of a marked thinning of 3–7 km in crustal thickness and 6–14 km in lithospheric thickness beneath the central rift zone relative to the surrounding un-rifted parts of the Kachchh rift zone. On an average, the Kachchh region is characterized by a thin lithosphere of 75.9 ± 5.9 km. The marked velocity decrease associated with the lithosphere–asthenoshere boundary (LAB), observed over an area of 120 km × 80 km, and the isotropic study of xenoliths from Kachchh provides evidence for local asthenospheric updoming with pockets of partial melts of CO 2 rich lherzolite beneath the Kachchh seismic zone that might have caused by rifting episode (at 88 Ma) and the associated Deccan thermal-plume interaction (at 65 Ma) episodes. Thus, the coincidence of the area of the major aftershock activity and the Moho as well as asthenospheric upwarping beneath the central Kachchh rift zone suggests that these pockets of CO 2-rich lherzolite partial melts could perhaps provide a high input of volatiles containing CO 2 into the lower crust, which might contribute significantly in the seismo-genesis of continued aftershock activity in the region. It is also inferred that large stresses in the denser and stronger lower crust (at 14–34 km depths) induced by ongoing Banni upliftment, crustal intrusive, marked lateral variation in crustal thickness and related sub-crustal thermal anomaly play a key role in nucleating the lower crustal earthquakes beneath the Kachchh seismic zone.

Stress-strength relationship in the lithosphere during continental collision

Lithospheric strength profiles generated for a shortening continental lithosphere generally predict excessively high differential stresses in the sub-Moho continental mantle; this seems inconsistent with the relative scarcity of earthquakes at this depth. This inconsistency was put forward as evidence for weak mantle rheology. However, this argument implicitly assumes that strength envelopes are valid in actively deforming regions. We test this assumption on two end-member model lithospheres having identical upper crustal rheologies, but with (1) a weak lower crust and strong mantle, and (2) a strong lower crust and weak mantle. For this purpose, we compare one-dimensional (1-D) with 2-D visco-elastoplastic numerical models of continental shortening. The 2-D models show that strongly heterogeneous deformation typically follows initially homogeneous deformation. Lithospheric-scale buckle folds and shear zones result in strain rate variations of as much as three orders of magnitude. Differential stresses in the upper crust are close to yield, as predicted by 1-D models. Stresses in deeper lithospheric regions, however, are significantly smaller than in 1-D models, especially in actively deforming regions. Systematic numerical simulations as a function of temperature and deformation rate reveal that 1-D models are reliable in hot and/or slowly deforming lithospheres only. The relative scarcity of earthquakes at mantle levels should thus be interpreted as an intrinsic consequence of strong lithospheric deformation rather than as evidence for a weak upper mantle.

Along‐strike differences in the Southern Alps of New Zealand: Consequences of inherited variation in rheology

Long‐ and short‐term strain variations along the Australian‐Pacific plate boundary through the South Island of New Zealand, including a 300% increase in orogen width, coexistence of oblique thrusting on orthogonal structures, and variability in the locus of orogenic gold deposits, coincide with rheologically relevant geological variation. Our model investigates the consequences of thin, strong lower crust in the north and thick, weak lower crust in the south. Solution of the full 3‐D mechanical equations reproduces the larger wavelength strain patterns of the orogen. A 3‐D perturbation‐based analytical solution leads to the identification of the sensitivity of displacement type to minor stress changes. Transition from boundary‐normal thrusting to boundary‐parallel thrusting occurs at the transition from strong to weak lower crust and is related to an increase in either τyz (shear stress in the yz plane) or the ratio of the coordinate normal stresses, (σyy/σxx), where x and y are in the horizontal and z is vertical. Both mechanisms are compatible with the geologically dependent rheological variation employed in our model.