Tertiary deformation history of southeastern and southwestern Tibet duringthe Indo-Asian collision

Geologic mapping and geochronological analysis in south,vest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary cru stal shortening along the classic Indus-Tsangpo suture in the Yalu River va lley The older event occurred between ca 30 and 23 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused e xtensive denudation of the Gangdese batholith in its hanging wall and under thrusting of the Xigaze forearc strata in its footwall. Examination of timi ng of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initi ation and de development of north-south shortening in the eastern Kunlun Sh an of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tag h fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionall y synchronous initiation of crustal shortening in and around the plateau ma y have been related to changes in convergence rate and direction between th e Eurasian plate and the Indian and Pacific plates. The younger thrusting e vent along the Yalu River valley occurred between 19 and 10 Ma along the so uth-dipping Great Counter thrust system, equivalent to the locally named Re nbu-Zedong thrust in southeastern Tibet, the Backthrust system in south-cen tral Tibet, and the South Kailas thrust in southwest Tibet. The coeval deve lopment of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal wea kening of the north Himalayan and south Tibetan crust, due perhaps to therm al relaxation of an already thickened crust created by the early phase of c ollision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya