PHYSICAL STATE OF HIMALAYAN CRUST AND UPPER MANTLE: CONSTRAINTS FROM VELOCITY AND ATTENUATION TOMOGRAPHY, SEISMICITY, AND ANISOTROPY

Results from recent seismic experiments provide new information on deep structure beneath the Himalaya. Receiver functions from the Himalayan Nepal Seismic Experiment (HIMNT) show an increase in crust-mantle thickness from 45 km beneath the Indo-Gangetic Plains to 75 km beneath the Tethyan Himalaya, and resolve a zone of strong anisotropy interpreted as the basal decollement of the Himalaya (Main Himalayan Thrust, MHT). Tibetan crust above the decollement has a seismic Vp/Vs ratio markedly lower than that of Nepal Lesser and Sub-Himalaya. We interpret the low Vp/Vs zone as orginal Tibetan crust, and the higher Vp/Vs beneath it as underthrust Indian crust. The high crustal velocities at the base of the Tibetan crust (60-75 km), along with a lower crustal seismic wave conversion, are interpreted as partial eclogitization of the underthrust Indian crust. The Southern Tibetan upper mantle has high Vp (8.4 km/s) and is isotropic, consistent with the presence of eclogite. Crustal seismic attenuation varies stongly in a north to south fashion, with higher attenuation (low Q) in southern Tibet relative to Nepal. While a subset of these results are consistent with previous interpretations of melt in the crust, all of the seismic variations taken together are not, at least assuming the current generation of laboratory scaling relations. A partially fluid-saturated crust, along with crustal compositional variations, is more consistent with our observations. The alpha-beta quartz transition (ABQT) may also play a role in the sharp increase in Vp/Vs with depth in the mid-lower crust of the Tethyan Himalaya. Beneath the region of highest relief along the Himalayan Front, strong alignment of shallowearthquakes concentrate on the hanging wall of the inferred MHT. The Tibetan crust shows a strongly bimodal distribution of seismicity with depth, and a nearly aseismic lower crust. A cluster of upper mantle earthquakes is found beneath southern Tibet (70-90 km depth), many with strike-slip focal mechanisms. Finite element modeling suggests that the observed pattern of seismicity and focal mechanisms is consistent with loading of a viscoelastic lithosphere where the main forces acting upon it are the weight of the mountains and thrust sheets, and a horizontal force associated with the plate convergence.