The Cenozoic Tectonic Evolution of the West Qinling: Constraints on the Uplift and Deformation of the Qinghai–Tibet Plateau

The Cenozoic tectonic and geomorphologic processes of the West Qinling as a part of Qinghai–Tibet plateau could provide some constraints on the uplift and deformation model of the Qinghai–Tibet plateau. On the basis of the analysis for the Cenozoic sedimentary basin and its deformation features, regional fault frameworks and its kinematics, and geomorphologic records, the following preliminary conclusions could be drawn as follows: (1) Late Mesozoic–Cenozoic red bed sedimentary strata in the West Qinling, based upon their rock composition, spatial distribution, and tectonic deformation characteristics, can be divided into three tectonic lithostratigraphic units, that are Late Jurassic–Early Cretaceous, Late Cretaceous–Paleogene, and Neogene, which are corresponded to three tectonic evolution stages; (2) The fault-thrusting and fold-shortening of K2–E in the West Qinling occurred at the end of Paleocene, synchronous with the crust shortening and thrusting in the entire Qinghai–Tibet plateau, which may indicate that the compressive tectonic stress from the collision convergence between the Indian plate and Eurasian plate had been transmitted to the West Qinling; (3) The West Qinling had entered a, relatively, stable tectonic period since the beginning of Neogene in which extensive erosion and planation occurred, and main planation surface and associated Neogene sedimentary strata consisting of red conglomerate and red clay finally formed before 3.6 Ma. This planation surface at present-day decreased gradually from 3200 m in the west to 1200 m in the east, which can be interpreted as continuous uplift of the West Qinling in the eastward expansion of the Qinghai–Tibet Plateau; (4) The structural deformation transition from thrusting-shortening to extension-strike slipping at 13–14 Ma or 8 Ma in the Southern Tibet have not appeared wide in the West Qinling, whereas fault thrusting predominated the total Cenozoic tectonic history and left-lateral strike-slip only occurred in the northern boundary fault of the West Qinling, which may indicate that block slip and thrusting coexisted in the West Qinling; (5) The regional displacement field determined by the Global Positioning System (GPS) observational data and tectonic stress directions deduced by earthquake mechanism solution showed the eastward motion of the West Qinling block and E-directed compressive tectonic stress, which may reflect that eastward creep flow of the lower crust drags brittle upper crust and uplift. The gradient zone of the crust thickness of the West Qinling could be interpreted as result of eastward expansion of the thickened lower crust, which controlled main landform features of the West Qinling.