ACTIVE FAULTING IN AND ALONG THE QINLING RANGE (CHINA) INFERRED FROM SPOT IMAGERY ANALYSIS AND EXTRUSION TECTONICS OF SOUTH CHINA

Since the beginning of the India-Asia collision, about 2500 km of convergence has been accommodated by crustal thickening in and around the Tibetan plateau and by extrusion of Tibetan blocks and of the South China Block. The relative amounts of convergence accommodated by the two mechanisms are, however, still largely a matter of debate. The main purpose of the present study is to determine the rate of left-lateral movement between south China and north China during the late Cenozoic. This movement is accommodated by offsets along the more than 1000 km long E-W trending Qinling fault system which separates the South China Block from the North China Block and terminates to the east in the right-lateral N-S trending Tanlu fault zone. Physiographic features typical of active faulting, such as triangular facets along scarps and drainage deviations, have been observed on SPOT images throughout the Qinling fault system. Recent active faulting involves both left-lateral strike-slip and normal dip-slip, which are partitioned laterally. East of the Weihe graben, two major faults (the Luonan and Danfeng faults) of the Qinling fault system slip left-laterally. A cumulative slip rate of about 7.2 +/- 2.2 mm/yr between the Qinling Shan (south China) and the Hua Shan (north China), has been estimated using Quaternary morphological offsets. In contrast, the Qinling frontal fault, which bounds the Cenozoic Weihe graben to the south, exhibits abundant evidence of normal faulting that accommodated large, post-early Eocene vertical displacement in the graben. Crustal extension, estimated from published seismic profiles and boreholes across the Weihe graben, contributes to a late Cenozoic rate of left-lateral movement of 2 +/- 0.5 mm/yr in a N140 degrees E direction between the Hua Shan and the Ordos Block. Combining these yields, a rate of left-lateral movement of 8.5 +/- 2.5 mm/yr in a N100 degrees E direction between south China and the Ordos Block is achieved. Additionally, the Ordos Block rotates counterclockwise relative to Gobi-Mongolia, resulting in NW-SE crustal extension along the Yinchuan-Hetao grabens. This is estimated from published seismic sections and adds a component of left-lateral movement of 4 +/- 1 mm/yr in a N125 degrees E direction between south China and Gobi-Mongolia. The total rate of left-lateral movement of south China with respect to Gobi-Mongolia is 12 +/- 4 mm/yr in a N110 degrees E direction. The eastward decrease of the left-lateral strike-slip on the active Qinling fault system may be explained by the partitioning of left-lateral movement on the faults bounding the Dabie Shan to the south and north and by a 'book shelf' mechanism of counterclockwise rotating blocks separated by active NE-SW right-lateral faults in the Dabie Shan.