Study On Tectonic Geomorphology In The West Kunlun Mountains And Active Tectonics In Ashikule Area

The West Kunlun Mountains, which are located in the northwestern part of the Qinghai-Tibet Plateau, have experienced much more intensive tectonic deformation, earthquake and volcanic activities than any other place, due to the collision between the Indian Plate and the Eurasian continent, and to the continuous convergence. The eastern end of the West Kunlun Mountains is an important area where the northern boundary of the Qinghai-Tibet Plateau changes its direction, and the Altyn Tagh strike-slip fault transforms to thrust fand normal faults. Surface activities are directly linked to deep processes, and structural characteristics are complex and not well-understood. Therefore, our study on active tectonics in this region will help us better understand the formation processes of the Qinghai-Tibet Plateau and the structure transformation here.Our approach is based on DEM analysis, interpretation of satellite images, field geological and geomorphic investigation and surveying, and cosmogenic nuclide 10Be dating. We studied:(1) Regional geomorphic characteristics in the West Kunlun area and their coupling relationship with tectonic activities; (2) Volcanic geomorphology and active faulting in the Ashikule Basin, located at the eastern end of the West Kunlun Mountains; (3) Slip-rates along the Ashikule segment of the Altyn Tagh fault and uplift of the northwestern Qinghai-Tibet Plateau; (4) Surface rupture characteristics and rupture mechanics of the 2008 Ashikule earthquake (Ms7.3), and its tectonic implications.The main results of this thesis are listed as below.(1) The regional geomorphic investigation of the West Kunlun Mountains indicates that Cenozoic tectonic activities produced two planation surfaces in our study area:the summit planation and the main planation surface, which were both higher in the eastern part of the West Kunlun Mountains than in its western part. We suggest that during the formation of the Western Himalayan Syntaxis, the convergence and squeezing that occurred between the Pamir and the southwestern Tienshan caused eastwards migration of materials, yielding height differences of the planation surfaces. The right-lateral strike-slip Karakorum fault located in the western end of the West Kunlun Mountains and Talas-Fergana fault, which is more to the north, may absorb part of the shortening in the northwestern Qinghai-Tibet Plateau, and contribute to the discrepancy in height of planation surfaces.(2) Through detailed active tectonic mapping on the satellite images and in the field, we precisely determined the distribution of the Quaternary volcanoes and their characteristics, in the Ashikule Basin, the high altitude uninhabited area in the eastern part of the West Kunlun Mountains. Combined with surveying of the active faults in the Ashikule Basin, we confirm that all the volcanoes were distributed along the normal fault strand of the main Ashikule fault. The formation and eruptions of these volcanoes were triggered by strike-slip movement of the Altyn Tagh fault.(3) Formation and development of the Ashikule Basin. The Ashikule Basin was formed in the local transtensional environment caused by arcuate and clockwise rotation of the Altyn Tagh fault and the northwestern Qinghai-Tibet Plateau, during the progressive northwards extrusion of the Pamir, caused by the India/Eurasian collision. A new type of strike-slip basin such as the Ashikule Basin is suggested here.(4) At least 7 terraces developed along the Akesu River, east of the Ashikule Basin. All of these terraces were offset by the Ashikule fault. Horizontal displacements of different terraces were measured by retro-deformation on satellite images and total station surveying, and are～11m,～33m,～105m,～220m, and～660m, for the T2/T0 riser, T3/T2 riser, T4/T3 riser, T5/T4 riser, and T6/T5 riser, respectively. Combined the displacements with cosmogenic nuclide 10Be exposure ages, which are 7.7±0.7 ka,32.7±3.1 ka,53.6±2.5 ka,115.7±23.2 ka,166.8±10.4 ka, and 195.1±8.5 ka, for T1, T2, T3, T4, T5, and T6 surface, respectively, slip-rates along the Ashikule fault range from 0.34 to～1.9 mm/yr, possibly up to 3.96 mm/yr. This slip rate is lower than that on other segments of the Altyn Tagh fault. We consider that the Ashikule fault is located in the local transtension environment where the Altyn Tagh fault changes direction and becomes the Karakax fault, with a series of strands splitting from the main fault. The normal faulting activities here absorb part of the horizontal slip, therefore producing a slower strike-slip rate.(5) The 7 terraces at Akesu River site attest of the tectonic uplift in this region. Combined with the cosmogenic ages on different terraces, and the height of each terrace surface compared to the modern river measured in the field with a total station, incision rates of 0.2-0.35 mm/yr were determined for the Akesu River. This may represent a Late Pleistocene uplift rate of the northwestern Qinghai-Tibet Plateau.(6) Based on detailed field investigation and surveying, we mapped the co-seismic surface rupture zone of the 2008 Ashikule earthquake, and determined characteristics of the surface rupture and co-seismic displacement. Considering a series of earthquakes and volcanic eruptions events along the Altyn Tagh fault since the beginning of the last century, we suggest that the next earthquake may migrate to the southwest.