Structural and Geochronological Constraints on the Early Mesozoic North Longmen Shan Thrust Belt: Foreland Fold-Thrust Propagation of the SW Qinling Orogenic Belt, Northeastern Tibetan Plateau

摘要

The NE striking Longmen Shan Thrust Belt (LSTB) forms the northeastern margin of Tibetan Plateau and merges northward with the southwestern Qinling orogenic belt. The LSTB is characterized by the Late Cenozoic SE-ward overthrusting and a current 60- to 70-km crustal thickness. Structural constraints link the Late Cenozoic deformation and crustal thickening of the LSTB to the formation of the Tibetan plateau. However, although older Mesozoic deformations are widely identified, including Triassic shortening, they remain underappreciated in the development of the LSTB as the Cenozoic history has been the recent research focus. Thus, it is important to document the early Mesozoic tectonics of the LSTB to better understand the Cenozoic orogeny along the northeastern margin of the Tibetan plateau. Here we synthesize existing and new mapping, structural analyses and geochronology of three regions of the northern LSTB: the Bikou, Tangwangzhai, and Longwangmiao regions. Our observations suggest that the Bikou, Tangwangzhai, and Longwangmiao thrust complexes have similar kinematic styles of southward foreland fold-and-thrust propagation. New Ar-40/Ar-39 dating results show that these thrust complexes have deformational ages at circa 237Ma to circa 180Ma, respectively, and diachronously emplaced before 201-174Ma. Triassic shortening structures involving metamorphic rocks are unconformably overlain by Lower to Middle Jurassic strata. We propose a new kinematic model in which Middle to Late Triassic, top-to-the south, in-sequence imbricate thrusts developed in the northern and central LSTB, suggesting that the LSTB was initially built in the early Mesozoic by the southward propagation of the foreland belt of the SW Qinling orogenic belt.Plain Language Summary The Early Mesozoic Longmen Shan is actually a part of the SW Qinling orogenic belt, because both of them have consistent thrusting kinematics within the orogenic thrust system. The present Longmen Shan thrust belt in the eastern margin of the Tibetan Plateau is the result that the Cenozoic deformation overprinted over the Early Mesozoic deformation.