关键词: 海原, 走滑断裂, 尾端挤压构造, 地壳缩短

Abstract: Haiyuan active fault zone is a main active fault in the northeast boundary of Qing-hai-Xizang plateau. Geologically, mapping of 1:50,000 has been made along the fault zone. The results show that Haiyuan active fault zone, 237km long, striking WNW in its west segment and striking NW in the east segment, has been a strike-slip fault sense since middle-late early Pleistoncene. The horizontal offset is 12-14.5km from middle-late early Pleistocene, with the slip rate of 11.7-19.2mm/a. The strong activity has been going on since the begining of the Holocene, with the sinistral strike slip rate of 6-10mm/a.Haiyuan strike slip fault zone consists of 11 secondary shear faults, mostly left stepping pinnate, partly right stepping pinnate. 8 pull-apart basins and 2 pushed-up blocks were formed along the fault zone. Pull-apart basins showing 2 types of romb and elongated, big ones began to develop in middle Pleistocene, small ones in late Pleistocene. The largest thickness of sediments is greater than 750m. A tensile-shear fault with the sinistral strike-slip was formed in some pull-apart basins.The tensile-shear fault links up two secondary shear faults which control pull-apart basin, intersecting them with low angle. After the formation of this kind fault, pull-apart process of the basin decreased and pull-apart basin gradually became feeble and die. The thrust faults intersecting the secondary shear faults with high angle was formed in the pushing area, appeared as an uplift in topography.A great earthquake of magnitude 8.6 occurred in Haiyuan, in December 16, 1920. The surface ruptures of earthquake can be divided as 15 fracture segments, which developed along secondary shear faults, tensile shear faults on pull-apart basin and boundary normal fault at both ends of pull-apart basin. The horizonal dislocation of 268 values have been measured. The maximum displacement of left-lateral strike-slip is 10-11m.At the southeast end of Haiyuan strike slip fault, a rare end compressional area, which consists of thrust fault zone of east pediment of Liupan Mountain, Madong Mountain fold zone and Xiaoguan Mountain thrust fault zone, striking nearly NS, was formed. The detail mapping and studying have been made on the deformation features of these compressional structure belts and calculation on the amount of crust shortening have been made, and the value is about 12.4-16.7km, which is about equal to the sinistral offset amount of Haiyuan active fault zone.

Key words: Haiyuan, Strike-slip fault, Compressional structure of the end, Crustal shortening.