Initial Cenozoic Exhumation of the Northern Chinese Tian Shan Deduced from Apatite (U-Th)/He Thermochronological Data

The present topography of Tian Shan is related to the India-Asia collision, whereas the mechanisms for the topographic growth of Tian Shan remain at the center of debate partly due to the poorly constrained onset timing of the Cenozoic tectonic deformation. Our new apatite (U-Th)/He (AHe) data in the northern Chinese Tian Shan suggest that rapid cooling commenced at 20.2 +/- 4.9 Ma along the northern margin, which is consistent with published fission-track data from the same area and confirms that the youngest component of fission-track has been totally annealed. Moreover, AHe data from the interior mountain suggest that enhanced cooling began at 28.0 +/- 2.3 Ma, which is slightly older than that from the northern edge of the mountain. Although thermochronological data suggest that both the interior and northern margin of the northern Chinese Tian Shan have undergone rapid cooling since the late Oligocene-early Miocene, well preservation of relict low-relief surfaces along the northern rim of the northern Chinese Tian Shan and published thermochronological data indicate that the northern Chinese Tian Shan may have experienced differential exhumation in the Cenozoic. Combining the thermochronological and geomorphological evidence, we propose a progressive northward growth model for the northern Chinese Tian Shan. During the late Oligocene and early Miocene, compressive deformation derived from the India-Asia collision have arrived at the northern Tian Shan to reactivate both the interior mountain and its northern margin, while intense exhumation was concentrated in the interior mountain range. Then, deformation extended northward into the foreland basin that may be initiated in the middle-late Miocene.