Tectonic models for the Oligocene-Miocene development of the Himalaya mountain range are largely focused on crustal-scale processes, and developed along orogen-perpendicular cross sections. Such models assume uniformity along the length of the Himalaya, but significant along-strike tectonic variations occur, highlighting a need for three-dimensional evolutionary models of Himalayan orogenesis. Here we show a strong temporal correlation of southward motion of the Indian slab relative to the overriding Himalayan orogen, lateral migration of slab detachment, and subsequent dynamic rebound with major changes in Himalayan metamorphism, deformation, and exhumation. Slab detachment was also coeval with South Asian monsoon intensification, which leads us to hypothesize their genetic link. We further propose that anchoring of the Indian continental subducted lithosphere from 30 to 25 Ma steepened the dip of the Himalayan sole thrust, resulting in crustal shortening deep within the Himalayan orogenic wedge. During the subsequent ~13 m.y., slab detachment propagated inward from both Himalayan syntaxes. Resultant dynamic rebound terminated deep crustal shortening and caused a rapid rise of the mountain range. The increased orography intensified the South Asian monsoon. Decreased compressive forces in response to slab detachment may explain an observed ~25% decrease in the India-Eurasia convergence rate. The asymmetric curvature of the arc, i.e., broadly open, but tighter to the east, suggests faster slab detachment migration from the west than from the east. Published Lu-Hf garnet dates for eclogite facies metamorphism in the east-central Himalaya as old as ca. 38-34 Ma may offer a test that the new model fails, because the model predicts that such metamorphism would be restricted to middle Miocene time. Alternatively, these dates may provide a case study to test suspicions that Lu-Hf garnet dates can exceed actual ages.

The Paleozoic-early Mesozoic geology of Tibet was controlled by the rift-drift, seafloor spreading and subduction zone tectonics of a Paleo-Tethyan realm, which evolved between the West Cathaysides (WC) and the East Cimmerides (EC). Different suture zones with ophiolites and ophiolitic mélanges, high-pressure metamorphic belts,

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[1] Predominant stretching structures in the Greater Himalayan Crystalline Complex (GHC) trend perpendicular to the belt and are linked to the southward exhumation or emplacement of the GHC between the South Tibet Detachment (STD) and the Main Central Thrust. However, our field investigations in southern Tibet reveal the widespread presence of gently dipping shear zones with a penetrative orogen-parallel stretching lineation, which separates the Tethyan Himalayan Sequence and the underlying GHC. The shear zones are well preserved in the upper part of the GHC, south to and structurally lower than the STD. Field criteria, microstructures, and quartz fabrics indicate top-to-the-east shearing in the Yadong shear zone (eastern GHC), coexistence of top-to-the-east and topto-the-west shearing in the Nyalam shear zone (central GHC), but top-to-the-west shearing in the Pulan shear zone (western GHC). Characteristic microstructures and slip systems of quartz in the high-grade GHC rocks resulted from the lateral flow under upper amphibolite (up to 650-700 C) to greenschist facies conditions. U-Pb ages of metamorphic zircon rims by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation multi-collector inductively coupled plasma mass spectrometry ( Ar cooling ages of biotite and muscovite suggest cessation of ductile sharing at 13-11 Ma on the Yadong shear zone, which is coeval with the activation of the STD. Combined with previous studies, we propose that initiation of orogen-parallel extension marks the transition from burial/crustal thickening to exhumation of the GHC. Due to lateral crustal thickness gradients in a thickened crust, orogen-parallel gravitational collapse occurred within the convergent Himalayan orogen in the late Oligocene-Miocene. This tectonic denudation triggered and enhanced partial melting and ductile extrusion of the GHC in the Miocene.

Classical equilibrium molecular dynamics simulations have been performed to investigate the diffusive properties of inter-cage hydrogen migration in both pure hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 0.05 kbar from 200 K and up to 250–260 K. For mixed H2-THF systems in which there is single H2 occupation of the small cage (labelled “1S1L”), we found that no H2 migration occurs. However, for more densely filled H2-THF and pure-H2 systems, in which there is more than single H2 occupation in the small cage, there is an onset of inter-cage H2 migration events from the small cages to neighbouring cavities at around 200 K. The mean square displacements of the hydrogen molecules were fitted to a mathematical model consisting of an anomalous term and a Fickian component, and nonlinear regression fitting was conducted to estimate long-time (inter-cage) diffusivities. An approximate Arrhenius temperature relationship for the diffusion coefficient was examined and an estimation of the hydrogen hopping energy barrier was calculated for each system.

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Sedimentological and geochronological studies along a north–south traverse across the Bangong‐Nujiang suture zone (BNSZ) in Gaize, Tibet provide evidence for a Late Triassic–Jurassic accretionary wedge accreted to the south margin of Qiangtang. This wedge, preserved as the Mugagangri Group (MG), records evidence for the northward subduction of the Bangong‐Nujiang Ocean (BNO) beneath Qiangtang. The MG strata comprise two coarser intervals (lower olistostromes and upper conglomerates) intercalated within sandy turbidites, which are consistent with timing and forearc stratigraphy during subduction initiation predicted by geodynamic modelling. Following the model, the northward subduction of the BNO beneath Qiangtang and subsequent arc‐magmatism are inferred to have begun, respectively, at ca. 220 Ma and ca. 210 Ma, with respect to depositional ages constrained by youngest detrital‐zircon ages. The initiation of arc‐magmatism is also supported by provenance transition reflected by sandstone detrital modes and age patterns of detrital zircons. Previously, evidence for an incipient arc was lacking, but the timing of Late Triassic BNO subduction and related arc‐magmatism is coincident with an important Late Triassic magmatic event in central Qiangtang that probably represents the ‘missing’ arc. Other Qiangtang events, such as exhumation of the Qiangtang metamorphic belt as a source area, and development of the Late Triassic Nadigangri deposits and bimodal volcanism, are more easily explained in the tectonic context of early northward subduction of the BNO beneath Qiangtang, beginning at about 220 Ma.

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