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Abstract

Numerous seismological experiments in central and southern Tibet have imaged the underthrusting Indian lithosphere and provided evidence of mid-crustal flow. A marked difference in mantle properties appears to occur in central Tibet, where body wave and Pn velocities become lower towards the north but with significant lateral variations. Surface wave studies tend to see velocities faster than the global average throughout Tibet in the deeper mantle (below ~150 km). Comparatively little seismological data had been collected in Northern Tibet. We combine data from the linear array of the INDEPTH-IV profile (2007-2008) and the ASCENT 2D deployment (2007-2009). We invert a high quality data set of more than 16,000 teleseismic P-wave arrival times and 3,100 teleseismic S-wave arrival times in a tomographic inversion to determine the velocity structure beneath NE Tibet in an area encompassing parts of the Qiangtang and Songpan-Ganzi terranes as well as the Kunlun Shan and part of the Qaidam. Major geological structures include the Kunlun Fault and the Jinsha and Bangong-Nujiang sutures. In total, 572 events at 80 stations were used for the P inversion, and 72 events for the S-inversion. Teleseismic regional tomography (ACH tomography) is most sensitive to lateral velocity contrasts, and in particular cannot resolve layer averages or very long range velocity variations; neither can it image variations in thickness or properties of the crust. In order to partially compensate for this, we utilise information from recent surface wave models, Acton et al (JGR, 2010) for the crust and shallow mantle, and a much extended version of the Priestley et al (JGR, 2006) model for the deeper mantle. The starting model for the body wave inversion combines both these models; in this way the well-established long wavelength background structure and absolute velocities provided by the surface waves are respected, but additional shorter range features can be imaged. For the P-wave starting model, we have translated the S-velocity variations into suitable P-velocities using a constant Vp/Vs ratio of 1.78. The resolution tests indicate that features more than a 100 km wide are reliably imaged throughout the study region; locally higher resolution is possible, but with significant smearing in the vertical direction. The resulting tomographic models show a number of large scale features. The lowest velocities are found below the Kunlun Shan, reaching depths of 300 km and more. There is a prominent high velocity zone extending across the south-west of the model below the Qiangtang terrane at approximately 130-220 km depth, whereas at shallower depths below the Qiangtang there appears to be more E-W variation. Further north, above 37°N, the velocity increases more smoothly with depth, and the high velocity zones are considerably less pronounced.