The Boundary Fault System In The Western Segment Of North China Craton:Geometry,Kinematics And Evolution

The Bayan Obo-Chifeng fault is the boundary fault between North China Craton(NCC)and the Xingmeng orogenic belt(XMOB).The structural features and evolution of the Bayan Obo-Chifeng fault are important to understand the relationship between the northern margin of the NCC and the southern part of the XMOB and their tectonic evolutions.The general features of the Bayan Obo-Chifeng fault have been preliminary srudied in the previous studies.However,the framework and detailed structures of the fault system as well as its evolution process remain unconstrained so far.In this study,Damoo country and the northern part of Bayan Obo in Inner Mongolia have been chosen to conduct detaile study on the Bayan Obo-Chifeng fault,including its geometry,kinematics and geochronology.The western part of the Bayan Obo-Chifeng fault system are composed of NWW oriented thrust faults,NWW striking normal faults,NW trending dextral strike-slip faults,and nearly E-W extending bedding parallel faults.Chronological data and the intersected relationships between the above faults and adjacent geological bodies reveal that,this region experienced four major tectonic events during late Paleozoic and Mesozoic:(1)From Early Permian to Late Triassic,nearly E-W extending synfolding bedding parallel thrust system(D1): The buckle folding associated faults developed along the lithologic boundaries,and the displacements were generally small.Nearly S-N compressional regional stress field may be related to the underthrust paleo-Asian ocean to the NCC.(2)From Late Permian to Late Triassic,nearly NWW extending thrust system(D2): With large displacement,the NWW extending thrusts were widespread throughout the study area.NNE-SSW compressional structures may be formed during the collision between the NCC and the Siberian Craton.(3)From Late Triassic to Early Cretaceous,NWW-NW dextral strike-slip fault system(D3): The system includes the NW-trending Damaoqi north dextral strike-slip fault system and the NWW extrending dextral strike-slip fault and the conjugate shear joints in the Gebusi granite.The above structures were formed at the same time under the NNW-SSE compressional stress field,and may be related to the intercontinental subduction between the North China Craton and the South China Craton,the collision between Qiangtang plate and Qaidam plate,and southern subduction of the Mongol-Okhotsk plate to Mongolia plate.(4)From Early Cretaceous to Late Cretaceous,NWW extrending normal fault system(D4): Represented by the Li Sangou normal fault,this normal fault with sinistral strike-slip component extends NWW.Li Sangou normal fault controls the depositional range of the Early Cretaceous faulted basin in north.The regional stress field is NNE-SSW stretching.The fault system may be related to the roll back of Pacific plate.Because of the influence of pre existing faults,the faults in the area show differences in extension direction.