Late Mesozoic Strike-slip Tectonic System And Its Evolution In The Western Segment Of Daqingshan,Inner Mongolia

The Daqingshan strike-slip tectonic system in Inner Mongolia is located on the northern margin of the North China Plate,belonging to the Yinshan-Yanshan Orogenic belt.It formed in the post-collision orogeny process which is a good place for the development of the strike-slip structure.The Mesozoic tectonic deformation and its evolution in this region records intraplate structural deformation process in the northern North China plate after the closure of the Paleo-Asian Ocean,and is significant for the systematic research.In recent years,the late Mesozoic thrust nappe structure and extensional structure have been deeply studied in this area,but the study of the strike-slip structure has rarely been reported.The latest structural mapping shows that numerous strike-slip fault develops in the western segment of Daqingshan.The partially inherited and strongly modified the early thrust system.Therefore,detailed geometric,kinematic and chronological studies of the strike-slip fault system in the area will help to clarify the tectonic evolution process in the Daqingshan area and further provide new geological evidence for exploring the remote effects resulted from the closure of Mongolian-Okhotsk Ocean.Detailed structural mapping and geometric and kinematic observations of important faults demonstrate that at least three structural deformations developed in the western segment of the Daqingshan during the Late Mesozoic.1)During Middle Jurassic,a series of D1 E-W thrust nappe structures formed under NS extrusion;2)During Late JurassicEarly Cretaceous,a large-scale D2 NW dextral strike slip system formed NNW-SSE and NNE-SSW compression;3)After early Cretaceous,under NE-SW compression,significant sinistral strike slip activity occurred along the early E-W-trending fault and reworked the NW dextral strike-slip system.The main structural combination styles of the strike-slip tectonic system in this area was preliminarily determined,including combination of the oblique-type strike-slip fracture similar to the R fracture in the Riddle shear system,the combination of the ramp thrust structural styles and lateral bulge in the strike-slip fault system.According to the paleo-stress inversion from fault-slip data,the paleo-stress field sequence for the early stage thrust fault system and the subsequent dextral and sinistral strike-slip fault systems is established.It is believed that these tectonic deformations mainly occurred after Middle Jurassic and may be related to the closure of MongolianOkhotsk Ocean.