Rheology Of Lithosphere Research Articles

Evolution of Microstructural Heterogeneity in the Deep Arc Lithosphere During Delamination

AbstractThe microstructural properties of deep arc cumulates (arclogites) are poorly understood, but are essential in gaining a comprehensive picture of the rheology of continental lithosphere. Here, we analyze 16 arclogite xenoliths, comprising a low MgO and a high MgO suite, from Arizona, USA using electron backscatter diffraction to map microstructures, clinopyroxene shape preferred orientations (SPO), and clinopyroxene crystallographic preferred orientations (CPO). The lower pressure (∼1 GPa) low MgO arclogites show a variety of different clinopyroxene fabrics (S, L, and LS‐type), whereas the high pressure (>2 GPa) high MgO arclogites show predominantly LS‐type fabrics. Furthermore, clinopyroxenes in low MgO arclogites all show a pronounced correspondence between the long axis of their grain shape ellipsoids with the [001] crystal direction, indicating an SPO control on the CPO. In contrast, high MgO arclogite clinopyroxenes lack such a correspondence. We propose that both arclogite types originated as igneous cumulates, consistent with previous studies, but that the high MgO suite experienced substantial recrystallization which diminished the original igneous SPO‐induced CPO. Using strain rates appropriate for arc settings, we calculate a strength profile for the lithosphere and argue that the deepest arclogite textures are consistent with lithospheric foundering through ductile deformation under high shear strain (10−14–10−12 s−1). Our study shows that there is a high degree of shear strain localization in deep arc roots while shallower portions are relatively undeformed.

The influence of lithospheric rheology and surface processes on the preservation of escarpments at rifted margins

The formation of rift flank topography along rifted margins is related to the flexural uplift of the footwall on the rift border fault during lithospheric stretching. The preservation of rift flank topography in mature margins can be explained as a feedback mechanism between differential denudation of the margin and regional isostatic response of the lithosphere. However, the contribution of the lithospheric rifting on the escarpment amplitude in both young and mature margins is not easily quantified. In the present work, we used a 2D thermo-mechanical numerical model to simulate lithospheric extension to evaluate the sensitivity of escarpment amplitude and its preservation under different rheological conditions over timescales larger than 100 Myr, comparable with the age of mature rifted margins. We found that the evolution of escarpment uplift and its preservation for tens of millions of years are sensitive to the degree of coupling between the crust and mantle and the regional isostatic response of the lithosphere to surface processes. The flexural uplift as a response to lithospheric stretching are limited to the first 100‐150 km from the rift flank. Without the influence of surface processes, the amplitude of the escarpment monotonically decreases through time due to the lateral flow of the lower crust and thermal cooling of the margin. In the scenarios with surface processes, topographic signature inherited from the rift phase in the post-rift escarpment elevation is possible only in margins where the erosive escarpment retreat is smaller than ∼100 km. In scenarios where the escarpment retreat exceeds this distance, the escarpment elevation is mainly preserved by the combination of the inherited topography (existent before the onset of lithospheric rifting) and flexural rebound of the lithosphere due to the erosion along the margin.

Structural Observations of the Northern North Sea: Insights into Rift Failure Dynamics

Lithospheric extension leads to rift formation and may continue to the point of breakup, with oceanic ridge initiation and the formation of two conjugate rifted margins. In some settings, extension can cease, and the rift may be abandoned. These so-called failed rifts archive snapshots of early phases of deformation, with geometries that may help better constrain the parameters that can prevent a rift from reaching breakup, such as lithospheric rheology, thermal state, rift opening direction and rate, inheritance. This contribution summarizes a study of the Norwegian Continental Shelf which includes the North Sea Rift and the Møre and Vøring rifted margins. We proceeded to the interpretation of a new dataset of deep penetrating seismic reflection profiles and worked at the regional scale, deliberately ignoring local particularities, to focus on the large-scale structural picture. The aim is to list architectural similarities and differences between the failed rift and the successful rifted margins. Our mapping shows that the North Sea structural geometries and basement seismic facies are very similar to the observations listed for the adjacent Møre and Vøring rifted margins. Various types of tectonic structures are observed, from thick anastomosing shear zones possibly evolving into core-complex geometries, to composite large-scale detachment faults and standard high-angle normal faults. These are categorized into five classes and interpreted as exemplifying the rift tectonic evolution through distinct generations of deformation structures that can activate, de-activate and re-activate. Based on these observations, rift failure dynamics are discussed, and it is proposed that the North Sea rift abandonment may not be related to pre-rift local conditions but rather to the ability to initiate specific tectonic structures such distal breakaway complexes.

Secular Evolution of Continents and the Earth System

AbstractUnderstanding of secular evolution of the Earth system is based largely on the rock and mineral archive preserved in the continental lithosphere. Based on the frequency and range of accessible data preserved in this record, we divide the secular evolution into seven phases: (a) “Proto‐Earth” (ca. 4.57–4.45 Ga); (b) “Primordial Earth” (ca. 4.45–3.80 Ga); (c) “Primitive Earth” (ca. 3.8–3.2 Ga); (d) “Juvenile Earth” (ca. 3.2–2.5 Ga); (e) “Youthful Earth” (ca. 2.5–1.8 Ga); (f) “Middle Earth” (ca. 1.8–0.8 Ga); and (g) “Contemporary Earth” (since ca. 0.8 Ga). Integrating this record with knowledge of secular cooling of the mantle and lithospheric rheology constrains the changes in the tectonic modes that operated through Earth history. Initial accretion and the Moon forming impact during the Proto‐Earth phase likely resulted in a magma ocean. The solidification of this magma ocean produced the Primordial Earth lithosphere, which preserves evidence for intra‐lithospheric reworking of a rigid lid, but which also likely experienced partial recycling through mantle overturn and meteorite impacts. Evidence for craton formation and stabilization from ca. 3.8 to 2.5 Ga, during the Primitive and Juvenile Earth phases, likely reflects some degree of coupling between the convecting mantle and a lithosphere initially weak enough to favor an internally deformable, squishy‐lid behavior, which led to a transition to more rigid, plate like, behavior by the end of the early Earth phases. The Youthful to Contemporary phases of Earth, all occurred within a plate tectonic framework with changes between phases linked to lithospheric behavior and the supercontinent cycle.

Quantifying continental collision dynamics for Alpine-style orogens

When continents collide, the arrival of positively buoyant continental crust slows down subduction. This collision often leads to the detachment of earlier subducted oceanic lithosphere, which changes the subsequent dynamics of the orogenic system. Recent studies of continental collision infer that the remaining slab may drive convergence through slab roll-back even after detachment. Here we use two-dimensional visco-elasto-plastic thermo-mechanical models to explore the conditions for post-collisional slab steepening versus shallowing by quantifying the dynamics of continental collision for a wide range of parameters. We monitor the evolution of horizontal mantle drag beneath the overriding plate and vertical slab pull to show that these forces have similar magnitudes and interact continuously with each other. We do not observe slab rollback or steepening after slab detachment within our investigated parameter space. Instead, we observe a two-stage elastic and viscous slab rebound process lasting tens of millions of years, which is associated with slab unbending and eduction that together generate orogenic widening and trench shift towards the foreland. Our parametric studies show that the initial length of the oceanic plate and the stratified lithospheric rheology exert a key control on the orogenic evolution. When correlated with previous studies our results suggest that post-detachment slab rollback may only be possible when minor amounts of continental crust subduct. Among the wide variety of natural scenarios, our modelling applies best to the evolution of the Central European Alps. Furthermore, the mantle drag force may play a more important role in continental dynamics than previously thought. Finally, our study illustrates that dynamic analysis is a useful quantitative framework that also intuitively explains observed model kinematics.

Fluid-enhanced grain-size reduction of K-feldspar from a natural middle crustal shear zone in northern Beijing, China

Grain-size reduction may lead to a transition from grain-size insensitive to grain-size sensitive deformation of rocks in the middle crustal level, thereby affecting the lithosphere's rheology significantly. However, the mechanisms of grain-size reduction of sheared rocks have long been debated. In this study, granitic mylonites from the Shuiyu shear zone of the Yunmengshan metamorphic core complex were sampled to investigate the mechanisms of grain-size reduction of K-feldspar during shearing in the middle crust. The mylonites are characterized by a typical microstructure of coarse-grained K-feldspar porphyroclasts surrounded by fine-grained matrix of oligoclase, K-feldspar, and quartz. Deformation temperatures are constrained from 400 °C to 550 °C, estimated using the feldspar and quartz microstructures, crystal-preferred orientation patterns of recrystallized quartz, and a two-feldspar geothermometer. Here, we show that the derivation of fine grains in the matrix from the K-feldspar porphyroclasts was accomplished via fluid-assisted subgrain rotation recrystallization. Walls of either well-organized or tangled dislocations constituted the boundaries of micron-scale subgrains along the margins of porphyroclasts. Hydrolytic weakening enhanced the dislocation motions to form subgrains. Furthermore, the subgrain rotation recrystallization promoted the replacement reaction of K-feldspar by sodium-rich fluid via increasing the areas of fluid/grain interfaces. The newly formed oligoclase and quartz were subsequently deformed by diffusion creep.

Crustal Shortening and Rheological Behavior Across the Longmen Shan Fault, Eastern Margin of the Tibetan Plateau

AbstractKnowledge of lithospheric rheology can provide fundamental insights into crustal deformation near the Longmen Shan fault (LMSF). Based on viscoelastic deformation models constrained by interseismic geodetic observations, we obtain an optimal crustal shortening rate of 4.8 ± 0.4 mm/a across the LMSF and an upper mantle viscosity of 5.0 × 1020−21 Pa · s beneath eastern Tibet. More importantly, we find a high‐viscosity zone (>1021 Pa · s) in the lower crust beneath the LMSF, where the steady‐state viscosity is significantly higher than the transient viscosity derived from postseismic deformation. Further investigations with a power‐law rheology suggest that, due to the stress loading of the Wenchuan earthquake and the relaxation afterwards, the effective lower crustal viscosity decreases to ∼1018 Pa · s immediately after the earthquake and finally recovers to interseismic level (∼1021 Pa · s). Our results highlight the stress‐dependent behavior and the viscoelastic effect of rheological structure beneath the LMSF during the earthquake cycle.

A lithospheric-scale Arrowsmith (2.4 Ga) detachment system with major Trans-Hudson (1.8 Ga) reactivation documented in the Howard Lake shear zone, Rae craton, Canada

This study characterizes the polyphase tectonometamorphic history of a major shear system in the Rae craton, the Howard Lake shear zone (HLsz), which exhumed high-grade rocks in the distant hinterland of the Paleoproterozoic Trans-Hudson Orogen. Prior work established a Mesoarchean mantle model-age discrepancy in basement rocks across the HLsz, which we interpret to have contributed a fundamental lithospheric rheology contrast that allowed for HLsz localization. Our work shows that the HLsz has a long-lived history starting in the early Paleoproterozoic as it abruptly separates 2.36 Ga granulite-facies metamorphic units to the east from 2.4 Ga greenschist-facies metamorphic basement rocks to the west. Subsequent reactivation during Trans-Hudson Orogen time between 1.86 and 1.82 Ga accommodated re-burial and re-exhumation of high-grade rocks in its footwall coeval with Sask craton collision. Strong penetrative overprinting deformation in metaigneous rocks and a newly discovered < 2.04 Ga schist preserve evidence for right-lateral transpression within the HLsz at ca. 1.82 Ga, which is coincident with the terminal collision of the Superior craton. Our work demonstrates that the HLsz is one of several major crustal-scale anisotropies in the south Rae craton and it is broadly analogous to major detachment fault systems documented in numerous Phanerozoic orogenic systems. Furthermore, the polycyclic deformation history of the HLsz, along which regional burial and exhumation was strongly localized, demonstrates the considerable influence crustal scale anisotropies can have on the evolution of craton architecture such as that presently found in the south Rae craton of the Canadian Shield.

Gondwana break‐up controlled by tectonic inheritance and mantle plume activity: Insights from Natal rift development (South Mozambique, Africa)

AbstractThe break‐up of Eastern Gondwana started during the Early Jurassic with the separation of Antarctica and Madagascar from Africa. While margins architecture has been described along the Western Somalia Basin and the Central Mozambique, the spatial extent of rifting further south—i.e. the Natal Valley—remains poorly documented. Seismic reflection profiles interpretation, 40Ar/39Ar ages and isotopic data show the existence of a plume‐related magma‐rich margin—the Natal segment—characterized by a large volume of seaward dipping reflectors inferred to be basalts. In particular, the contrast in lithosphere rheology, probably caused by a Meso‐Neoproterozoic belt between the Kaapval and Grunehogna Archean cratons, favoured extension and upwelling from a deep mantle thermomechanical anomaly, the so‐called Karoo superplume that started at around 180 Ma. Furthermore, the presence of stretched continental crust at around 30°S implies a more southerly position of Antarctica which will need to be considered in future kinematic models.

Plate corner subduction and rapid localized exhumation: Insights from3Dcoupled geodynamic and geomorphological modelling

AbstractRapid, localized exhumation has been reported at many plate corners between adjacent subduction/collision segments. Here we use a fully‐coupled geodynamic and geomorphological modelling approach to investigate overriding plate deformation and resulting rock uplift patterns in these narrow, cuspate regions. In this study, we focus on the effects of internal deformation within a subducting convex‐upward‐shaped indenter and the strength of the interface between the upper and downgoing plate. The strongest localization of high rock uplift rates in the region above the indenter apex is predicted in experiments with a deformable lower plate, a weak interface layer and lateral shortening accommodated only by subduction (i.e., without an upper plate advance component). Our results suggest that bull’s eye shaped structures characterized by young thermochronological ages can, in principle, be reproduced numerically when taking into account a non‐rigid subducting plate together with complex brittle‐ductile rheology and stratification of the overriding lithosphere and realistically implemented fluvial erosion at its surface.

Effects of asthenospheric flow and orographic precipitation on continental rifting

Asthenosphere-lithosphere interactions modulated by surface processes generate outstanding topographies and sedimentary basins, but the nature of these interactions and the mechanisms through which they control the evolution of extensional tectonic settings are elusive. Basal lithospheric shearing due to plume-related mantle flow leads to extensional lithospheric rupturing and associated magmatism, rock exhumation, and topographic uplift away from the plume axis by a distance inversely correlated to the lithospheric elastic thickness. When moisturized air encounters a topographic barrier, it rises, decompresses, and saturates, leading to enhanced erosion on the windward side of the uplifted terrain. Orographic precipitation and asymmetric erosional unloading facilitate strain localization and lithospheric rupturing on the wetter and more eroded side of an extensional system. This simple analytical model is validated against thermo-mechanical numerical experiments where a rheologically stratified lithosphere above an asthenospheric plume is subject to fluvial erosion proportional to stream power during extension. Our modeling results are consistent with Paleogene mantle upwelling and flood basalts in Ethiopia synchronous to distal initiation of lithospheric stretching/rupturing in the Gulf of Aden, which progressively propagates into the Red Sea. The present-day asymmetric topography and extensional structures in the Main Ethiopian Rift may also be an effect of a Neogene-to-present orographic erosional gradient. Although inherently related to the lithosphere rheology, the evolution of continental rifts appears even more conditioned by the mantle and surface dynamics than previously thought.

Crustal thickening versus lateral extrusion during India–Asia continental collision: 3-D thermo-mechanical modeling

Continental collision between the Indian and Eurasian plates in the past ~55 ± 5 Ma has led to the development of the Tibetan Plateau, where nearly doubled crustal thickness indicates intense lithospheric thickening during the collision. On the other hand, geological evidence indicates significant lateral extrusion of Tibetan materials around the edges of the plateau, especially around the Eastern Himalaya Syntaxis. Two end-member models, “pure shear thickening” and “tectonic escape”, emphasize respectively the vertical and lateral deformation of Tibetan lithosphere during the collision. However, their competing roles in accommodating the Tibetan continental collision remain debated. Here, we developed a series of 3-D high-resolution numerical models to systematically study strain partitioning between crustal thickening and lateral extrusion during the collision. The model focuses on eastern Tibetan Plateau, where most of the escaping tectonics occurred. The model results indicate that the amount of crustal thickening is generally several times higher than that of lateral extrusion in accommodating the lithosphere shortening. The partitioning between crustal thickening and lateral extrusion depends on the rheological contrast between the Tibetan and neighboring blocks, whereas the relic suture zones within the Tibetan lithosphere do not control the strain partitioning but affect the morphology. The southward motion of the Indochina block, resulting from extension caused by trench retreating, has major impacts on the development of large-scale lateral extrusion tectonics. The vertical variation of lithospheric rheology leads to faster motion of the crust than the mantle lithosphere in the extruding block, hence mechanical decoupling between the extruding crust and lithospheric mantle.

Reconciling lithospheric rheology between laboratory experiments, field observations and different tectonic settings

SUMMARYRecent modelling studies have shown that laboratory-derived rheology is too strong to reproduce observations of flexure at the Hawaiian Islands, while the same rheology appears consistent with outer rise—trench flexure at circum-Pacific subduction zones. Collectively, these results indicate that the rheology of an oceanic plate boundary is stronger than that of its interior, which, if correct, presents a challenge to understanding the formation of trenches and subduction initiation. To understand this dilemma, we first investigate laboratory-derived rheology using fully dynamic viscoelastic loading models and find that it is too strong to reproduce the observationally inferred elastic thickness, Te, at most plate interior settings. The Te can, however, be explained if the yield stress of low-temperature plasticity is significantly reduced, for example, by reducing the activation energy from 320 kJ mol−1, as in Mei et al., to 190 kJ mol−1 as was required by previous studies of the Hawaiian Islands, implying that the lithosphere beneath Hawaii is not anomalous. Second, we test the accuracy of the modelling methods used to constrain the rheology of subducting lithosphere, including the yield stress envelope (YSE) method, and the broken elastic plate model (BEPM). We show the YSE method accurately reproduces the model Te to within ∼10 per cent error with only modest sensitivity to the assumed strain rate and curvature. Finally, we show that the response of a continuous plate is significantly enhanced when a free edge is introduced at or near an edge load, as in the BEPM, and is sensitive to the degree of viscous coupling at the free edge. Since subducting lithosphere is continuous and generally mechanically coupled to a sinking slab, the BEPM may falsely introduce a weakness and hence overestimate Te at a trench because of trade-off. This could explain the results of recent modelling studies that suggest the rheology of subducting oceanic plate is stronger than that of its interior. However, further studies using more advanced thermal and mechanical models will be required in the future in order to quantify this.

Seismic faults triggered early stage serpentinization of peridotites from the Samail Ophiolite, Oman

Serpentinization of mantle peridotites has first order effects on the rheology and tectonic behavior of the oceanic lithosphere, on the global water cycle, and on the biosphere at mid-oceanic ridges. Investigating serpentinization of abyssal peridotites is limited by the scarce occurrences of peridotites at or close to the ocean floor at slow and ultra-slow ridge environments where peridotite is exposed by long-lived detachments. The processes controlling hydration of the upper mantle below a thick magmatic crust at fast spreading ridges are poorly constrained. Here we present results based on samples from cores drilled in peridotites from the Samail ophiolite obtained during the Oman Drilling Project. We describe an early generation of highly localized brittle faults ubiquitous through all the peridotite cores and investigate their relation to the main serpentinization event represented by mesh-textured serpentinites. We combine microstructural observations with mineral and bulk chemical analyses as well as oxygen isotope microanalyses obtained by secondary ion mass spectrometry (SIMS). Asymmetric wall rock damage, weakening of crystal preferred orientation (CPO) in small fault clasts, and intense fragmentation within the fault zones even in association with very small displacements suggest that the early stage faults represent seismic events and predate mesh formation. Hydration and mesh texture formation follows in the wake of this faulting. Serpentinization is associated with moderate enrichment of fluid mobile elements including B, Li, Rb and U, indicative of fluid rock interaction characterized by relatively low fluid/rock ratios. This is consistent with a scenario where serpentinization took place below a thick magmatic crust following an earthquake-induced permeability increase. The oxygen isotope compositions of mesh serpentine are consistent with off-axis serpentinization at temperatures in the range 200-250°C

Fault interaction and active crustal extrusion in the southeastern Tibetan Plateau: Insights from geodynamic modeling

• The 3D model is used to investigate the fault interaction and crustal deformation . • Faults in southeastern Tibet are kinematically linked and mechanically coupled. • Fault interaction has affected fault activities and crustal extrusion rates. In the southeastern Tibetan Plateau, the Xianshuihe-Xiaojiang fault system (XXFS) and its neighboring fault systems collectively accommodate the crustal extrusion of the Tibetan Plateau. However, we do not clearly understand how these faults interact with each other and how the fault interaction impacts fault slip rate, strain partitioning and crustal extrusion in this region. Through building a 3D viscoelastoplastic finite element model, we investigate the effects of fault geometry, fault configuration, lithospheric rheology and topographic loading on fault slip rate and crustal deformation. We find that the geometrical irregularity of Anninghe-Zemuhe faults in XXFS (a concave bend) prevents the crustal extrusion of the Chuandian (Sichuan-Yunnan) block, and tends to cause the inception of their subparallel Daliangshan fault on their east. On the other hand, the active Daliangshan fault decreases the slip rates on the Anninghe-Zemuhe faults, causing faster crustal extrusion of the Chuandian block. The active Lijiang-Xiaojinhe fault decreases the slip rates of faults in the south and hence hinders the crustal extrusion of the Chuandian block. Besides, the topographic loading plays an important role in the crustal extrusion of the southeastern Tibetan Plateau, by redistributing regional deformation and fault slip rates. Modeling results demonstrate that all faults in this region are kinematically connected and mechanically coupled: the slip rate change of one fault, both induce strain repartitioning and variations of slip rates on all other faults in such a system. Our research reveals the significant impact of fault interaction on fault activity and crustal extrusion in the southeastern Tibetan Plateau.

Passive margin inversion controlled by stability of the mantle lithosphere

Contractional deformation of passive continental margins may resemble early stages of induced subduction initiation. Mechanical instabilities are required to permit underthrusting of the oceanic plate. Therefore, the success of developing a new subduction zone will largely depend on the rheology of the mantle lithosphere. In this physical analogue modelling study, a range of mantle viscosities subject to different convergence rates serve as a proxy to simulate contraction of differently aged passive margins with the purpose of describing and quantifying passive margin deformation and mantle stability using the buoyancy number. The experiments illustrate distinct differences in geometry and length-scale of deformation as a function of lithospheric mantle strengths. The results indicate that deformation occurs at passive margins for intermediate strength lithospheric mantle and high strain rate. In contrast, low and high strength mantle lithospheres lead to dominantly intra-oceanic deformation or long wavelength buckling of the entire model, respectively. Our experiments portray an evolution in which early-stage deformation commences at the ocean-continent transition and is controlled by the ductile lower crust of the continent. In the next stage, shear localization through the formation of a decollement within the ductile passive margin crust favors underthrusting of the oceanic lithosphere, leading to a reduction of the area affected by deformation. Prior to underthrusting, the primary response of the lithosphere to compression is by folding at scaled wavelengths of 100–300 km and 500–1000 km, controlled by the strength of the mantle lithosphere.

Lithosphere Weakening During Arctic Ocean Opening: Evidence From Effective Elastic Thickness

AbstractEvolution of the Arctic Ocean lithosphere has involved multiple stages of opening with crustal stretching and thinning prior to lithospheric breakup. Mapping lateral variations in lithospheric rheology can help unravel the detailed tectonic history of the Arctic. Here we perform a wavelet analysis of gravity and bathymetry data to map the effective elastic thickness () of the lithosphere in the Arctic. The low overall suggests that large shear stresses and serpentinization weakened the lithosphere at Arctic passive margins during a multistage opening process. Moderately low values along the Gakkel Ridge imply a relatively cold ultraslow‐spreading center compared to typical mid‐ocean ridges.

Influence of Shear Heating and Thermomechanical Coupling on Earthquake Sequences and the Brittle‐Ductile Transition

AbstractLocalized frictional sliding on faults in the continental crust transitions at depth to distributed deformation in viscous shear zones. This brittle‐ductile transition (BDT), and/or the transition from velocity‐weakening (VW) to velocity‐strengthening (VS) friction, are controlled by the lithospheric thermal structure and composition. Here, we investigate these transitions, and their effect on the depth extent of earthquakes, using 2D antiplane shear simulations of a strike‐slip fault with rate‐and‐state friction. The off‐fault material is viscoelastic, with temperature‐dependent dislocation creep. We solve the heat equation for temperature, accounting for frictional and viscous shear heating that creates a thermal anomaly relative to the ambient geotherm which reduces viscosity and facilitates viscous flow. We explore several geotherms and effective normal stress distributions (by changing pore pressure), quantifying the thermal anomaly, seismic and aseismic slip, and the transition from frictional sliding to viscous flow. The thermal anomaly can reach several hundred degrees below the seismogenic zone in models with hydrostatic pressure but is smaller for higher pressure (and these high‐pressure models are most consistent with San Andreas Fault heat flow constraints). Shear heating raises the BDT, sometimes to where it limits rupture depth rather than the frictional VW‐to‐VS transition. Our thermomechanical modeling framework can be used to evaluate lithospheric rheology and thermal models through predictions of earthquake ruptures, postseismic and interseismic crustal deformation, heat flow, and the geological structures that reflect the complex deformation beneath faults.

Influence of crustal rheology and heterogeneity on tectonic stress accumulation characteristics of North China constrained by GNSS observations

Abstract North China is characterized by significant lithosphere heterogeneity and numerous faults, with the occurrence of many historical and ongoing devastating earthquakes. To extend our understanding of the mechanisms of seismicity initiation and fault activity due to lithospheric rheology and lateral difference, we first established a three-dimensional viscoelastic finite element model based on the lithospheric lateral heterogeneity of physical properties across the north–south gravitational lineament (NSGL), spatial distribution of faults, and interseismic global navigation satellite system (GNSS) velocities (1999–2018). We then analyzed the stress accumulation characteristics across the NSGL and its relationship with seismicity. Finally, we explored the temporal and spatial variations of stress along major faults and further analyzed potential relationships between the stress components and rupture mechanisms of typical faults. The results showed that high maximum shear stress, mainly distributed in eastern North China (ENC) and western North China (WNC), corresponding to the focal depth, which suggests that the different brittle crustal thicknesses across the NSGL may be one of the major factors that dominates earthquake depth in North China. Significant maximum shear stress is mainly accommodated on faults around the Ordos Block in WNC and northern faults in ENC, which is consistent with frequent seismic activity in these regions. The relationship between the calculated stress components and rupture mechanisms of typical faults imply that the differential tectonic loading from neighboring blocks may be one of the major dynamic factors for seismogenic processes in North China.

Role of Lithospheric Buoyancy Forces in Driving Deformation in East Africa From 3D Geodynamic Modeling

AbstractDespite decades of investigation, the origin of forces driving continental rifting remains highly debated. Deciphering their relative contributions is challenging due to the nonlinear and depth‐dependent nature of lithospheric rheology. Recent geodynamic studies of the East African Rift (EAR) report contradicting results regarding the relative contribution of horizontal mantle tractions and lithospheric buoyancy forces. Here, we use high‐resolution 3D regional numerical modeling of the EAR to isolate the contribution lithospheric buoyancy forces to observed deformation. Modeled surface velocities closely match kinematic models of the Somalian Plate, Victoria Block, and Rovuma Block motions, but provide poor fit to along‐rift surface motions in deforming zones. These results suggest that lithospheric buoyancy forces primarily drive present‐day ∼E‐W extension across the EAR, but intrarift deformation may result from viscous coupling to horizontal asthenospheric flow.