STRUCTURAL DEFORMATION CHARACTERISTICS OF BO-A FAULT IN THE SOUTHWESTERN MARGIN OF TURPAN BASIN

The Bolokenu-Aqikekuduke Fault ( Bo - A Fault) is a large-scale right-lateral strike-slip fault zone, which starts in Kazakhstan in the west, enters China along the NW direction, passes eastward through Alashankou, Lake Aibi and the southwestern margin of Turpan Basin, and terminates in the Jueluotage Mountain, with a total length of about 1 000km. At present, researches on the fault mainly focus on the area from Lake Alakol to Jinghe. Through satellite images, it can be found that the Bo-A Fault enters the southwestern margin of the Turpan Basin in the SE direction, and offset various landforms such as river terraces and alluvial fans, forming clear linear features on the surface, which indicates that there have been obvious activities since late Quaternary in this fault section. However, no detailed research has been carried out on the tectonic deformation characteristics of the Bo - A Fault in this area. The active characteristics of the faults in the southwestern margin of the Turpan Basin are studied, and the results are helpful to understand the role of the Bo - A Fault in the Cenozoic tectonic deformation of the Tianshan Mountains. The study area is located in the southwestern margin of the Turpan Basin, where three stages of alluvial-proluvial fans are developed. The first-stage alluvial-proluvial fan is called Fan3, which was formed earlier and its distribution is relatively limited, formed roughly in the early late Pleistocene; The second-stage alluvial-proluvial fan is called Fan2, which is the most widely distributed geomorphological surface in the study area. The geomorphic surface in this period was roughly formed from the late Pleistocene to the early Holocene. The third-stage alluvial-proluvial fan is called Fanl, which belongs to the Holocene accumulation, most of which are located at the outlet of gullies near the mountain passes, forming irregular fan-shaped inclined surfaces. To the west of Zulumutaigou, the fault offset the Fan3 alluvial-proluvial fan, forming dextral dislocation and fault scarp of the gully on the surface. The measurement shows that the amount of the dextral dislocation produced by the fault is between 22m and 40m. The height of the scarp is 3. 9~ 4. 2m. The section exposed by the fault shows that the Paleozoic bedrock thrust northward onto the Quaternary gravel layer, and the fault fracture width is about lm, which reflects that the Bo-A Fault also has a certain thrust component. On the east bank of Zulu Mutaigou, the fault offset the Fan3 alluvial-proluvial fan, and the measurement results show that the offset of the gully is between 46. 3m and 70.2m. To sum up, the movement mode of the Bo-A Fault in the study area is dominated by dextral strike-slip. On the Fan2 alluvial-proluvial fan at the northwest of Zulu Mutaigou, there are two secondary faults arranged in a right-step en-eehelon pattern, forming high scarps with a height of 1.6-3. 9m on the surface. Trench profiles reveal that both faults are SW-dipping thrust faults, thrusting from south to north, and they are preliminarily judged to be formed by the expansion of the Bo-A Fault into the basin. There are mainly three stages of alluvial-proluvial fans developed in the study area. Although no specific dating results have been obtained in this work, we believe that the age of the Quaternary landforms in the study area is the same as that in the Chaiwopu Basin, which is only separated by a mountain. Quaternary geomorphological ages are basically the same. Through geomorphological comparison, we believe that the age of Fan2 alluvial-proluvial fan is 12 ~ 15ka, and the age of Fan3 alluvial-proluvial fan is 74ka. It is estimated that the dextral slip rate of the Bo-A Fault is about lmm/a since the formation of Fan3, and the vertical movement rate of the fault is about 0.13 - 0. 32mm/a since the formation of Fan2. According to GPS observations and geological data, the NS-direction shortening rate in the East Tianshan area can reach 2 ~ 5mm/a. Through this study, it can be found that the Bo-A Fault also plays a role in regulating the near-NS-trending compressive stress in the East Tianshan area by accommodating the compression strain inside the Tianshan Mountains mainly through the NWW-directed right-lateral strike-slip motion. In addition, in the study area, the youngest fault scarp is located on the Fan2 alluvial-proluvial fan at the north of the main fault. It is preliminarily judged that the latest activity of the Bo-A Fault has a tendency to migrate from the mountain front to the basin. © 2022 State Seismology Administration. All rights reserved.