Crust and mantle beneath western Himalaya, Ladakh and western Tibet from integrated seismic data

To understand deep structure of the western Himalaya, Ladakh and western Tibet, we conduct an integrated analysis of teleseismic body wave recordings from the linear array of 16 portable broadband seismographs along a profile running through the Himalaya to the Karakoram. The database includes P and S receiver functions, teleseismic P and S residuals and observations of shear wave splitting in SKS. The analysis reveals beneath the Himalaya a cold region at a depth of several hundred kilometers. This anomaly can be caused by remnants of Tethys subduction and, perhaps, is unrelated to the ongoing process. The present day process is reflected in velocities beneath the lesser Himalaya, intermediate between those of the crust and the upper mantle. This observation can be explained by scraping off the ductile lower crust of the underthrusting Indian plate and accumulation of the high-velocity crustal material in the frontal region of the thrust zone. A representative value of the crustal thickness at the profile is 65 km, and at most stations there is no evidence for a low-velocity layer at mid-crustal depths, which makes crustal channel flow unlikely. Seismic waves in the upper 200 km of the mantle are faster than in global IASP91 model, and the structure includes a low-velocity layer sandwiched between two high-velocity layers. We interpret the lower layer as an image of subducted mantle lithosphere of the Indian plate. Shear wave splitting is different in the south and the north. In the south the fast direction of anisotropy is normal to the trend of the Himalaya and can be interpreted as an effect of the NE motion of the Indian lithosphere. In the north the fast direction is oriented E-W and can be explained by the fabric left in the presently extinct subduction zones. In eastern Tibet a similar analysis of the P and S receiver functions reveals upper mantle with the S velocities about 5% lower than in the west. The mantle in the vicinity of station LSA (Lhasa) contains a low S velocity layer between 160-km and 230-km depths with a velocity reduction of 0.2 km/s, underlain by the Lehmann discontinuity. (C) 2008 Elsevier B.V. All rights reserved.