内蒙古狼山地区南北向断裂构造应力场特征及其与区域构造演化关系

Characteristics of the Tectonic Stress Field of the South-North Oriented Fault of the Langshan Mountain Region of Inner Mongolia and Its Relationship with Regional Tectonic Evolution

内蒙古狼山地区断裂构造十分复杂,主要发育有南北、东西、北东和北西走向的断裂构造.从南北向断裂的几何形态、运动性质、构造应力场特征入手进行研究,结合野外实地调查与测量,运用极射赤平投影方法,求出构造应力场的主应力轴方位,进而对本区的构造演化进行了探讨.初步认为,研究区发育的近南北向断裂至少受到过两期构造应力场的作用,第一期是在晚二叠世,由于华北克拉通向北、西伯利亚板块向南活动而形成碰撞拼贴运动所产生的近南北向近水平挤压构造应力场,此时构造应力场的主应力轴σ1为北偏东10°左右,向北倾伏,倾伏角为15°~20°.在这一期构造应力场的作用下,狼山地区发育了一套破裂系统,它们分别表现为近东西走向的挤压构造带和逆断层、近北东走向的以左行为主的走滑断层、近北西走向的以右行为主的走滑断层以及近南北走向的张性断层.这些早期的断裂系统也制约着该区域后来的构造活动,第二期构造应力场是侏罗纪以来古太平洋板块向亚洲大陆俯冲而产生的.此构造应力场的主应力轴σ1为北西-南东向,倾伏向为150°左右,倾伏角为10°~20°.第二期构造应力场的作用,使早期南北向断裂由原来的张性破裂面转为左行走滑,早期东西向断裂转为右行走滑,早期北东向左行滑动面转为压性面和褶皱轴方向,而早期的北西向破裂面则转为张性破裂性质.

The fault structure is very complex,with several extension directions including the north-south,east-west,north-east and north-west in Langshan area,Inner Mongolia.In this study,we conducted the geometrical morphology,fault movement property and structural stress field analyses on the N-S trending fault,combining with field investigation and stereographic projection,to reveal the regional structural stress field and tectonic evolution history in Langshan area.Our preliminary results show that the N-S trending fault in the study area has experienced at least two phases of significant tectonic events.The first phase is in the Late Permian,because a collision collage movement due to the northward movement of North China Craton and the southward movement of Siberia plate produced nearly horizontal compressive tectonic stress field with principal stress axis sigma 1 for north by east 10°or so,plunging to the north,and with dip angle of 15°-20°.As aresult the Langshan mountain region developed a rupture system,which consisted of nearly east-west direction compressive tectonic belt and reverse fault,near the north-east direction of the predominantly left strike-slip faults,near north-west direction of the predominantly right lateral strike-slip faults and nearly north-south extensional fault.These first formed fault systems also have controlled and constrained subsequent tectonic activity in the region.The second phase of tectonic stress field was generated by the subduction of the palaeo-pacific plate from the Jurassic to the Asian continent.The tectonic stress field of the principal stress axis sigma 1 is northeast-southwest,plunging to about 150°and with dip angle of 10°-20°.The effect of the second phase of the tectonic stress field made the early fault tectonics move again,so that the early north-south fault was turned from the original tensile fracture plane to the left-lateral strike-slip,early east-west fracture from compressional structural surface to left-lateral strikeslip,early north-east left-lateral strike-slip surface shifted to the compressive surface and the fold extended direction.The early north-west was converted to a rupture property.