南大巴山冲断构造及其剪切挤压动力学机制

The Thrust Tectonics and It's Transpressive Geodynamics in Southern Dabashan Mountains

南大巴山是一个形成于T_3—K_1，滑脱深度小于8-10km的扬子板块北缘薄皮冲断锲。它主要由发育在显生宙地层中台阶状逆断层及断层相关褶皱构成的边冲岩席、双重推覆体和冲起构造等组成。变形扩展以前列式为主。经平衡地质剖面制作，因冲断南大巴山地壳缩短率平均达49．3％。并以每年约1．28mm的速率总体缩短约64km。它的成因受控于秦岭碰撞造山过程中扬子板块北缘A型俯冲所提供挤压应力，在向南扩展时，由于古大陆边缘形态不一所诱发的右旋剪切挤压动力学机制。

The deformation belt of Southern Dabashan mountains, located between the northeast border of Sichuan craton of Yangtze plate and the Qinling collisional orogenic belt,is of a southward verging thin - skinned thrust wedge which is characterized by superimposition of dextral transpressive deformation. The thrust wedge is mainly composed o f the stepped - thrust sheets, the fault - related folds and duplex in the cover strata.Kinetically, the propagation of stepped thrusts is in the piggy-back style from the root zone named the Chengkou -zhongbao thrust which is the northern boundary fault of southern Dabashan mountains where the largest depth of detachment layer is about 8-- 10km to the frontal zone of Wenquanjing thrust which is the southern boundary fault of southern Dabashan mountains where the largest depth of detachment layer is less than 1. 5kin,at the end of Triassic,the thrust deformation was first initiated in the root zone near the Chengkou -zhongbao thrust. Since then the thrusting had propagated southwestward in the form of piggy - back and in the beginning of Cretaceous the deformation had cxtended to the southmost front of southern Dabashan mountains. Thus the total time span of deformation is about 50Ma. By mapping of balanced -- cross sections and restoration of structures, the crustal shortening,the shortening rate and the thrusting velocity arc estimated about 64km,49. 3 % and 1. 28mm/y respectively. Furthermore the geodynamics to develop the deformation belt is dominantly related to the Qinling collisional orogenic processes. In the Mesozoic, the continent -continent collision between North China plate and Yangtze plate resulted in the formation of Qinling collisional orogenic belt and the A -type subduction of Yangtze plate beneath the middle Qinling volcanic are through the southern Qinling and northern Dabashan tectonic belts. As a result, the A -type subduction led to the horizontal tectonic stress which caused the southwestward propagation of the thrusting toward southern Dabashan mountain deformation bait. Besides, with the existence of Michang mountain uplifting Protcrozoic metamorphic complex in the southwest of Dabashan mountains, the morphology of northern margin of Yangtze plate is in the form of northward convex. Under the controlling of this boundary condition, the northward A -- type subduction deduced the dcxtral transpression to the east of Michang mountains uplifting Protcrozoic metamorphic complex along the Chengkou -Zhongbao thrust and northern Dabashan mountains extruded eastward. It is this geodynamics that had caused the dextral transpressive thin -skinned thrusting deformation in southern Dabashan mountains.