STRUCTURAL CHARACTERS AND MECHANISM IN THE MICANG SHAN AND SOUTHERN DABA SHAN MOUNTAINS FRONT
LI Yanfeng1,QU Guosheng1,LIU Shu2 and ZHANG Hong2(1.National Earthquake Response Support Service, Beijing 100049, China;2.Southwest Research Institute,China Petroleum & Chemical Corporation, Deyang 618000, Sichuan, China)
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.
【CateGory Index】： P542
【CateGory Index】： P542