Mapping out the recent exhumation pattern in Lhasa Terrane, South Tibet, with multiple thermochronometers on modern detritus

Detrital zircon fission track (ZFT) and Ar-Ar (K-feldspar) analysis of the Lhasa Terrane, Southern Tibet, have both displayed a significant age discrepancy between the Lhasa River and Nyang River. Detritus collected from the Lhasa River drainage produced a high portion of young ZFT age population (peak ages < ca. 15 Ma, up to 80%) while from the Nyang River only 10% of the grains displayed this young age component. To clarify the origins of the young thermal ages, the ZFT and Uranium-Lead (U/Pb) double dating analysis were conducted on the same zircon grains from each catchment. Grains from the Lhasa River generated a distinct age period which clearly represents a recent exhumational episode (i.e. younger than ca. 15 Ma). This young exhumation, however, is not observed within the Nyang River. Instead, the grains from Nyang River exhibited strong influences inherited from the basement strata and suggest that the Nyang River catchment has not experienced large scale interferences of surface process for a prolonged period of time. Although the two catchments have shown different records of recent exhumational activities, the ZFT age population analysis imply that they might have shared similar thermal history before 20 Ma. Previous studies in conjunction with our thermal age results (ZFT, U/Pb and apatite fission track ages) show that a large portion of the studied catchments has gone through similar cooling paths under cooling rates < 10 °C/Myr for the past 40 Ma. Evidently, the Ar (K-feldspar) age comparisons of upstream and downstream detritus strongly suggest that the majority of the young grains came from the western boundary of the Lhasa River basin, where situated the active Gulu Rifting Belt. Our results show that the E-W extension has played an important role in the denudation history of the central Lhasa Terrane. Thermal age results from multiple thermochronometers indicate that the extension might have superimposed the accelerated cooling rates to > 20 °C/Myr along the rifting belt and induced focused exhumation in the structurally active region.