A REVIEW ON THE LITHOSPHERIC STRUCTURES IN THE TIBETAN PLATEAU AND CONSTRAINTS FOR DYNAMICS

In the last decade, several international joint projects were conducte d in the Tibetan plateau by Chinese, American and French geophysicists and geologists. In the present review, the results from vertical refl ections, wide-angle reflections and broadband digital seismic recordin gs are reviewed and compared. Constraints for the dynamics of continen t-continent collision from the lithospheric structures, seismicity, fo cal mechanism and anisotropy are discussed. The velocities of Pn, Sn, (P) over bar, (S) over bar were accurately determined by using their t ravel times from local events. They evidenced that the uppermost mantl e underneath the Tibetan Plateau was similar to that of the ordinary c ontinental mantle. The reflection profile from INDEPTH-I furnishes con vincing evidence that the Indian crust penetrates into the Tibetan low er crust. The results from teleseismic waveform inversion reveal that the Moho discontinuity dips northwards, and an offset of Moho occurs n ear Bangong suture. The fact that materials within the Tibetan Plateau escape laterally has been proposed by several authors. Recent data an d studies provide further convincing evidence that eastward mass trans fer does occur, and their paths and natures are investigated. Some aut hors suggested that the large strike slip faults (Kun Lun, Xianshuihe) in the eastern plateau may be related to the lateral extrusion. Howev er, most of the strike slips are left-lateral, and extrusion could not occur without right-lateral strike slips. Recent observations of the focal mechanisms and geological structure indicate that the earthquake s in the Yanshiping-Changdu belt are left-lateral strike slip. It is t he southeast zone of the left-lateral slip faults in the eastern Tibet an plateau. Geological and seismological evidence show that the Bencuo -Jiali belt is the only large right-lateral fault in the eastern plate au. It was proposed that the present eastward extrusion occurs between the Yangshiping-Changdu left-lateral strike slip and the Bencuo-Jiali right-lateral strike slip. The other left-lateral strike slips north of the Yangshiping-Changdu belt are considered to be the fossils of th e ancient flow paths.