Northwest-trending, middle Cenozoic, left-lateral faults in southern Yunnan, China, and their tectonic significance

Abstract

The northwest-striking Jianshui, Qujiang, and Chuxiong faults and probably the Zhongdian fault, south China, show evidence for different amounts of middle Cenozoic (pre-Pliocene and post-early Paleogene) left-lateral displacement that range from 6 to 25 km. The age and orientation of the left-lateral faults suggest that the faults are related to a regional deformational event associated with important left-lateral shear on the Ailao Shan shear zone which lies ∼50–70 km to the south. Identification of these faults extends left-lateral shear during this deformational event farther to the northeast than previously recognized. Unlike the Ailao Shan shear zone, these faults do not appear to be part of a through-going NW-trending shear zone. These faults are interpreted to be the northernmost part of a NW-trending boundary whose main structure, the left-lateral Ailao Shan shear zone, forms a broad zone of mylonitic rocks bounding a crustal fragment that rotated clockwise around the eastern Himalayan syntaxis. Following middle Cenozoic left-lateral displacement, only the Jianshui and Qujiang faults exhibited a reversal of displacement in post-Miocene time. This reversal is interpreted to be the result of counterclockwise rotation within the broad, N–S-striking, left-lateral Xiaojiang fault zone. None of the post-Miocene displacement on these two faults appears to be related to post-Miocene right-lateral displacement on the Red River fault system. The Chuxiong fault may have reversed its early left-lateral displacement, but evidence from surface geology for young and active right-lateral displacement is not clear. The Zhongdian fault appears to have undergone only left-lateral displacement, some of which may be middle Cenozoic in age and some post-Miocene in age. Active displacement on the Zhongdian fault is interpreted to mark the eastern boundary for a small crustal fragment that rotates clockwise around the eastern Himalayan syntaxis.