An early unroofing in northeastern Tibet? Constraints from Ar-40/Ar-39 thermochronology on granitoids from the eastern Kunlun range (Qianghai, NW China)

In order to better constrain the mechanisms accommodating the India-Asia co llision along the whole Kunlun range (northern Tibet), which is essential t o better understand the evolution of the entire deformations area, a Ar-40/ Ar-39 thermochronological study has been carried out on a suite of granitoi ds collected in the vicinity of Golmud (Eastern Kunlun range). Biotite and K-feldspar have been analysed and the cooling histories of the latter have been modelled. The ranges of Ar-40/Ar-39 data (Bt = 128-257 Ma; Kf = 102-22 8 Ma) are similar to previous Kb/Sr and K/Ar data, indicating both a middle -Paleozoic unit and a Mesozoic unit, including a Triassic series and a Jura ssic-Cretaceous series. This suggests that the two geological units previou sly reported in the Western Kunlun are also present in the eastern part of the range, suggesting that Eastern and Western Kunlun ranges might have bee n left laterally offset by as much as 500 km along a strike-slip fault pres ently parallel to the Altyn-Tagh fault. The cooling histories based upon K- feldspar modelling show that samples from the southern part of the range re corded an important cooling event (9-15 degrees/Myr) around 30 Ma, which is thought to reflect an increasing rate of denudation related to the tectoni c activity in this area; minimum denudation rates of 0.2-0.3 km/Ma have bee n estimated. In contrast, samples from the northern part of the range did n ot record any variation in their cooling history. This heterogeneous denuda tion across the range is thought to be related to ramp stacking and normal faulting with associated uplift mainly affecting the southern part of the r ange, suggesting that the Eastern Kunlun underwent an important crustal thi ckening period 30 Ma ago. This crustal thickening event is suggested to be related to the motion along the great Asian strike-slip faults in a way whi ch remains problematic but possibly associated to the Cenozoic propagation of the Altyn-Tagh fault. Finally, unroofing in the Eastern Kunlun range app ears to predate that in the Western Kunlun range where it is suggested to h ave started around 20 Ma ago. This diachronous denudation could be related to a higher denudation rate in Western Kunlun range and it is thus highly p robable that the important unroofing event outlined around 20 Ma ago in the Western Kunlun has also affected the eastern part of the range, even thoug h the signature is not available at the moment. (C) 1999 Elsevier Science B .V. All rights reserved.