Abstract:

The Youshashan fault, whose lower side is Ⅺ fault with south-dip and upper side is Yingbei fault paralleling to Ⅺ fault, is one of the major faults in western Qaidam Basin. In recent years, there were different views on activity period and formation mechanism of Yingbei fault. Some researchers thought that it was the upper branch of Ⅺ fault with early formation period and ongoing activity since Cenozoic. And some of them thought that it was a derived fault with the same period in the formation of Youshashan fault in Himalayan Movement, and the exact time is during the deposition of Youshashan Formation in Pliocene. In this paper, according to outcrop survey, ESR dating data and 3D seismic data, we did some discussion on activity period and formation mechanism on the base of growth strata development, fault growth index analysis and the relationship between principal stress directions change and tectonic deformation. And then we conclude that: (1) Yingbei fault was a P fault generated from right lateral torsion slip activity by Youshashan fault in late Himalayan Movement, and it has nothing to do with Ⅺ fault; (2) Its formation period is mostly the Middle Pleistocene, about 500,000 years ago, later than that of Youshashan fault; (3) Yingbei fault is not involved in early oil and gas migration and accumulation, but it plays a certain role in late oil and gas adjustment.