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Abstract

Recent studies have suggested that uplift of the northern Tibetan plateau may be related to removal of lithospheric mantle, resulting in emplacement of hotter, less dense asthenosphere. Other studies propose that plateau uplift and crustal thickening have occurred through a process of lateral mid-crustal flow or coherent deformation between crust and lithospheric mantle. Some authors attribute the geophysical properties of upper mantle beneath the plateau to either delamination of thickened lithosphere, or asthenospheric counterflow associated with subduction of continental Indian lithosphere beneath central Tibet. In order to study the evolution and dynamics of the Tibetan plateau, we deployed 74 broadband seismic stations throughout northeastern Tibet within the scope of ASCENT/INDEPTH-IV experiment. In conjunction with Namche Barwa data, we have calculated fundamental mode Rayleigh wave phase velocities utilizing two-plane wave approach, for periods between 20-143 seconds. We also obtained preliminary phase velocities using Love waves. To invert for shear wave velocities, we use partial derivatives from Saito (1988), assuming a constant Poisson’s ratio. Our azimuthal anisotropy measurements agree well with SKS splitting results; both indicate significant (>2%) average azimuthal anisotropy throughout the upper mantle down to depths exceeding 250 km, with a dominantly EW fast directions. Although we observe variations of fast directions with depth, they are generally consistent (i.e., within 15 degrees) up to ~200 km, indicative of vertically coherent deformation. Furthermore at crustal depths, azimuthal fast directions tend be sub-parallel to the strikes of major strike slip faults, suggesting that shearing is the dominant deformation mechanism in eastern Tibet. Our tomographic models show an uppermost mantle low velocity anomaly north of Bangong-Nujiang Suture (BNS) in northeastern Tibet, and a high velocity anomaly extending ~200 km centered on the BNS. We suggest that the low velocity zone is due to warmer, thinner lithosphere in the northern Qiangtang and Songpan-Ganzi terranes. At depth, we observe high velocity bodies to the south both in our phase velocity and shear wave velocity maps, indicative of an underthrusting Indian lithosphere. Our phase maps and shear wave velocity-anomaly isosurfaces strongly indicate that underthrusting of Indian plate is sub-horizontal, has variable geometry in EW direction, and does not extend north of the BNS. We propose that underthrusting is accompanied by lateral tearing along old weak zones into at least two fragments, and subsequent break-off of the western-most portion. Our models reveal low velocity zones concentrated along major strike slip faults, which we attribute to strain heating. Furthermore, the Qiadam Basin is characterized by high velocities in the mantle, and moderate to slow velocities below the Kunlun Shan and northern Qiangtang argue against continental subduction.