Surface Geomorphological Processes In The Northern Piedmont Of Tian Shan Responding To Climate Change And Tectonic Deformation Since Middle Pleistocene

Crust of the northern pediment of Tian Shanwas tilted and uplifted from south to north with continuing and intense deformation since middle Pleistocene. Mass ofconglomerates and loess mantled in northern blank of Tian Shan and a large scale of fluvial fan developed to accumulate the elevation of the pediment. With the drier climate, incision was limited in the stiff canyon trenched by the main stream. The surfaces keep a stable statue of equilibrium between incision and accumulation. Tian Shan had propagated to north with the impact by climate change and tectonic deformation in deferent time scale and undergone intercontinental orogeny since middle Pleistocene.Two styles of deformation existed in the crust of northern pediment of Tian Shan on vertical direction. One is thrusting that the crust of Tian Shan overthrusted to the Zhunger basin along the South Zhunger Fault (SZF). The other is the upper layer of rock moved to north along the deep decollement surface and the rock folded and thrusted as the same time. As a result, the crust of the pediment of Tian Shan was tilted and uplifted from south to north. There are three row of folds‐thrusts belts in north blank of Tian Shan. The Qingshuihe‐Shitizi Fault (QSF) in the first row of folds‐thrusts belt propagated to west and made the surface uplift. The Huoerguosi folds, as well as Manas folds and Tugulu folds, of the second rows of folds‐thrusts belt propagated from the middle to the both side. The three folds uplifted and consolidated to a low mountain. The Dushizi folds, as well as Halaande folds and Anjihai folds, of the third rows of folds‐thrusts belt propagated from the middle to the both side. The three folds uplifted to became three isolated mountain. The rock of upper crust in the northern pediment of Tian Shan propagated along the NW direction with the folding and thrusting deformation due to multi‐level subduction that the crust of the Zhunger Basin subducted to the crust of Tian Shan along the SE direction.The catchment and the fan is a coupling system. Mass of loose materials covered the upper catchment. In the warm and humid climate stage, the storage materials in mountain were transported to the main stream and carried to the front of mountain. With repeatedly transporting, the fan was formed. For the drier climate, the runoff was so lack that the surface could be hardly eroded. Consequently, incision was limited in the stiff canyon trenched by the main stream and the incision surface only was10percentage of the whole surface. The surfaces keep a stable statue of equilibrium between incision and accumulation. The pediment of the north Tian Shan was tilted and partitioned several parts by the folds and thrusts. Materials aggraded in the partitioned basins and surface accumulated elevation of the basins. On the other hand, partitioned basins were distorted accompanying with deformation of thrust faults and folds. As a result of shortening, the partitioned basins tilted along the northern flank of Tian Shan. Uplift of crust tilt almost totally converted to uplift of surface because of slight erosion. The pediment near the mountain became higher as a part of Tian Shan. As a result, the Tian Shan propagated to north.Study of downcutting of the Kuitun River shows that intense river downcutting in northern pediment of Tian Shan controlled by tectonic deformation and drier climate change at the long time scale, and also controlled by warm and humid climate change at the short time scale. The slope was supplied by long‐term deformation and the additional river power was supplied by the short‐term climate change. At the stage of high temperature and mass precipitation about30ka B.P. in the Last Glacial period, the stream power increased. The materials in mountain were transported to the northern blank of Tian Shan and formed fluvial fans. With supply of the river sediment decreasing, the stream power was used to downcut river bed. The fluvial fan in middle Pleistocene was entrenched and the terrace T3formed. At the warm and humid climate stage in early Holocene about11ka B.P., the loose materials in the mountain were carried to deposit on the strath terrace. With supply of the river sediment decreasing, the stream power was used to downcut river bed at10ka B.P. and the terrace T2formed. Since early Holocene, the surface runoff strongly decreased with the more arid climate and the storage of loose materials in upper catchment could be hardly carried to the northern front of Tian Shan. Therefore, all of stream power was used to entrench river bed and the river downcutting became more intensely. At the stage of humid in Holocene, small amounts of materials was transported to the downstream and deposited. At the end of humid stage, the river had not enough deposit materials and the width of river bed was decreased to increase stream power. The river became a new stage of downcutting. For the reason, more arid climate change since early Holocene led to intense river downcutting in the northern pediment of Tian Shan. Moreover, the additional slope was supplied by the deformation of folds and thrusts. It increased additional stream power for entrenching river bed many times and a lot of terraces formed where the river crossed the folds.