An Analysis Of Tectonic Geomorphology In The Lingqiu Basin And Its Tectonic Activities

The Lingqiu basin, located in the northeast of the Shanxi fault-depression zone and east of the Xinding basin, is a small subsidiary half-graben basin. This area began to warp downward and to receive sediments in Quaternary when the dextral shear on the Shanxi fault-depression zone caused horizontal rotation and tilting at its end. According to the local county history, an Ms7.0earthquake shook this area on28June1626. Some seismologists and geologists have conducted detailed research on this historical earthquake, and acquired a lot of new data, including the distribution of the foreshocks, investigation of the destroyed historic sites, intensity distribution, related seismic parameters and geophysical characteristics, and seismogenic structure for this earthquake, which is very helpful to the subsequent research.Due to the limited conditions10-20years ago, such as little satellite images available, some problems remain to be solved in this region, including geologic structure and distribution of the main active faults in the basin. Because of the particularity of the location, the research on the geologic structures within this basin can not only raise the level of understanding the characteristics of the ruptures in the Shanxi graben system, but also be helpful to clarify the characteristics of the fault development in conjunction with data of other basins. In addition, the research on the seismogenic structure of the1626earthquake can be used to enhance the understanding of recurring models of the strong shocks in the Shanxi graben system as well as modes of stress concentration in the North China block, where the crust is broken.On the basis of the SPOT5images of this basin, combining the up to date SRTM3data and field work, this work divides this basin into several geomorphic units and finds that the topographic surface development across the Tanghe river is asymmetry. There are T1, T2terraces and pluvial platforms on the south of the river. However, on the other side of the river, there are only T1and T2river terraces. This work recognizes2linear subsidence centers in front of the pluvial terrace in the western and southern basin, and a relative smaller subsidence center in the northern basin via aspect map of the basin. According to the slope map of the basin, it is obvious that the slope of the pluvial terrace surface is steeper than that of the surface in the northern basin, up to an angle of5-11°. Furthermore, according to the SPOT5image, there are many deep ’parallel streams’ developing on the pluvial terrace in the west, which are the result of the terrace uplift and head erosion. All the topographic features show the shape of the terrace in the south is controlled by active structures. The amount of uplift of the terrace via SRTM3data is30-40m in the west, whereas it is20m in the east on average.Much field work has been conducted to confirm the speculations. The profile to the south of the Zuoxin village shows the T1on the south of the Tanghe river is2.3-2.4m higher than that on the north, T2on the south is about4.5m higher than that on the north. The profile to the south of the Xifutian village shows T1on the south of the river is0.8-0.9m higher than that on the north, and T2on the south is about1.9m higher than that on the north. This thesis deduces a conclusion that the Shuijian-Luoshuihe fault has an average rate of0.38-0.4mm/a since6000a, and0.5mm/a since9000a. The Huashanhe fault, building a trough valley in the northern basin, leads to the fact that the river terraces across the Huashan river are asymmetric, where the differential height comes to about8-13m.This work deduces the location of the Shuijian-Luoshuihe fault on the basis of the topographic features, and excavated a trench to the south of Xianandi village. The result shows that the attitude of the fault is320°∠60°, the fault extends up almost to the surface, suggesting the latest activity of this fault was probably in1626, for the0.8-1m high fault scarp still maintains. Geochemical prospecting results show not only the amount of Rn near the fault exceeds u+2a, but also points to the extension of the fault. Some profiles near the Shilei village show the latest activity of the piedmont fault before Late Pleistocene-Holocene. A profile near the Gaojiazhuang village shows the Huashanhe fault is a normal fault with a strike-slip component. The fault also extends up to the near surface, and its latest activity was in1626for the maintaining of the2m high fault scarp. Near the Zhangwanggou village, the fault built a trough valley, the latest wedge is10cm beneath the surface, it is considered to be one of the seismogenic structures for the1626earthquake.On the basis previous studies and the efforts of this work, this thesis suggests that the Shuijian-Luoshuihe fault and Huashanhe fault are the main active faults in the Lingqiu basin right now, and these conjugate faults are the seismogenic structures for the1626earthquake. The Shuijian-Luoshuihe fault inherits the activity from the range-front fault, and expresses the latest activity of the basin. The Huashanhe fault is a transfer fault, which dips west in the north and dips east in the south, accommodating nonuniform extension of the basin.