New fission track (FT) thermochronologic data from plutonic rocks at the Gangdese magmatic arc support the view that there was a pulse of rapid cooling (> 80°C/m.y.) and unroofing (> 2 mm/m.y.) around 20-15 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°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°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 from the fission track data and previously reported 40Ar/39Ar data. These relatively slow unroofing rates can be viewed as a regional “background” in this mountain-building area, and the rapid unroofing recorded in the Quxu area as a “pulse” superimposed on it. Apatites from the granitic gneisses of the Nyainqentanglha range, west of the Yangbajain graben, yield exceptionally young FT ages of 3.3–5.1 Ma. These ages together with 40Ar/39Ar data, allow cooling rates of 20 to ∼ 200°C/m.y. (9-0 Ma) to be deduced at several locations. These young ages and fast cooling rates are interpreted to be the result of recent rapid uplift and exhumation of the foot-wall of a major normal-faulting detachment zone accompanying the Yangbajain graben.
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