The Uplift Of Mijia Shan And Its Link To The Geometrical Complexity Of Haiyuan Strike-slip Fault

The geometrical complexities have a significant controlling effect on the activity of the strike-slip fault, especially on the generation and occurrence of large seismic events. Thus, it could be evaluated as a tectonic syntaxis along the active faults. The geometrical complexities are common on large strike-slip faults in the world, for instance, the San Andreas Fault in America,the Anatolian Fault in Turkey, as well as the active faults along northeastern Tibetan plateau.Due to the intersection between different branches of faults, a series of uplift zones have developed along the Haiyuan fault in northeastern Tibetan plateau, among which the Mijia Shan is the most prominent one and could be evaluated as a tectonic syntaxis. Most previous research has concentrated on the deformation of the fault in the horizontal perspective, while less attention has been paid on the uplift rate and mechanism of these uplift zones and the relationship between tectonic uplift and fault geometric complexity.Based on integrating bedrock geology, earthquake reflection profile, as well as GPS rates published adjacent to the study site, we have ascertained regional tectonic framework in an attempt to explore the uplift mechanism of the Mijia Shan. We have recognized at least twelve levels of fluvial strath terraces capped with no more than 5 meters of alluvial deposits along the western bank of the Yellow River, also on the eastern side of the Mijia Shan. In order to constrain the uplift rate of the Mijia Shan, we measured these terraces using RTK and UAV survey. Based on detailed field mapping, we collected both quartz-rich pebbles and fine silt for cosmogenic radio nuclide(CRN) and optimally stimulated luminescence(OSL) dating to constrainthe terrace formation ages. Quartz-rich pebbles have been sampled through the man-made pit with depth-profile method at the site where is pretty flat and supposed to have suffered from minor erosion. The CRN age results were corrected in terms of inheritance and shielding by loess.Further comparisons of terrace ages with deep ocean oxygen isotopic curves were made to reveal no direct link between the terrace development and climatic variations. Thus, we think tectonic uplift act as the first-order factor for terrace development. Based on terrace elevation above the present Yellow River and terrace ages from various levels, we confidently derived an average uplift rate of 2mm±0.34/yr, which is well consistent with that calculated from the GPS horizontal movement imposed on the fault plane by the far-field bock motion. We argue that the uplift of Mijia Shan resulted from the oblique shear of positive flower in the deep crust of theleft-lateral Haiyuan Fault. Owing to the remarkable denudation of the Mijia Shan and the superposed drainage network of highly effective transport capacity, we tend to believe that mass unloading and isostatic rebound concurrent with tectonic deformation might have exerted an obvious positive effect on the uplift of Mijia Shan, that is, magnifying surface uplift resulting from tectonic deformation.