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Recycling of oceanic lithosphere and serpentinized peridotites into the mantle leads to dehydration melting and volcanic arcs. However, the mechanism of the occurrence of long‐term volcanic gap in subduction zones remains poorly understood. Two‐dimensional thermomechanical numerical models focusing on resisting the transport of the major hosts of hydrous minerals to mantle depths, show that thin oceanic crust together with rheologically weak and buoyant serpentinized mantle could result in hydrated lithologies piling up in the accretionary wedge and no or sparse occurrence of arc magmatism. This scenario may occur during subduction of oceanic lithosphere formed at (ultra)slow‐spreading margins characterized by thin crust and extensive mantle serpentinization, which is a plausible explanation for the "arc gaps" in the Alps and southern Tibet. Plain Language Summary: Volcanic arcs in subduction zones are widespread and are mainly attributed to slab dehydration and subsequent mantle hydrous melting. However, the magmatic history of the Alps and southern Tibet is characterized by two periods of "arc gap" (i.e., no volcanism) that lasted for 40–50 Myr and 10–30 Myr, respectively, with no or only very sparse magmatism and simultaneous exhumation of serpentinized mantle slivers. Our numerical models indicate that thin oceanic crust, such as that formed at (ultra)slow‐spreading centers, together with an extensively serpentinized lithosphere facilitates the removal of the subducting crust at shallow depths resulting in limited arc magmatism. Key Points: The influence of abyssal and slab‐bending serpentinites on magmatism in subduction zones are numerically investigatedAmagmatic subduction zones may occur where abyssal serpentinites are abundantModel results can explain sparse magmatism during subduction in the Alps and southern Tibet [ABSTRACT FROM AUTHOR]

This study used SKS waveforms from the International Deep Profiling of Tibet and the Himalayas (INDEPTH) III dataset and a new 2D method for modeling seismic waves in anisotropic media to construct an image of anisotropic structures beneath central Tibet. A preferred model revealed three-segment anisotropic structures in the upper mantle beneath the study region. Waveform modeling demonstrated that the anisotropy was mainly generated by the lithosphere but not the asthenosphere, and that an anisotropic model with a flatter axis of symmetry provides a more consistent interpretation of the observations than models having steeply dipping symmetry axes. A relatively low velocity zone may underlie or intermingle with the anisotropic structures in the northern portion of the region. Synthetic tests also indicate that variations in the elastic constants and depth extent of the anisotropy assumed by the calculations do not affect the general conclusions, although trade-offs exist among certain model parameters. The modeling results suggest that the complex seismic structures in central Tibet were associated with underthrusting of the Indian lithosphere beneath the Asian lithosphere; the inferred flat symmetry axis of the anisotropy was likely generated during this collision process. If this were not the case, the inherited anisotropy would exhibit a steeply dipping axis of symmetry, parallel to the direction of underthrusting. [ABSTRACT FROM AUTHOR]

The mechanism of crustal deformation and the development of large offset strike‐slip faults during continental collision, such as the India‐Eurasia zone, remains poorly understood. Previous mechanical models were simplified which are either (quasi‐)2‐D approximations or made the a priori assumption that the rheology of the lithosphere was either purely viscous (distributed deformation) or purely localized. Here we present three‐dimensional visco‐elasto‐plastic thermo‐mechanical simulations, which can produce both distributed and highly localized deformation, to investigate crustal deformation during continental indentation. Our results show that large‐scale shear zones develop as a result of frictional plasticity, which have many similarities with observed shear zones. Yet localized deformation requires both a strong upper crust (>1022 Pa·s) and a moderately weak middle/lower crust (~1020 Pa·s) in Tibet. The brittle shear zones in our models develop low viscosity zones directly beneath them, consistent with geological observations of exhumed faults, and geophysical observations across active faults. Plain Language Summary: Large offset strike‐slip faults are one of the key surface features of continental collision, such as in Tibet. These narrow belts of strike‐slip faults accommodate strong deformation, and deciphering their mechanism of formation helps to understand the complex dynamics of India‐Eurasia collision. Yet previous models developed to address this either assumed simplified rheologies or employed low numerical resolutions that is insufficient to simulate the spontaneous formation of localized zones. Here, we present high‐resolution 3‐D visco‐elasto‐plastic thermo‐mechanical models that simulate the formation of large‐scale strike‐slip faults during Indian indentation, while also taking distributed deformation into account. Our simulations show that a combination of a strong upper crust and a moderately weak middle/lower crust produces faults that are, to first order, consistent with observed ones in the Tibet region. Localized deformation usually initiates at the brittle‐ductile transition, and a weak middle/lower crust facilitates subsequent shear localization such that the shear zones cut through the whole crust. The craton‐like strong terranes can sustain large stresses and initiate large offset faults along their boundaries. Key Points: 3D VEP high‐resolution thermomechanical models accounting for both localized and distributed deformation during India‐Eurasia collisionMajor strike‐slip faults develop preferentially in models with a strong upper and a moderately weak lower crustLower crustal flow and strong terranes surrounding Tibet facilitate fault formation [ABSTRACT FROM AUTHOR]