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 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.