Shear wave structure in the northeastern Tibetan Plateau from Rayleigh wave tomography

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The northeastern Tibetan Plateau has undergone active shortening due to the India-Asia collision and has been an ideal place for studying the growth of the plateau. To reveal deep structure that is responsible for geological observations in NE Tibet, we have constructed a 3-D shear wave velocity model from Rayleigh waves recorded by 36 stations in the Northeast Tibetan Plateau Seismic experiment. First, the two-plane-wave tomography method was applied to compute the average and 2-D variations of phase velocities in a period range of 20 to 100 s. Then 1-D and 3-D shear wave velocity models were developed from the obtained Rayleigh wave dispersions. The entire region is characterized by a weak upper mantle with a relatively thin lithosphere of ~80-100 km thick. This observation helps to explain the active deformation at the Kunlun Mountains and the Qinling Orogen in NE Tibet. Relative low velocity anomalies appear along and to the south of the Kunlun fault (KF) in contrast to high velocity anomalies in the north and northeast. One pronounced slow anomaly centered at ~120 km depth is imaged right beneath the restraining bend of the Kunlun fault, which might be a northern extension of the large-scale slow anomaly to the south of the fault. We interpret it as a local feature due to its large magnitude and coincidence with the restraining bend. It is probably related to anomalously high temperature and associated partial melting resulted from localized asthenosphere upwelling after the delamination of a thick lithosphere root. The formation of the thick lithosphere root would require coherent shortening in the crust and mantle lithosphere caused by local compression due to the bend of the strike-slip fault, indicating that the Kunlun fault is probably on lithosphere scale and has played an important role in accommodating the eastward extrusion of the Tibetan Plateau. © 2013. American Geophysical Union. All Rights Reserved.

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Journal of Geophysical Research: Solid Earth