Kinematic Processes Of The Paleozoic Accretion Along The Northern Margin Of The North China Craton And Southern Xing'an-Mongolia Orogen Belt:Insight From Tectonic Deformations

Studies on current plate tectonics kinematics show that oblique plate convergence is more common than orthogonal plate convergence,while the kinematic characteristics of plate convergence that preserved in ancient accretionary and collisional orogenic belts have rarely been reported.The northern margin of the North China Craton?NCC?adjacent to the Central Asian orogenic belt experienced a complex arc-continent collision and oceanic plate subduction during Paleozoic.Although a couple of tectonic models have been proposed previously to account for the accretionary process along the northern margin of the NCC,few of them are concerned with the kinematic and dynamic process of the accretion due to the lacking of detailed tectonic deformation analysis.The research area is located at Bayan Obo-Damao area,where spans North China Craton and Xing'an Mongolia Orogenic Belt?XMOB?.Combining detailed structural mapping and geological interpretation of satellite images from Google Earth,the relative tectonic deformation sequences were established following the identification of the tectonic deformation styles and the superimposed transformation relationships.Based on the radiometric age dating of key intrusive rocks,dikes and sills,the timing of the main deformations?D₁,D₂,D₄?was defined.Accordingly the Paleozoic tectonic deformation sequence has been established at the northern margin of the NCC and the southern XMOB.These results are compared with the Paleozoic accretionary orogenic process proposed by previous workers,and then a kinematic model is established.Main achievements of this study are summarized as follows.?1?An alternative method for qualitatively and quantitatively identifying,the shape and orientation of the early folds are proposed in the cross superposed buckling.It can be applied to the identification of early folds with large wavelength and small amplitude that are strongly modified by late tight folds.?2?The early regional NNW trending folds in the Bayan Obo Group were discovered for the first time and a three-generation fold deformation sequence was redefined.The deformation sequence includes?1?NNW trending open folds;?2?nearly E-W trending tight to isoclinal folds and thrust faults;?3?NNW trending folds.The main stage of nearly E-W trending folds were superimposed on the early NNW trending open folds,forming Mode 1cross superimposed fold patterns of different scales.The stratigraphic age,relationships between geological bodies and isotope dating suggest that the first two stages of folds were formed after the Early Cambrian and before the Early Permian.?3?The fold deformation styles and their relative deformation sequences of the Lower Paleozoic in the Bainaimiao arc belt,southern XMOB are identified.A NWW trending tight fold was discovered in the Ordovician Baoerhantu Group,and it was formed before the Late Silurian.Three stages of folds developed in the Late Silurian-Early Devonian Xibiehe Formation.The early NW trending tight folds developed locally.The main folds were NE trending tight folds,superimposed on the early NW folds,forming Type 2 superimposed fold patterns.The late NW trending gentle folds caused the significantly curving of the axial surfaces and axial traces of the main stage folds,showing Type 2 superimposed fold patterns.The isotopic dating of sills involved in the main folds suggests that the main fold deformation of the Xibiehe Formation occurred after the end of the Middle Permian??262 Ma?.?4?The geometry and the deformation periods of the Carboniferous Amushan Formation were found out,and the timing of the main folds was determined.Two stages of folds were determined.The early folds were NEE-NE trending tight folds,which constituted the main tectonic framework of Amushan Formation.The late folds were NWW-NW trending gentle folds.They superimposed on the early folds,and slightly transformed their plane distribution.According to the isotopic dating results of sills involved in the eary folds,the early NEE-NE trending folds formed after the end of the Middle Permian??264 Ma?.?5?Tectonic deformation sequence was established in the northern margin of the NCC and the southern XMOB during Paleozoic,and its corresponding relationship with the accretionary orogen process was discussed.?1?The D₁ folds include the NNW trending open folds of the Bayan Obo Group?BOG?and the NWW trending folds of the Baoerhantu Group before Late Silurian,which may record the left-lateral oblique arc-continent collision between the Bainaimiao arc and the NCC.?2?The D₂ folds are early NW trending tight folds locally developed in Xibiehe Formation before Late Carboniferous,which may indicate that the previous left-lateral oblique arc-continent collision between the Bainaimiao arc and the NCC was still continuing.?3?The D₃ folds are represented by the E-W trending tight to isoclinal folds of the BOG before the Permian,which may be related to the southward orthogonal subduction of the Paleo-Asian Ocean plate beneath the amalgamated Bainaimiao arc and the NCC in late Carboniferous and Early Permian.?4?The D₄ folds include the NE trending folds of the Xibiehe Formation and the NEE-NE trending folds of the Amushan Formation in the Bainaimiao arc.They formed after the end of the Middle Permian and before the Early Triassic,and may be the result of the final closure of the remaining Paleo-Asian Ocean and the continent-continent collision between the South Mongolia continent and the NCC.?6?The study of tectonic deformations indicates that a dynamic regime transition from sinistral oblique convergence to the orthogonal convergence and then to the right-lateral convergence occurred at the northern margin of the NCC during Paleozoic.