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.

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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.

[1] Shear wave velocities (Vs), anisotropy, and shear wave splitting have been measured at pressures up to 600-800 MPa for ultrahigh-pressure (UHP) metamorphic rocks from the Dabie-Sulu orogenic belt, China, with a focus on three types of eclogites. Type 1 eclogites are coarse-grained, unaltered samples showing high densities and high Vs values (4.85 ± 0.06 km/s at 600 MPa); type 2 eclogites are fine-grained, sheared samples with intermediate Vs values (4.53 ± 0.04 km/s at 600 MPa); and type 3 eclogites are overprinted by amphibolite facies metamorphism and display low Vs values (4.33 ± 0.09 km/s at 600 MPa). The compositional layering and retrograde metamorphism can result in significant anisotropy and shear wave splitting in eclogites, suggesting their plausible contribution to seismic anisotropy in the lower crust, upper mantle, and particularly in subducted slabs. Integrating our P and S wave velocity results with reliable data from previous studies, we estimated the pressure and temperature derivatives of Vp, Vs, and Poisson's ratios for common rock types in the UHP metamorphic belt. The geometric means were used as a mixture rule to invert the lithological and chemical compositions of the layered crust from seismic refraction velocities. The inferred crustal composition suggests that the eclogite-bearing UHP rocks are tectonic slices of crust that have been thrust along a series of shear zones during the continental collision between the north China and Yangtze cratons, over a normal UHP-free middle lower crust with overall intermediate composition.

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