Abstract:

The Cenozoic tectonic and geomorphological processes of the west Qinling as a part of QinghaiTibetan plateau could provide some constraints on the uplift and deformation model of the QinghaiTibetan plateau. Based on the analysis of the Cenozoic sedimentary strata and its deformation features, regional fault framework and its kinematics, and geomorphological records, we could reach the following preliminary understanding. (1) Based on their rock composition, spatial distribution and tectonic deformation characteristics, we can divide the Late MesozoicCenozoic red bed sedimentary strata in the west Qinling into the Late JurassicEarly Cretaceous, the Late CretaceousPaleogene and the Neogene three tectonic strata units, which corresponds to three tectonic evolution stages. (2)The faultthrusting and foldshortening of K2E strata in the west Qinling occurred at the end of Eocene, synchronous with the crust shortening and thrusting in the QinghaiTibetan plateau, which may indicate that the compressive tectonic stress from the collisional convergence between the India plate and the Eurasia plate have been transmitted to the west Qinling. (3)The west Qinling had undergone a relatively tectonic stable period since the Neogene, in which extensive erosion and planation occurred, and finally main planation surface and associated Neogene sedimentary strata consisting of red conglomerate and red clay were 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 in the eastward expansion of the QinghaiTibetan plateau. (4) The structural deformation transition from thrustingshortening to extensionstrike slip at 1314 Ma or 8 Ma in the south Tibet have not appeared widely 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 GPS observational data and tectonic stress directions deduced by earthquake mechanism solution showed eastward motion of the west Qinling block and Edirected tectonic stress, which may reflect that eastward creep of low crust drags upper crust movement and uplift. The gradient zone of the crust thickness of the west Qinling could be interpreted as the result of eastward expansion creep of the thickened lower crust, which controlled main landform features of the west Qinling.

Key words:  northeast margin of QinghaiTibetan Plateau; west Qinling; Cenozoic red strata; fault zone; tectonic deformation; planation; uplift