MIDDLE MIOCENE ONSET OF RAPID RIVER INCISION ALONG THE CENTRAL ALTYN TAGH FAULT SYSTEM (NORTHERN TIBETAN PLATEAU) IN RESPONSE TO TECTONIC REORGANIZATION

Rapid river incision or rock exhumation since Miocene has been identified in both the eastern and the northeastern Tibetan Plateau through thermochronological dating of samples collected along vertical transects, and is interpreted as either the result of tectonic loading or the response to climate change (e.g., high monsoon precipitation in the middle Miocene). By far, it remains difficult to tell whether the tectonics or the climate was the major factor controlling the Miocene rapid river incision along the margins of the Tibetan Plateau, because the well-studied eastern and northeastern Tibetan Plateau are both tectonic active and obviously impacted by Asia monsoon system at present. In this study, we attempted to solve the problem by investigating the history of river incision in the Altyn Shan, which is far away from the areas impacted by Asian monsoon system, and tectonically controlled by the ~1600-km long, left-lateral, active Altyn Tagh fault system defining the northern margin of the Tibetan Plateau.

Based on geomorphological analysis, we discovered some High Elevation Low Relief Surfaces (HELRSs) in the Altyn Shan. These HELRSs are now dissected by deeply-incised, narrow rivers flowing northward into the Tarim Basin, with the local relief up to 2000 m. As the geographic condition is very tough there and it is nearly impossible to collect samples along a complete transect from the top of the HELRSs down to river outlets, we instead collected a river sand sample near the outlet of a river that solely drains the area of HELRSs. We conducted Apatite U-Th/He (AHe) dating for the sample to get a full cooling history of the selected drainage basin. Thirty apatite grains were dated and the ages range from 5.9 Ma to 144.1 Ma with most ages <20 Ma. We used the Bayesian simulation to model the age-elevation/depth relationship and bedrock erosion rate through time based on dated detrital ages and DEM. The result indicated >30 times increase in erosion rate from ~10 m/Ma to ~350 m/Ma at ~15 Ma, indicating rapid river incision since the middle Miocene. As our study area is devoid of influence of Asian monsoon system, we interpreted the rapid river incision as the result of increased elevation and relief by intense tectonic loading, rather than high monsoon precipitation by climate change. Together with publish results of the middle Miocene events nearby, our result highlighted a middle Miocene tectonic reorganization of the Altyn Tagh fault system, which finally established the present-day tectonic framework of the northern Tibetan Plateau.