The Crust Structures and Tectonics of Ürümqi Depression Revealed by Deep Seismic Reflection Profile in the Northern Margin of Tianshan Mountains

Abstract

AbstractIt is known from the research of active tectonics that the northern margin of Tianshan mountains is characterized by typical intra‐continental active tectonics, and has multiple thrust faults and anticlines parallel to the mountains. In order to investigate the fine crustal structure and the geometry of major faults, as well as the deep‐shallow tectonic relations in the Ürümqi depression, a deep seismic reflection profile of 78 km long in near‐SN direction was completed in 2004. This profile is located in the transition zone between Tianshan Mountains and Junggar Basin to the west of Ürüumqi. The results show that the crust beneath the investigated area is divided into upper and lower crusts by a strong reflective zone at about 9~10.5 s TWT. The thicknesses of the upper and lower crusts are about 26~28 and 23~25 km, respectively. There are rich reflective layers and clear structural patterns above 5 s TWT as well as obviously different structural features along the profile. In the southern region of Xishan, the stacked deep seismic reflection section shows a series of EW‐striking thrust anticlines arranged in SN direction as well as a group of faults thrusting from south to north. All of these are influenced by a deep detachment zone. In the Xishan and Wangjiagou area, there is a set of steeply north‐dipping reflective layers and a group of faults slipping along the layers. The northern part of the profile shows the image of a typical sediment basin and its deepest depth is about 10~12 km. Between 6 and 9 s TWT, the stacked deep seismic reflection section shows disordered reflections with comparatively short continuation time and weak energy. These indicate that this part of the crust is evidently possessed of “reflection transparence”. The Moho transition zone occurs at 14~17 s TWT, and the zone thickness is about 9~10 km. In the studied area, the Moho discontinuity gradually deepens from north to south. Its depth is about 50~52 km at the northern segment of the profile and is about 54~55 km near north Tianshan. In the neighborhood of Xishan at the middle profile, the reflections from the boundary between upper and lower crusts as well as the Moho transition zone become misty while the shallow stratums show signs of uplift and fold, which may be related with the compression between Junggar basin and Tianshan mountains.