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Earthquakes in the dry lower continental crust are enigmatic, as they require very high deviatoric stresses. Field observations suggest that stress amplification in rigid blocks surrounded by ductile shear zones leads to seismic failure: the jostling block model. Here we quantify this model by systematically testing numerically how variations in geometry, material properties, and loading conditions impact magnitude and rate of stress amplification. We demonstrate that bulk strain rate is the dominant factor controlling stress amplification. High strain rates of 10−10–10−12 s−1 cause stresses on the 102–103–MPa level within 100–102 years, while lower strain rates are insufficient to generate the stresses required for lower‐crustal earthquakes. While geometries and material properties play a subordinate role in causing stress amplification, tests with varying loading conditions show that pure shear is more effective in generating high stress amplifications compared to simple shear in the case of the given geometry.

Arc lower crust plays a critical role in processing mantle-derived basaltic melts into the intermediate continental crust, yet can only be studied indirectly or in exposed arc sections. Compared with the relatively well-studied oceanic arc sections (e.g., Kohistan and Talkeetna), the composition and formation mechanisms of continental arc lower crust remain less clear. Here we present a geochronological and geochemical study on the Lilong Complex and the Wolong granitoids from the Gangdese arc deep crustal section in southern Tibet. The Lilong Complex is composed of the early (85–95 Ma) mafic-intermediate sequence and late (85–86 Ma) ultramafic sequence. The Lilong crustal section exposed crustal depth extending from ~ 42 to 17 km based on the geobarometry. The mafic-intermediate sequence is a damp (low H2O) igneous differentiation sequence characterized by the subsequent appearance of pyroxene → plagioclase → amphibole → biotite. The ultramafic sequence represents a wet igneous differentiation sequence composed of olivine → pyroxene → amphibole → plagioclase. The 74–84 Ma Wolong granitoids were formed by fractional crystallization of wet magma and intra-crustal assimilation. Calculated seismic properties of the Gangdese deep arc crust are comparable to the average continental crust at a similar depth. The average composition of the Gangdese arc lower crust is basaltic andesite with SiO2 of ~ 54 wt%. The highly incompatible elements in the Gangdese arc lower crust are systematically higher than those of the oceanic arc and are comparable with the estimates of lower continental crust, suggesting continental arc magmatism significantly contributes to the formation of continental crust.

We carried out seismic tomography study to reveal three-dimensional (3D) seismic velocity structures in the Noto peninsula, Japan, where swarm-like seismic activity started in December 2020. The obtained results reveal a highly heterogeneous structure in the crust. The most striking feature is the existence of a low-velocity anomaly in the lower crust beneath the Noto earthquake swarm. Although the data set used in this study cannot resolve the upper mantle structure, previous regional tomographic studies suggest that a low-velocity anomaly exists at depths of 50–150 km around the Noto peninsula that is probably interpreted as a fluid-rich region. We infer that fluids have been supplied from the uppermost mantle to the lower crust over a geological time scale and a large volume of fluids have accumulated below the seismogenic zone beneath the Noto peninsula. A further upward migration of fluids to the upper crust, which may have suddenly started in December 2020, probably triggers numerous earthquakes at depths of 10–15 km. Since major active faults exist at shallower extensions of the hypocenters of the Noto earthquake swarm, we consider that the earthquake swarm occurs along pre-existing and weak fault planes. Dense temporary seismic observations will highlight a smaller-scale (5–10 km) 3D seismic velocity model and finer hypocenter distribution, which provide additional information for better understanding of the generation mechanisms of the Noto earthquake swarm.

During the late‐stage of continental rifting, lower crustal rocks can be exhumed to the Earth's surface. Such exposure has been identified especially in rifts that developed within former orogens. These rifts exploited zones of lithospheric weakness created during previous mountain‐building events. Here we investigate whether lower crustal exhumation during rifting could result directly from structural inheritance of prior orogeny. To this aim we use Wilson‐Cycle numerical models where we impose a prior shortening and subsequent extension. Our models incorporate the effects of thermal and structural inheritance as well as surface processes. We find that the erosion of mountain belts thins the upper crust, reducing the upper‐to‐lower crustal ratio prior to rifting. This process facilitates the exhumation of lower crustal material during extension, exposing them at the Earth's surface along the footwall of normal faults. We suggest a new conceptual model to explain lower crustal exhumation observed at rifted margins worldwide.

Troctolites were recovered during Integrated Ocean Drilling Program Expedition 345 at the Hess Deep Rift, next to fast-spreading East Pacific Rise. These troctolites are divided into three groups based on textural differences: coarse-grained (1–10 mm in length) troctolite, fine-grained (~ 2 mm in length) troctolite, and skeletal olivine-bearing troctolite. All troctolites exhibit a magmatic fabric. The major-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites are intermediate between those of Hess Deep gabbros and harzburgites. The trace-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites overlap with those of troctolites from slow-spread crust, but they record no petrographic evidence indicating assimilation of mantle peridotite. Thermodynamic calculation for mineral chemistry showed that fractional crystallization of melt is the dominant process responsible for the formation of the troctolites. The fine-grained troctolite was crystallized with high crystallization rate resulting from hot melt injection into colder wall gabbro. In contrast, interactions between the unsolidified troctolite containing interstitial melt and newly injected melt resulted in the formation of the skeletal olivine-bearing troctolite. While our results demonstrate that the troctolites exhibit multiple melt injections and partial dissolution of a troctolite precursor, fractional crystallization is the dominant process for the creation of the lower crust in the Hess Deep Rift.

To constrain the behavior of Mg isotopes during deep crustal processes and the Mg isotopic composition of the middle and lower continental crust, 30 composite samples from high-grade metamorphic terranes and 18 granulite xenoliths were investigated. The composites derive from eight different high-grade metamorphic terranes in the two largest Archean cratons of China, including 13 TTG gneisses, 5 amphibolites, 4 felsic, 4 intermediate, and 4 mafic granulites. They have variable bulk compositions with SiO2 ranging from 45.7 to 72.5%, representative of the middle crust beneath eastern China. The δ26Mg values of these samples vary from -0.40 to +0.12 ppm, reflecting heterogeneity of their protoliths, which could involve upper crustal sediments. The granulite xenoliths from the Cenozoic Hannuoba basalts also have a diversity of compositions with MgO ranging from 2.95 to 20.2%. These xenoliths equilibrated under high temperatures of 800-950°C, corresponding to depths of the lower continental crust (>30 km). They yield a large δ26Mg variation of -0.76 to -0.24 ppm. The light Mg isotopic compositions likely result from interactions with isotopically light metamorphic fluids, probably carbonate fluids. Together with previously reported data, the average δ26Mg values of the middle and lower continental crusts are estimated to be -0.21±0.07 ppm and -0.26±0.06 ppm, respectively. The bulk continental crust is estimated to have an average δ26Mg of -0.24±0.07 ppm, which is similar to the average of the mantle. The large Mg isotopic variation in the continental crust reflects the combination of several processes, such as continental weathering, involvement of supracrustal materials in the deep crust, and fluid metasomatism.

Zircon grains from the metasedimentary lower crust of the Rio Grande Rift, New Mexico, preserve a metamorphic record of the transition from Laramide compression to Eocene extension. Zircon U‐Pb isotopes and trace‐element concentrations from five two‐pyroxene metaigneous granulite xenoliths define discrete populations: older zircon cores (∼15–50 Ma) that are depleted in heavy rare‐earth elements (HREE) but Ti‐rich, and younger zircon rims (∼3–15 Ma) with elevated HREE and lower Ti concentrations. Coupled phase equilibria and garnet‐melt‐zircon trace‐element partitioning calculations show that the older zircon cores equilibrated in thick (>40 km), hot (800–900°C), garnet‐bearing lower crust during the cessation of compression at the end of the Laramide orogeny. Zircon rim domains equilibrated at lower pressures, consistent with >9 km of thinning of the lower crust. Thermal‐kinematic calculations show that these pressure‐temperature‐time constraints require thinning of the lithospheric mantle prior to and during regional Cenozoic extension. Convective erosion of the mantle lithosphere over tens of millions of years, possibly facilitated by dynamics of the Farallon slab, provides a mechanism to facilitate lower crustal heating and extension. Key Points Zircon U‐Pb dates from five granulite xenoliths, Rio Grande Rift, range between ∼50 and ∼3 Ma Zircon cores equilibrated in thick (>40 km), hot (800–900°C), garnet‐bearing lower crust during the Laramide orogeny P‐T‐time constraints require thinning of the lithospheric mantle prior to and during regional Cenozoic extension

The Re-Os isotopic system is largely considered the geochronometer of choice to date partial melting of terrestrial peridotites and in constraining the evolution of Earth's dynamics from the mantle viewpoint. While whole-rock peridotite Re-Os isotopic signatures are the core of such investigations, the Re-Os dating of individual peridotite minerals - base metal sulfides (BMS) and platinum group minerals (PGM) - that are the main hosts for Re and Os in the mantle peridotites came into play two decades ago. These nanometric-micrometric BMS and PGM display an extreme complexity and heterogeneity in their 187Os/188Os and 187Re/188Os signatures that result from the origin of the BMS±PGM grains (residual vs. metasomatic), the nature of the metasomatic agents, the transport/precipitation mechanisms, BMS±PGM mineralogy, and subsequent Re/Os fractionation. Corresponding whole-rock host peridotites, typically plot within the 187Os/188Os and 187Re/188Os ranges defined by the BMS±PGM, clearly demonstrating that their Re-Os signatures represent the average of the different BMS±PGM populations. The difference between the 187Os/188Os ratios of the least radiogenic BMS±PGM and the respective host peridotite increases with the fertility of the peridotite reflecting the increasing contribution of metasomatic BMS±PGM to the whole-rock mass balance of Re and Os concentrations and Os isotope compositions. Corollaries to these observations are that (1) BMS may provide a record of much older partial melting event, pushing back in time the age of the lithospheric mantle stabilization, (2) if only whole-rock peridotite Re-Os isotopic measurements are possible, then the best targets for constraining the timing of lithospheric stabilization are BMS-free/BMS-poor ultra-refractory spinel-bearing peridotites with very minimal metasomatic overprint, as their 187Os/188Os signatures may be geologically meaningful, (3) while lherzolites are \"fertile\" in terms of their geochemical composition, they do not have a \"primitive,\" unmodified composition, certainly in terms of their highly siderophile elements (HSE) and Re-Os isotopic systematics, and (4) the combined Re-Os isotopic investigations of BMS and whole-rock in BMS-rich mantle peridotites would provide a complementary view on the timing and nature of the petrological events responsible for the chemical and isotopic evolution and destruction of the lithospheric mantle. In addition, the 187Os/188Os composition of the BMS±PGM (both residual and metasomatic) within any single peridotite may define several age clusters - in contrast to the single whole-rock value - and thus provide a more accurate picture of the complex petrogenetic history of the lithospheric mantle. When coupled with a detailed BMS±PGM petrographical study and whole-rock lithophile and HSE systematics, these BMS age clusters highlight the timing and nature of the petrological events contributing to the formation and chemical and isotopic evolution of the lithospheric mantle. These BMS±PGM age clusters may match regional or the local crustal ages, suggesting that the formation and evolution of the lithospheric mantle and its overlying crust are linked, providing mirror records of their geological and chemical history. This is, however, not a rule of thumb as clear evidence of crust-mantle age decoupling also exist. Although the BMS±PGM Re-Os model ages push back in time the stabilization of lithospheric mantle, the dichotomy between Archean cratonic and circum-cratonic peridotites, and post-Archean non-cratonic peridotites and tectonites is preserved. This ability of BMS±PGM to preserve older ages than their host peridotite also underscores their survival for billions of years without being reset or reequilibrated despite the complex petrogenetic processes recorded by their host mantle peridotites. As such, they are the mantle equivalents of crustal zircons. Preservation of such old signatures in \"young\" oceanic peridotites ultimately rules out the use of the Re-Os signatures in both oceanic peridotites and their BMS to estimate the timescales of isotopic homogenization of the convecting mantle.

To test Eastern Tibet crustal thickening modes, we compare 2‐D numerical models of two emblematic end‐member models, with either an obstacle in the low viscosity lower crust or a thrust embedded in the high viscosity one. We show that the obstacle halts the viscous lower crustal flow potentially initiated by the weight of the high Central Tibet, generating a smooth exhumation gradient at the edge of the plateau, not observed in Eastern Tibet. On the contrary, including a low viscosity discontinuity in the upper crust, mimicking a shallow steep listric fault as inferred in the region, reproduces a sharper exhumation profile, as constrained from thermo‐kinematic inversions of thermochronological data, and the lack of foreland basin, as observed in the field. Moreover, such fault drives deformation throughout the entire crust, suggesting a deep crustal ductile shear zone limiting the more ductile deformation in the lower crust although no discontinuity is imposed. Plain Language Summary The role of thrusting in crustal thickening during the formation of Tibet, the world's largest and highest orogenic plateau, constitutes one of the main controversies in earth sciences. In Eastern Tibet in particular, two end‐members based on two contrasting controversial hypotheses can be tested: the thickening is dominated either by the flow of the lower Tibetan crust halted by the hard Sichuan craton, or by thrusting of the Tibetan upper crust. Here, we present 2‐D crustal numerical models of a shallow steep listric thrust (as inferred in the region) embedded in the high viscosity upper crust, and we show that such model reproduces the exhumation profile constrained from thermochronological data and the lack of foreland basin observed in the field. Interestingly, we also show that such upper crustal thrust drives upward the more ductile lower crust albeit no discontinuity is imposed. On the contrary, by using a model driven by an overpressure in the lower crust, we show that the obstacle halts the viscous lower crustal flow and generates a smooth exhumation gradient at the edge of the plateau, not observed in Eastern Tibet. Key Points 2‐D numerical models of thrusts embedded in the high viscosity upper crust, to test thermo‐kinematic models based on thermochronology data accommodation in the lower crust by ductile flow of the deformation induced by the high angle thrust in the upper crust predicting exhumation rates and subsidence patterns that are compatible with the measured ones in Eastern Tibet

Deeply exhumed crustal terranes of continental arcs worldwide commonly contain metasedimentary components, but the nature of these lithologies and how they became incorporated into the lower crust are not fully understood. Here, we present petrological, geochemical and geochronological data from exhumed deep‐crustal metapelites and orthogneisses from the eastern Gangdese magmatic arc, southern Tibet. Geochemical and geochronological affinity between metapelites and forearc sediments indicates that their protoliths were probably deposited in an Early Cretaceous (<120 Ma) forearc basin of the Gangdese arc, while the orthogneisses represent metamorphosed equivalents of Late Jurassic (157–140 Ma) arc‐type magmatic rocks. Petrological modeling and zircon U‐Pb dating show that metapelites and orthogneisses simultaneously experienced high‐pressure granulite‐facies metamorphism at peak pressure‐temperature conditions of 810–840°C and 12–16 kbar at 87–85 Ma. Our findings suggest that both the Mesozoic forearc sediments and igneous rocks that were initially emplaced into the upper crust of the Gangdese arc were subsequently transported to its lower crust within c. 25 Myr by crustal shortening, folding and underthrusting during the Late Cretaceous. When combined with previous data collected from the region, we propose that tectonic burial within arc crust and slab subduction‐related mechanisms most likely operate together in magmatic arcs, promoting crustal recycling. The transport of surficial silica‐rich materials into the lower crust is likely a basic process that has driven the growth and differentiation of continental arcs throughout geological time.