SEISMIC TOMOGRAPHY OF NORTHERN TIBET AND KUNLUN - EVIDENCE FOR CRUSTAL BLOCKS AND MANTLE VELOCITY CONTRASTS

Although the crust and mantle of the Tibet Plateau reveal vital inform ation for understanding the interplay of dynamic processes that has go verned its recent uplift and growth, their deep, physical and thermal structure remains poorly understood. In order to throw light on the st ructure and, hence, to constrain the processes and models tied to them , we performed a teleseismic experiment on a 600 km long profile acros s the northern part of the plateau, the Kunlun range and the Qaidam ba sin, The 400 km deep tomographic image we obtain has a resolution < 50 km, over one order of magnitude better than achieved in previous, bro ader-scale studies. At relatively shallow depth (< 100 km), the tomogr aphic cross-section clearly reveals that the mid-lower crust and upper lithosphere of northern Tibet is an assemblage of blocks with differe nt velocities and thicknesses; hence different natures, histories and ages. The crust of the Qiantang block appears to be thickest (approxim ate to 70 km), with the lowest velocity. Along the north edge of that block, the Jinsha suture, although not remarkable in the local geology and topography, stands as a particularly sharp and prominent crustal boundary. The crust and lithosphere of the region between that suture and the Qaidam (Bayan Har-Songpan, South Kunlun) appear to be thinnest (approximate to 50 km) and with the highest velocity. At greater dept h (> 150 km), our experiment confirms the existence of a bulky low-vel ocity zone in the mantle beneath the northernmost reaches of the plate au. The tomogram helps assess with unprecedented accuracy the location and shape of this low-velocity anomaly, which forms a 250-300 km wide dome rising to about 150-200 km, south of the Kunlun range. roughly c oincident with the region where the strongest shear-wave splitting, he nce horizontal anisotropy, has recently been found. That dome lies bet ween two relatively high velocity zones with opposite dips, one toward s the north, under the Qaidam basin, the other towards the south, bene ath the Qiangtang platform. The implications that this new, detailed i mage of the deep structure of northern Tibet has upon the uplift and t hermo-mechanical evolution of the plateau are discussed.