Cenozoic Geomorphic Evolution Of The Shiquanhe Area, Western Tibetan Plateau : Insights From Thermochronological Ages

The uplift and exhumation of the Qinghai-Tibet Plateau are considered to be the triggering factors of many geological events since Cenozoic,and also have formed the unique topographic and geomorphological characteristics of Tibet,which is quite special in western Tibet for the development of several large-scale faults and the existence of external and internal drainage at the same time.Previous studies were mainly focused on the eastern and central Tibet,the exhumation history of western Tibet still remains largely unknown.I conducted systematic geochronological and low-temperature thermochronological studies to understand the uplift processes and exhumation mechanisms of western Tibetan Plateau,and these results were modelled with HeFTy and QTQt program for thermal history.The zircon U-Pb ages from intrusive rocks of Shiquanhe and Gegyai areas ranges from 118.3±3.0 Ma to 160.2±3.8 Ma,which suggests that they were formed in themiddle Jurassic-early Cretaceous.The apatite(U-Th)/He(AHe)ages display three groups according to their sampling locations:(1)The sample collected far away from faults and rivers has a weighted mean age of 37.69±1.06 Ma.The modelled thermal history indicates a rapid cooling stage during 45-40 Ma which was followed by a slow cooling since 40 Ma,resembling to that of central Tibetan Plateau.(2)The AHe ages of samples collected at the hanging wall of Zaduo thrust has weighted mean ages of 23.06±0.63 Ma and 23.73±0.65 Ma,respectively.The modelled thermal history reveals a relatively fast cooling episode around 25 Ma,which is related to the slip of the Zaduo thrust,and constrained the youngest movement of the forward thrusting nappe from Jiagang thrust to Zaduo thrust.(3)The weighed mean AHe ages of samples in a valley near Gegyai are 17.4±0.49 Ma and 14.91±0.46 Ma.The modelled thermal history implies a 17-15 Ma fast cooling stage,consistent with moderate cooling from 19-17 Ma reported in a valley south to Bangong Lake,near Shiquanhe,suggesting that this phase of relatively fast cooling is widely documented in this region.We interpret the regional exhumation as responding to surface isostatic uplift triggered by the delamination of deep-lithosphere in western Tibet,which is implied by the abundance of potassic volcanic rocks dated at 24-16 Ma.The ages and modelled thermal history discussed above suggested that the Shiquanhe-Gegyai area,western Tibet,share a similar uplift history with central Tibet,and have reached a relatively high elevation with low topographic relief before 40 Ma.The great relief was related to regional uplift and exhumation induced by local tectonic activity and deep-crustal motion.The western Tibet was partially influenced by external drainage,which was an important factor of the development of great topographic relief in Shiquanhe-Gegyai area.