Basic characteristics of the Taihu thrust system in eastern Hunan

Abstract：The Taihu thrust system in eastern Hunan was defined in this paper on the basis of geological and borehole observations. The NE-trending thrust system is composed from northwest to southeast of such structural elements as frontal thrust fault (Luosiqiao fault), frontal belt of nappe and upper decollement structure, low ramp (Tiepuao-Longfengqiao hidden fault) and back belt of nappe, back-thrusting fault (Zhongwan fault), and rear syncline. There occurs Qingdong reversed syncline to the northwest of the frontal fault. The axial trace of the rear syncline shows an arc bulging to the northwest, and there also exist Yajiangqiao domal uplift and Indosinian Yajiangqiao pluton in the middle segment of the syncline. The midpoint of the Qingdong syncline, the bulging part of the arc rear syncline and the center of the Yajiangqiao pluton (or the Yajiangqiao domal uplift) lie on a NW-trending line coinciding with the long axis of the Yajiangqiao pluton. Based on these geological characteristics and using structural correlation analysis, the authors put forward the formation mechanisms of the Taihu thrust system, the Yajiangqiao domal uplift and the Yajiangqiao pluton as follows: Under the NW(W)-trending compressive stress during the Indosinian movement, deep and shallow detachment and thrusting took place along the low ramp, resulting in the formation of the Taihu thrust system. There also existed a middle belt extension from the Qingdong syncline to the Yajiangqiao pluton where compression and slip were more violent than the two sides when the thrust body moved northwestward. This led to the formation of the Yajiangqiao domal uplift and, controlled by back hauling, made the axial trace of the rear syncline bulge to the northwest. More intense compression-shearing and crust-thickening existed in the Yajiangqiao domal uplift, and accordingly the temperature of the deep crust rose. These factors caused the melting of the deep crust with the magmatic intrusion, and the Yajiangqiao pluton was formed with the weakening of the stress after strong compression. The Taihu thrust system gives some constraints on Mesozoic tectonic evolution in South China.