EXPLOSION SEISMIC P AND S VELOCITY AND ATTENUATION CONSTRAINTS ON THE LOWER CRUST OF THE NORTH-CENTRAL TIBETAN PLATEAU, AND COMPARISON WITH THE TETHYAN HIMALAYAS: IMPLICATIONS ON COMPOSITION, MINERALOGY, TEMPERATURE, AND TECTONIC EVOLUTION

Abstract

P and S velocity and attenuation estimates in the lower crust are obtained from a set of wide angle reflection-refraction profiles in the region of active tectonics at the NE edge of the Tibetan Plateau and discussed together with respect to similar data at its Himalaya-south Tibet edge. The quality factor is estimated in the lower half of the crust by accounting for the differential effect on amplitude-frequency observed between waves of different penetrations, and both in P and S modes. Attenuation values allow to exclude a significant proportion of partial melt and to estimate the homologous temperature, ratio of in situ to solidus absolute temperatures. The latter depend on the physical conditions being of dry, wet or dehydration melting, which are found different among the regions of the northern Bayan Har and northern Qang Tang boundaries between blocks, as well as the Tethyan-Himalayas, south of the Indus-Tsangpo suture. Their in situ temperatures differ also as estimated from their different Vp for a similar felsic composition. Joint measurement of several parameters, Vp, Vs, Qp and Qs reveals the composition, mineralogy, temperature and hydration conditions of the lower half of the thickened crust of Tibet that may be discussed in terms of evolution. The material presently in the thickened crust, even its lower part, has a felsic composition, upper to middle crustal lithology, and the temperature conditions estimated suggest that basic material that could have underlain it could be eclogitized and not appear anymore above the seismic Moho. Under northern Qang Tang, the felsic material in the lower half of the crust appears as hot and dry. Its burial may have occurred earlier or may have been moderate in the postcollisional phase. This is consistent with a model of indentation of the Qang Tang crust by an originally thinner Bayan Har crust to bring part of its crust to greater depth, suggested from imaging the crustal architecture. Under northern Bayan Har, the material in the lower half of the crust appears as felsic, at low temperature and not dry conditions. This is evidence that it has been transported from a shallower depth, and this recently enough not to be yet dehydrated and temperature equilibrated in a conductive geotherm. It supports a model of recent overriding of the middle crust of the north Kun Lun block to the north independently suggested from the image of crustal architecture. The Tethyan Himalayas case appears bracketed by these two cases in northern Tibet for Vp and temperature conditions, but shows highest attenuation in the lower crust that is colder but less dry than under northern Qang Tang. © 2005 Elsevier B.V. All rights reserved.