四川盆地喜马拉雅期张扭性断裂构造特征及形成机制

Structural Features and Deformation Mechanism of Transtensional Faults in Himalayan Period,Sichuan Basin

前人对四川盆地盆缘冲断带及川中高-磨地区断裂系统进行了系统性研究,但对于盆地其他区块断裂发育特征、形成演化及应力背景的认识尚不充分.发现了梓潼-成都-威远-华蓥山-广安地区震旦系-下三叠统地层中发育的一套区域性张扭性断裂,主要自震旦系及以下地层向上延伸切穿二叠系地层,多为高陡、小断距正断层,部分形成负花状构造.根据该断裂系统的剖面产状、纵向穿层特征及盆地范围内体现出的分异性,推断该断裂系统形成于喜马拉雅期.物理模拟实验研究发现该断裂体系应发育于扭张性应力环境中,该断裂体系的发育指示了四川盆地内部新生代存在南西-北东向张应力,可能与四川盆地新生代发生的逆时针旋转有关.

Previous studies on fault systems mainly focused on thrust belts around the basin and Gaoshiti-Moxi area in central basion.For other areas of the basin,the features,evolution and stress field of fault system is not well understood.In this study,it is newly discovered that a regional transtensional fault system is developed in Zitong-Chengdu-Weiyuan-Huayingshan-Guangan area based on seismic data interpretation.The faults have similar features in the basin,mainly extending from strata of the Sinian system to the Permian system.The faults have vertical occurrence,small normal displacement and negative flower structure.Based on three evidences,it is inferred that the faults are developed in the Himalayan Period:(1)In the Weiyuan-Ziyang area with oblique formations,the fault are still vertical.So the faults are developed after the formation of the Weiyuan anticline which is developed in the Himalayan perioid.(2)Some faults,extending to strata of the Triassic-Jurassic,are of the same system of transtensional faults in the Sinian-Permian strata.Therefore the faults are developed later than the Late Indosinian Period.(3)The distribution of transtensional faults varies from the central part to the compressional areas around the basin,and the stress field of the basin started to vary from the Late Indosinian Period.Based on physical simulation,it is inferred that the faults were developed under torsional stress field.The development of the faults indicates that a NE-SW direction tensional stress field existed in Sichuan basin in the Cenozoic Period.The stress field may originated from the clockwise rotation of the Sichuan basin in the Cenozoic Period.