松树沟“入”字型构造演化分析

EVOLUITON OF THE “λ” TYPE STRUCTURE IN THE SONGSHUGOU AREA

松树沟“入”字型构造体系是包容在秦岭纬向带中的低级序构造。其主干构造为商丹深断带,分支构造为其北侧的控制区内基性或超基性岩体的北西向褶皱断裂系。燕山期商丹深断带受左行直扭应力场的作用,形成该“入”字型构造。

The lambda-type tectonic system in the Songshugou area is a low-sequence structure,which is held in the Qinling latitudinal zone. Its main structure is the Shangdan deep fault zone and its branch structure consists of a series of fauls, folds and basic-ultrabasic rocks controued by them in the northern part of the main fault. The Shangdan deep fault zone is a supercrust fault zone with an important geological significance. It controls the different sedimentary formations in this area and the evolution of the Qinling marine trough. Its dimensions are very large, its activities are frequent, and the mechanical characteristics of the structural levels have been reformed many times.It is of the complexed mechanical properties: compression, tensile, tensile-shear and compresso-shear structures, etc. It strictly controls the tectonic evolution and the tectonic stress activities in the area.Through the early Caledonian,Caledonian,Hercynian-Indosinian,Yanshanian and Himalayan tectonic movements, the structural patterns of the fault zone are extremely complicated. The branch structure is formed by a series of the northwestern compression-shear faults and folds, the Songshugou ultrabasic rock mass and water-rich basic complex rocks which existed in the places of axes of folds and faults. The left-lateral slip of the Shangdan fault zone in the Yanshanian is a tectonic stress field conditions for the formation of the lambda-type structure system.The three main stress orientations determined by equatorial projection analysis are σ1-52°∠18°, σ2-146°∠78° and 3-318°∠3°.In the Himalayan period,a stress rebound took place in the deep fault zone, thus forming a right-lateral fault and the Tertiary gentle folds in the NEE--SWW strike direction. The study of the Songshugou lambda-type structure system has a sertain significance for exploration of chromium and nickel ores in this area further.