Folded fluvial terraces and the deforming of a new uplifted region in the mountain front the Qilian Shan Mountain, China

Abstract:

How the Tibetan Plateau is extended is one of the key problems to understand the earth crust evolution in the frame of plate tectonics. A newly uplifting area, the Dahe region, locating between the Yumu Shan Mountain and the Qilian Shan Mountain, in the northeastern Tibetan Plateau, would supply us a fresh sight on the process that how the plateau is extended to a new region. The Dahe region was a relatively depressing or stable area before late Pleistocene, and received thick fluvial sediment derived from the Qilian Shan in the south. In late Pleistocene, the old depositing surface Sp (alluvial fan surface) was deeply cut by the Dahe River. Below the old depositing surface, four staircases of strath terraces (strath is the old fluvial deposition) are formed by the Dahe River, and each terrace surfaces are buried by aeolian loess. By the OSL dating on overlying loess on the terraces and correlating to climate records, we obtain formation ages (terrace surface abandoning time) of the four terraces (from high to low): 128.2 ±9.8 ka, 109.6±20.8 ka, 96.3 ±9.0 ka, and 15.9 ±2.5 ka. We obtain the extrapolated Sp age of 160 ±25 ka, which represents the time when the fan depositing was end and river cutting and eroding was started in the Dahe region. By the uplifted terrace staircases and warped long profiles of terraces, we can find that the region is not only experiencing regional uplifting but also folding deformation. Through analyzing the geometry of the deforming terrace surfaces, we propose that a new blind thrust fault was derived from the main decollement in the upper crust, and thus the growing fault deduced the uplift of the Dahe region and the folding near the fault tip. The growth of the Dahe region, which is sandwiched by the Yumu Shan and the Qilian Shan, both uplifted millions years ago, suggests that northeastern extending of the plateau is in the form of new fault-fold system growing in mountain front and back.