Study On Morphotectonic Features And Uplift In The Front Of The West Kunlun Mountains

West Kunlun Mountains locate in the northwestern Tibetan plateau, southern margin of Tarim basin. The continuous collision of Indian plate and Euro-Asia plate since Cenozoic era formed the West Kunlun Mountains. This process also resulted in a series of deformation and large-scale strike-slip and thrust faults along the mountain front. The typical morphotectonics of the piedmont recorded the uplift and propagation information of the northwestern Tibetan Plateau.Based on the interpretation of satellite images, processing of DEM data, analysis of large scale topographic maps, combined with field morphotectonic investigations and observations, a qualitative and quantitative morphotectonic features of this area was investigated. Typical features such as the structure, tectonic and propagation process of the thrust belt in the piedmont of West Kunlun, the location of the frontal thrust zone, the uplift rates of the western Kunlun Mountains since Cenozoic and the shortening amounts of the piedmont structural belt was also studied. Some cognitions are presented as follows:(1) There are two planation surfaces: summit planation surface and main planation surface in northwestern Tibetan plateau. The inconsistencys of the regional uplift process result in the different heights of planation surfaces between the east part and western part of western Kunlun. At the east part of the West Kunlun, the altitude of the summit planation surface and the main surface are 5800-6400m and 5200-5800m respectively; At the west part of the West Kunlun, the altitutude of the planation surface and the main surface are 5000-5200m and 4000-4500m respectively.(2) A series of typical morphotectonic features of West Kunlun piedmont record information about the uplift process of the west Kunlun and intense activity of the thrust belt in the piedmont of West Kunlun. These morphotectonic features also indicate that the West Kunlun piedmont migrated from west to east in the process of its propagation.(3) Based on study of the formation age and deformation amount of the Guman Anticline and the Yingjisha Anticline, the uplift rates of the West Kunlun during the Cenozoic were alculated: The minimum uplift rates were 0.21-0.25 mm/yr from late Pliocene-early Pleistocene and 0.23mm/yr from middle Pleistocene. These two rates are similar, indicating that a slow and uniform uplift process might be exist of West Kunlun from late Pliocene to middle Pleistocene. The study on the formation ages and the uplift heights of the piedmont river terrace shows the uplift rate of the West Kunlun Mountain was 1.5mm/yr from the late Pleistocene, this indicates that the West Kunlun uplifted more quickly from late Pleistocene.(4) By the area-balance theory, shortening amount of Guman Anticline and Yingjisha Anticline in the piedmont of West Kunlun are calculated, which are 300m and 108m respectively. Based on the previous data, it is estimated that the average shortening rate of Guman Anticline ranges 0.14-0.17mm/yr since the Late Pliocene-Early Pleistocene and the average shortening rate of Yingjisha Anticline is about 0.11mm/yr since the Middle-Pleistocene.(5) The south-vergent thrusting in the piedmont of West Kunlun has a strike-slip component. The thrusting of Upar Fault accompanied by a large right-lateral strike-slip component, which offsets the Paleogene and rivers that flow through the fault. According to the measured offset of drainages caused by single earthquake event and pervious research that the recurrence interval of large earthquake is about 1000 years, the average strike-slip rate of the fault is estimated as 4-6.8mm/yr, and the fault is estimated to start at 2.2-3Ma.(6) Based on detailed field survey and measuring work in Kashi area located between West Kunlun and Southwestern Tianshan, the locations of the thrust front in the piedmont of the West Kunlun and the Southwestern Tianshan are located. Mingyaole Anticline and Kashi Anticline are the most forefront of thrust belt in the piedmont of West Kunlun and Atushi-Talanghe Anticline are the most forefront of thrust belt in the piedmont of Southwestern Tianshan. The two large tectonic systems of West Kunlun and Southwestern Tianshan have collided in the west of this region.(7) Both the morphotectonics and surface deformation of the Mazartagh in Tarim basin indicate that south-vergent thrusting exists, and deep geophysical data also show the south-vergent thrusting of the Mazartagh fault. Combination with regional tectonics, the Mazhatage thrust fault is considered as forefront of the thrust belt in the piedmont of West Kunlun. Hetianhe gasfield located on the top of anticline in the front of the thrust, and the forefront of the thrust may exist northward. Deformations in the Cenozoic have obviously changed the structures of the Palaeozoic and Mesozoic in the southern and middle of the Tarim basin. Although Hetianhe gasfield locates in the Carboniferous, intense deformation in the Cenozoic promoted the formation of the gasfield.