PARTITIONING OF CRUSTAL SHORTENING IN THE NORTHERN TIBETAN PLATEAU: CONSTRAINTS FROM THERMOCHRONOLOGY STUDIES

The accommodation of India-Asia collision in the Northern Tibetan Plateau has been a dynamic and controversial topic for decades. One area of contention focuses on the timing and mode of strain partitioning between strike-slip motion along the >1,600 km long Altyn Tagh fault (ATF) and thrust faulting in the Qilian Shan fold-thrust belt. Here we present results of multiple-thermochronology studies, including apatite and zircon (U-Th)/He, apatite fission track analysis and ⁴⁰Ar/³⁹Ar modeling, from six vertical transects across the Qilian Shan. The thermochronology studies revealed Eocene deformation restricted to the south margin of Qilian Shan and then the northward sweeping of Neogene extensive exhumation that started in the south of Qilian Shan prior to 18-16 Ma and in the central and north Qilian Shan at ca.12-8 Ma. The onset of basement rapid exhumation—a proxy for range-bounding thrust faulting—along with Holocene slip rates of ATF, was applied to constrain crustal thickening. The estimation is equal to the post-Middle Miocene slip motion along ATF, implying the predominance of distributed crustal thickening since 18-12 Ma. In comparison to the total displacement of ATF, the apparent deficit of crustal shortening absorbed through crustal thickening in the Northern Tibetan Plateau requires a mechanism of accommodating the India-Asia collision out of plateau before 18 Ma. This change in regional tectonics is synchronous with increasing confinement of plate boundaries.