米仓山、南大巴山前缘构造特征及其形成机制

在对四川盆地东北部盆山结合部地表地质和石油地球物理资料综合分析的基础上,阐述了米仓山前缘构造、南大巴山前缘构造的几何学、运动学特征;发现了二者的共性和不同,二者均以双重构造为主,通过古生代构造层的叠置,而迅速抬升出露地表,米仓山前缘以被动顶板双重构造为主,即典型的"三角带"构造,南大巴山前缘以主动顶板双重构造为其显著特征;初步分析了原因,区域滑脱层,特别是嘉陵江组-雷口坡组膏盐岩滑脱层及古生界泥页岩滑脱层,构成了顶板和底板逆冲断层,其间的台地相碳酸盐岩构成了断夹块,受米仓山早期基底隆升和侧向挤压,形成了被动顶板双重构造,南大巴山递进挤压变形,形成了主动顶板双重构造。

STRUCTURAL CHARACTERS AND MECHANISM IN THE MICANG SHAN AND SOUTHERN DABA SHAN MOUNTAINS FRONT

Based on field investigation,reflecting seismic profiles and other data,the Meso-Cenozoic structural style in the transition zone between the mountain and basin in the northeastern Sichuan basin is discussed in this paper,and the deformation mechanism and tectonic model are outlined as well.Two principal types of duplex structures have been recognized or inferred, including passive-roof duplex (namely typical "triangle belt") and active-roof duplex, both of them have the duplex structure as the main structure style with the uplift Palaeozoic strata by overlapping. In the northern transition zone, the passive-roof duplex dominates the buried area in the front of the Micang Shan Mountains. A passive-roof duplex is bounded top and bottom by thrust faults with opposite senses of vergence that form a triangle zone at the leading edge of the advancing thrust sheet. The north-dipping reverse faults forming the leading edge of the Micang Shan Mountains uplift, a 15km-wide fold-and-thrust belt. The south-dipping reverse fault disclosed by seismic profiles recently is interpreted as a back thrust at the bottom of the Mesozoic foreland basin strata, making the belt doubly vergent. In the eastern transition zone, the active-roof duplex dominates the buried area in the front of the southern Daba Shan Mountains. A active-roof duplex is bounded top and bottom by thrust faults with same senses of vergence that form a multi-overlapping zone at the buried leading edge of the advancing thrust sheet. The northeast-dipping staircase thrust faults at the bottom of the Palaeozoic strata forming the floor fault, the thrust faults with the same dip at the bottom of Mesozoic foreland basin strata forming the roof fault. The relative strength of the decollements and surrounding rocks impacts the development of active- or passive-roof duplexes (triangle zones).The presence of relatively weak and multiple regional decollements promotes forethrusting of the cover, antiformal stacks, coeval growth of structures and low internal strain, with the exception of significant early layer-parallel shortening.Regional detachment layers, especially the thick gypsum decollement interbedded in the Jialingjiang and Leikoupo Formations and multiple Palaeozoic mud shale decollements, constitute the roof and floor detachment faults, and the platform carbonate rock constitutes the horse. In the Micang Shan Mountains, the southward thrusting battered by strong basement rocks of the Micang Shan Mountains, and caused back thrusting detachment from thick gypsum interbedded in the Jialingjiang and Leikoupo Formations at the front of the thrust belts, forming the passive-roof duplex. While in the southern Daba Shan Mountains, the thrusting was propagated southwestwards and extended to the southmost part of southern Daba Shan Mountains,and caused thrusting deformation to be preferentially developed along major decollements and to progress toward foreland from lower Palaeozoic to Mesozoic foreland basin strata, forming the active-roof duplex.