Nappe tectonic of Xianshuiquan on southern edge of Altun, Qinghai, China

Well Zhong 3 is located in the Xianshuiquan structure of Qaidam Basin and comes across the bedrock when drilled through 70 m thickness N~~1__2 formation. Well Zhong 9 and Well Zhong 6 stand in the south of Well Zhong 3 and in the high position of Huayan mountain. They are still in the upper of N_1 formation when drilled more than 1 000 m. So the bedrock elevation varies very much. According to this study, near Well Zhong 3 in the top position of the Shiyougou of the Xianshuiquan structure, there exists nappe tectonic formed from NW pression. This kind of nappe belongs to the former place nappe consisting of a series of faults along the foreland of Altun, including F_9, F_8 and the imbricated nappe structure formed from the faults in the foreland. Its front edge is situated near Well Zhong 3 at the top part of the Shiyougou. By the analysing bedrock buried depth data come from magnetic information and fault tilt angle, the author points out that the nappe translation was about 9 km and came into being in latter N_1 period. It is just the nappe that supplies the sediments to the upper formation of N_2. This nappe is proved by the balanced cross-section result. The standpoint of nappe gives a reasonable interpretation to the structure style in this area and has the guide meaning for the petroleum exploration in this area.