Oligocene-Early Miocene Topographic Relief Generation of Southeastern Tibet Triggered by Thrusting

The timing and mechanisms of uplift in southeastern Tibet remain disputed. To address this debate, we conducted structural and morphological analyses of the Yulong thrust belt; we also reconstruct the cooling and exhumation history of the Jianchuan basin in the hanging wall of the thrust system using inverse thermal modeling of apatite fission-track and (U-Th)/He thermochronology data. Our results provide evidence for 2.3-3.2km of rapid exhumation in the Jianchuan basin between 28 and 20Ma, followed by limited exhumation of less than 0.2km since then. The magnitude of basin exhumation is consistent with the present-day topographic step of 1.8-2.4km across the Yulong and Chenghai thrust belts, as shown by morphometric analysis. We thus infer that the present-day morphology of the southeastern margin of Tibet results partly from thrusting along the Yulong thrust belt during the Late Oligocene-Early Miocene. This structure may be the southwest continuation of the Longmen Shan thrust belt, offset by the Xianshuihe fault in the Late Miocene. On a regional scale, the approximate synchronicity of exhumation in the hanging walls of the Yalong-Yulong and Longmen Shan thrust systems indicates that widespread crustal shortening and thickening took place in southeastern Tibet during the Late Oligocene-Early Miocene. Plain Language Summary The timing and mechanisms for the high-elevation, low-relief landscape in southeastern Tibet remain at the center of debate. Using apatite fission track and apatite (U-Th)/He thermochronology and thermal modeling, this study sheds new lights on this issue by reconstructing the cooling and exhumation history of the Jianchuan basin, western Yunnan. One major finding is that the Jianchuan basin experienced rapid exhumation during 28-20Ma at a rate of 0.57-0.80km/Myr, coinciding with the absence of coeval sedimentation within the basin. Furthermore, morphometric and structural analyses point out a reasonable causal link between production of the topographic relief in the SE Tibet, exhumation of the Jianchuan basin, and thrusting of the Yalong thrust belt. These results allow us to propose a regional-scale tectonic scenario of widespread crustal shortening and thickening took place in southeastern Tibet during the Late Oligocene-Early Miocene.