Structural and(U-Th)/He thermochronological constraints on the Longmen Shan thrusting-gliding klippes, eastern margin of Tibetan Plateau

The NE-striking Longmen Shan thrust belt(LSTB) in the eastern margin of the Tibetan Plateau is located at the junction of the Songpan-Ganze Terrane and the Yangtze block. This belt experienced the Late Triassic Qinling-Dabie and Songpan-Ganze orogenesis and the Cenozoic eastern propagation of the Tibetan Plateau. A series of klippes were produced along the foreland thrust belt of the LSTB. Traditional research proposed that the klippes are the front zone of the modern NE-trending Longmen Shan foreland thrust belt, while recent studies suggested that the klippes might be the result of Mesozoic thrusting of the LSTB. Nonetheless, there still remains a lack of crucial constraints regarding the emplacement time and formation mechanism of the klippes. In this study, we conducted systematic structural measurements on the Tangbazi klippe and Woniuping klippe, and performed petrographic analysis for the brecciated limestone. The results confirm a typical structural breccia with top-to-the-SSE kinematics of thrusting at the bottom of the klippes. Structural kinematics reveals the ultimate emplacement of the klippes was under NNE to SSW gliding, and the tail-like brecciated limestone in the southeast edge of the Tangbazi klippe was produced by the Pliocene gravitational collapse. Systematical samples were collected from both the hanging wall and footwall of the klippes and apatite and zircon were separated for(U-Th)/He dating. The(U-Th)/He ages of apatite from the Woniuping klippe are dated at 66.8–72.3 Ma, corresponding to the rapid uplift of the LSTB during the Late Cretaceous; the(UTh)/He ages of apatite from the footwall of the klippe are dated at 33.2–25.2 Ma, corresponding to the rapid uplift and denudation of the LSTB during the Oligocene. Integrated structural analysis and geochronological constraints, the northern segment of the LSTB, including the Tangwangzhai and Longwangmiao thrust complexes, might be produced by in-sequence thrusting during the Late Triassic to Early Jurassic. The northern segment of the LSTB and the northern Sichuan region experienced rapid uplift during the Late Cretaceous. During the Late Eocene to Early Miocene, gravity-driving klippes gliding from the Tangwangzhai and Longwangmiao thrust complexes toward the Sichuan basin occurred along the Silurian mudstone or phyllite and formed gravitational collapse.