苏州西南部的推覆和伸展构造

THE NAPPES AND THE EXTENSIONAL TECTONICS IN THE SOUTHWESTERN SUZHOU AREA, JIANGSU PROVINCE

根据大量的地质、钻孔资料,结合地球物理资料和岩组分析研究,发现苏州西南部中生代以来至少经历了两期重要的构造变动:先期的推覆、后期的伸展。推覆时代确定在早、中三叠世之后,晚侏罗世之前。其后该区便转入伸展构造发育期。推覆构造具有台阶状逆断层及其相关褶皱组合的几何学特征。推覆动向大体从北西(340°±)向南东,并有左旋逆冲性质。根据平衡剖面恢复,确定最小推覆缩短量为18—20公里,缩短率为45.6—47.6％。伸展拉张为表层的重力滑动,浅层形成NNE走向为主的“堑—垒”式对称正断层体系。区域盆地构造走向,野外地质和岩组的研究表明拉张主方向大体为NW305°—SE115°,并具右旋运动性质。挤压和拉张、推覆和伸展的叠加改造,使得苏州西南部的地层,包括大部分岩体在内,均成为无根构造岩片的叠置。这对研究该区高岭土和多金属矿床的成矿机理和指导找矿有重要意义。

Based upon extensive field geology, borehole and geophysical data, and the petrofabric analysis of the deformed rocks, the Southwestern Suzhou area shows evidence of at least two significant tectonic events since the Mesozoic: an early thrusting event and a later extensional event. The thrusting took place between T_(1-2) and J_3, with the extensional event occurring afterwards. The stepped thrusts and associated folds are characteristics of nappes. The directions of the thrusts' motion accompanied by sinistral motion range from NW 840° to SE 180°. Restoration of the balanced-sections provides an estimatc for the minimum thrust shortening to be 18—20 km (shortening ratio is 45—48%). The extensional event is revealed by gravity gliding of shallow- layers which form an NNE-trending symmetric normal fault-bounded 'graben-horst' system. The main direction of extensional motion accompanied by dextral motion ranges from NW305° to SE 115°. The combined product of the compression and extension (thrusting and stretching) produce rootless tectonic slices of strata and the magmatic rock bodies. Understand the nature of these nappes may help us better understand the origin of the polymetallic deposits and kaoline deposits in the research area and furthermore provide a guide to their exploration.