AMOUNT AND STYLE OF LATE CENOZOIC DEFORMATION IN THE LIUPAN SHAN AREA, NINGXIA AUTONOMOUS REGION, CHINA

The structures of the Liupan Shan area are characterized by numerous active thrust and strike-slip faults that suggest thin-skinned deformation. The structural history of this area can be divided into three phases that probably overlap one another in time and are parts of a single protracted deformation. The oldest Cenozoic deformational phase occurred probably between late Pliocene and early Quaternary time and produced some of the folds and thrust faults in the Liupan Shan and Yueliang Shan. During this phase, deformation was the result of approximately N50-degrees-E shortening, and the amount of shortening seems to have been about 1-2 km. The second phase of deformation was dominated by left-lateral strike-slip faulting (left slip) on the N60-degrees-W striking Haiyuan fault zone and shortening on north-south trending structures; shortening was associated with a transfer of the left-slip displacement on the Haiyuan fault zone to shortening in areas farther east. Shortening occurred by thrust faulting in the Liupan Shan and Xiaoquan Shan and by folding in the Madong Shan. During this phase the orientation of shortening changed to N60-degrees-W. The average amount of shortening on the north-south trending folds in the Madong Shan is about 6.3-7.8 km. Most of the shortening on the Liupan Shan and Xiaoguan Shan thrust faults also occurred during this phase and amounted to a minimum of 4.8-6.3 km and 6.6-7.6 km, respectively, also with an orientation of N60-degrees-W. During the third phase of deformation, about 1-1.5 km of late Pleistocene to Recent left slip occurred on the Xiaokou fault, which was transferred into oblique left-slip thrusting in the Liupan Shan. At this time, deformation in the Madong Shan and Xiaoguan Shan ceased or was reduced to a very slow rate. The present, active left-slip on the Haiyuan fault zone is accommodated by shortening in the Liupan Shan area. The total displacement along the Haiyuan fault is essentially the same as the total amount of shortening in the Liupan Shan area. The sequence and interaction of strike-slip and thrust faults in the Liupan Shan area seem to apply to the folds and thrust faults farther north in the southern Ningxia region. Thus the northeastern margin of the Tibetan Plateau appears to grow by shortening oriented northeast and by left slip faulting that transfers material from farther to the west. The total left-slip in the entire southern Ningxia region probably is less than 20-25 km and may be absorbed by shortening within this region. Thus the eastward translation of crustal fragments in the northern part of the Tibetan Plateau may not extend east of southern Ningxia, and if large-scale eastward displacement has occurred, it must lie farther south.