关键词: 天山北麓, 库松木契克山山前断裂, 几何结构, 断层陡坎, 垂直位错

Abstract:

Thrust fault is the basic model of crustal deformation and also one of the major structural forms of orogenic belts, indicating the tectonic environment of compression. Most of the catastrophic earthquakes that affect human activity occur within the plates. In the interior of the plate, reverse faults are likely to develop as long as there is compressive stress in the regional sense or under some local tectonic conditions. It is considered that the NS compression resulting from collision of the Indian plate and the Eurasian plate is the main cause for the formation of the present tectonic framework in both north and south sides of Tianshan Mountains. The continuous crustal shortening and thickening has made the Quaternary active structures in the front margins of Tianshan Mountains well developed. Meanwhile, the new nappe structures in front of Tianshan Mountains are also the main sites for the preparation of medium-strong earthquakes in the Tianshan Mountains area, and their seismogenic mode is mostly in the forms of blind fault ramp-decollement plane-surface fault ramps.
The northern Tianshan inverse fault-fold belt is located at the junction between the northern foothill of Tianshan Mountains and Junggar Basin, where the Kusongmuqike piedmont fault is located in the south of Jinghe County, and is an important active thrust fault belt in the western northern Tianshan Mountains. In recent ten years, there were many earthquakes with magnitude 5.0 or above occurring in the eastern section of the fault zone. A detailed study of the geometric distribution and tectonic geomorphologic features is helpful to understand the tectonic deformation characteristics and regional strain distribution in the Tianshan area since the late Quaternary. The results of high-resolution remote sensing image interpretation, UAV aerial survey and differential GPS terrain profile survey combined with field geological survey show that the eastern segment of the Kusongmuqike piedmont fault is composed of two secondary reverse faults. Among them, the south branch, the Xinlongkou Fault, is composed of 5 en echelon-arranged sub-faults, with an overall trend of NW, dipping S, steep dip angle, and a length of about 48km. The fault offset the two-stage piedmont alluvial-pluvial fan and 5 river terraces, the activity time of terrace T1/T2 and fan3 is the latest, and the fault scarps are 3.6m to 4.7m high, being the product of concurrent fault activities. The vertical displacement of terrace T3 and T4 is 13.5m and 20.3m, respectively, and the vertical displacement of terrace T5 is roughly the same with that of the surrounding pluvial fan2, which is about 30m. On the fan1, there is no tectonic deformation observed in places where the fault passes through, and the initial landforms are retained on the surface. The north branch, the Hydrographic Station Fault, is distributed in an intermittent manner. The overall strike of the fault is near EW, with a total length of about 44km, and the fault offset multi-stage alluvial-pluvial fans. On the alluvial-pluvial fan of Fan3, two near-parallel normal scarps are developed in the northern margin of the alluvial-pluvial fan, while other faults cut through the alluvial-pluvial fan and the surface gully, forming steep reverse scarps on the surface. According to the cumulative height of the normal scarps, the maximum vertical displacement is 17.2m and the minimum vertical displacement is 0.3m, the scarp height is concentrated between 4.7~9.9m. On the reverse fault scarps, the maximum vertical displacement is 7.8~9.8m, the minimum scarp height is 2.4~3.1m, and the scarp height concentrates between 3.3~9.2m. Several sub-faults are developed scatteredly between the two sets of faults, with scarp heights ranging 0.5~1.0m. As far as the scarp height distribution is concerned, its vertical displacement shows a distribution law of decreasing from west to east. These results may contribute to the further understanding of the strain partitioning pattern in the western part of the northern Tianshan.

Key words: the northern foot of Tianshan, Kusongmuqike piedmont fault, geometry, fault scarp, vertical displacement