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We have located a total of 232 local earthquakes using data recorded by the SANDWICH seismic network from November 2013 to October 2014 in central Tibet across the Bangong-Nujiang suture (BNS). The focal depths of all earthquakes are shallower than 30 km and therefore are in the upper crust. The absence of lower crust earthquakes may imply a weak, ductile lower crust in central Tibet. Moreover, these earthquakes are dispersed throughout conjugate strike-slip fault zones, indicating that evenly distributed upper crustal deformation might predominate in central Tibet. This observation agrees with the hypothesis that conjugate fault zones accommodate coeval east-west extension and north-south contraction via continuous deformation. Moreover, the focal mechanisms show that strike-slip and normal faulting are the dominant types of deformation and that the extension in central Tibet is oriented approximately east-west. Despite some anomalies, the kinematics implied by most of the focal mechanisms correlate well with those of the surface structures. [ABSTRACT FROM AUTHOR]

Abstract: Tectonic models for the evolution of the Himalayan orogen interpret the Greater Himalayan crystalline complex (GHC) to be the result of either thick-skinned thrusting involved Indian basement, thin-skinned thrusting involving exotic terranes, middle-crustal ductile flow, or wedge extrusion of the Indian crust during India–Asia collision. Two key pieces of information needed to test the validity of these models is the temporal–spatial distribution of, and the identification of the dynamic mechanisms involved in, regional melting under southern Tibet. Here, we document an Eocene–Oligocene melting event in southern Tibet, which forms a 150-km-long, NW–SE-trending granitoid belt along the Zedong–Lhunze traverse between the Indus–Yarlung suture (IYS) and the south Tibetan detachment (STD). U–Pb dating of magmatic zircons indicates that this granitoid belt youngs northward from ∼46Ma (in Lhunze) to ∼30Ma (in Zedong). 40Ar/39Ar dating of deformed biotite within 42–46Ma granitoids constrains the timing of shearing to ∼39–41Ma. The granitoid belt of southern Tibet is dominated by Eocene two-mica granites in the Tethyan Himalaya, with minor ∼30Ma granodiorites along the IYS and ∼35Ma granites in the Yelaxiangbo dome, where Indian mid-crustal rocks are exposed. The ∼35Ma granites are characterized by variable Na2O/K2O ratios (1.03–4.44), relatively high Sr concentrations, and high Sr/Y (14.0–126.3) and La/Yb (11.1–42.8) ratios, which distinguish these granitoids from Miocene leucogranites in the Himalaya. Comparison of the Sr–Nd isotopic compositions of these granites with mid-crustal amphibolites exposed in the Yelaxiangbo dome suggests that the granites were derived from melting of the amphibolites at ∼880°C and ∼10kbar. The ∼30Ma granodiorites and ∼42–46Ma two-mica granites are Na-rich and peraluminous, and are adakitic. They contain inherited Proterozoic zircons, and have a much wider range in ε Nd(t) of –14.9 to –2.5 and (87Sr/86Sr) i of 0.7062–0.7188, and have a Nd isotopic model age of 1486–1978Ma, indicating that these magmas were derived from a thickened Indian lower crust and were subsequently mixed with amphibolite-derived granite melts or were contaminated by the middle crust under southern Tibet. An apparent northward-younging age trend and shearing of the Eocene–Oligocene granitoids requires the southward migration of slices of middle crustal material, in which the Eocene granitoid magmas were emplaced and stored. Our data, along with structural, metamorphic, and intrusive histories of the Himalaya, lead us to propose a model for crustal anatexis and tectonic evolution of the Himalayan orogen, controlled by a number of large-scale events, such as slab break-off, buoyancy-driven uplift, lateral movement, and subsequent exhumation of slices of the subducted Indian crust during Indo-Asia collision at 55–40Ma. [Copyright &y& Elsevier]