Thermotectonic history of the Longshou Shan: From Paleozoic Tethys subduction to Cenozoic Tibetan Plateau growth

Abstract

Constraining exhumation and tectonic processes along an orogenic plateau's boundary provides important insights into the mechanisms leading to plateau expansion and crustal evolution. The Longshou Shan thrust belt (LSSTB) is located in the foreland of the northern Qilian Shan thrust belt, which is commonly regarded as the northeastern margin of the Tibetan Plateau. The LSSTB is thus ideal for decoding the recent expansion of the Tibetan Plateau by tracking deformational pattern at its northeastern margin. In this study, the spatiotemporal characteristics of exhumation and deformation along the LSSTB are investigated by detailed analysis and numerical modeling of published and new thermochronological data. Five Proterozoic basement and intrusion samples yielded Cretaceous apatite fission-track central ages (126–74 Ma, with mean track lengths of 12.6–13.3 μm), and Late Cretaceous to Eocene apatite (UTh)/He mean ages (84–51 Ma). Inverse thermal history modeling reveals multi-stage exhumation of the LSSTB in the Permo-Triassic, late Mesozoic, Paleogene, and post-middle Miocene. Permo–Triassic exhumation hints at a > 250 Ma-old peneplain surface that may have formed in response to the closure of the Paleo-Tethys and Paleo-Asian oceans. Late Mesozoic exhumation likely resulted from intracontinental extensional deformation associated with tectonic processes at the Eurasian continental margin. Exhumation during the Paleogene was likely triggered by the India-Asia collision. Post-middle Miocene periods of uplifts along the reactivated Longshou Shan thrusts (no later than 10 Ma and 5 Ma on the southern and northern Longshou Shan Thrust, respectively) were driven by the northeastward expansion of the Tibetan Plateau. Our results support the LSSTB as a long-lived block boundary since the Permo-Triassic and an emerging plateau boundary that has lately been reactivated by the Tibetan Plateau expansion.