南大巴前陆冲断带构造变形几何类型、分布特征及其成因分析

Geometrical Types,Distribution Characteristics and Genetic Mechanism of Structural Deformation in Southern Dabashan Foreland Fold-Thrust Belt,Central China

南大巴前陆冲断带位于秦岭造山带向四川盆地过渡的部位,是晚三叠世扬子-秦岭俯冲碰撞与中新生代以来陆内造山形成的。根据构造变形的几何学特点,自北东向南西发育紧靠城口断层的根带、坪坝断裂与鸡鸣寺断裂之间的中带、镇巴-鸡鸣寺断裂与铁溪-巫溪隐伏断裂之间的锋带。各带发育不同的构造变形:根带的冲断层系统以逆冲叠瓦构造为主控构造组合,同时发育构造三角带、冲起构造和双重构造等组合;中带的冲断褶皱系统以发育断层相关褶皱组合为主;锋带发育为滑脱褶皱系统,以类侏罗山式褶皱为主控构造,同时发育了箱状背斜、膝折构造、倒转背斜、平卧褶皱、紧闭背斜、同斜背斜、虚脱不协调背斜等滑脱褶皱。垂向上发育3套区域性滑脱层:震旦系泥页岩和寒武系泥页岩、志留系泥页岩、下三叠统膏盐岩;在南秦岭自北而南的推覆作用下,依次沿这3套滑脱层逐级抬升而向南滑脱,存在"推覆作用(叠层滑动)→冲断作用(切层滑动)→层滑作用(顺层滑动)"的滑脱变形序列,从而造就南大巴的多种构造变形类型及其有序分布特征。

The southern Dabashan foreland fold-thrust belt lies between the Qinling orogenic belt and Sichuan cratonic basin, which is formed by the combined action of Late Triassic plate subduction and collision of the Qinling orogenic belt and subsequent intracontinental orogeny. From NE to SW, it can be divided into three sub-belts based on geometrical characteristics of its structural deformation. （1）The inner-beh adjacent to the Chengkou fault is characterized by an intense thrust system. The dominant structural assemblage is imbricate faults, accompanied by such subordinate structural assemblages as duplex, the triangle zone and pop-up structure. （2） The middle-belt between the Pingba fault and Jimingsi fault is characterized by the thrust-fold system with the fault-bend fold as its dominant structural assemblage. （3） The outer-beh between the Zhangba-Jimingsi fault and Tiexi-Wuxi concealed fault is characterized by the deformation-fold system. The dominant structural assemblage is the Jura-type-fold with different types of deformation-folds such as box anticline, kink structure, over anticline, recumbent fold, tight anticline, isoclinal anticline and detachment-disharmonic anticline. Three major regional detachment layers are found vertically： Sinian to Cambrian clay shale, Silurian clay shale and Lower Triassic halite-bearing gypsum. As the southern Qinling belt propagated southward, the fold-thrust system developed from nappe thrusting, with elevation of the three main deformation layers and detachment toward south within these layers. Actually it grows on the detachment evolvement sequence： errace slipping--＊secant slipping--＊interlayer-slipping. This model provides new insights into evolution of folding and thrusting by cap rock detachment, and is helpful to understand the deformation mechanism of the southern Dabashan foreland fold-thrust belt.