THERMAL AND UNROOFING HISTORY OF THE LHASA AREA, SOUTHERN TIBET - EVIDENCE FROM APATITE FISSION-TRACK THERMOCHRONOLOGY

New fission track (FT) thermochronologic data from plutonic rocks at t he Gangdese magmatic arc support the view that there was a pulse of ra pid cooling (> 80-degrees-C/m.y.) and unroofing (> 2 mm/m.y.) around 2 0-15 Ma in the Quxu area, southern Lhasa terrane. The average cooling rate prior to 20 Ma and post to 15 Ma was only about 5-6-degrees-C/m.y . in this area. A fast cooling and unroofing event was not detected in other studied areas in the Lhasa terrane. Average cooling rates of 4- 10-degrees-C/m.y. since the India-Asia continental collision (45-0 Ma) and unroofing rates of 0.1-0.3 mm/y can be deduced for these areas fr om the fission track data and previously reported Ar-40/Ar-39 data. Th ese relatively slow unroofing rates can be viewed as a regional ''back ground'' in this mountain-building area, and the rapid unroofing recor ded in the Quxu area as a ''pulse'' superimposed on it. Apatites from the granitic gneisses of the Nyainqentanglha range, west of the Yangba jain graben, yield exceptionally young FT ages of 3.3-5.1 Ma. These ag es together with Ar-40/Ar-39 data, allow cooling rates of 20 to approx imately 200-degrees-C/m.y. (9-0 Ma) to be deduced at several locations . These young ages and fast cooling rates are interpreted to be the re sult of recent rapid uplift and exhumation of the foot-wall of a major normal-faulting detachment zone accompanying the Yangbajain graben.