The Mesozoic Tectonic Deformation And Evolution Of The Northern And Southern Margins Of The Northeast China

The tectonic deformation and evolution of the north and south margins of northest China are important which controlled the tectonic deformation of the inner part of the northeast China and they are also the important segments of the central Asia tectonic belt. We choosed3important areas to research these two margins:Mongolia-Okhotsk collisional belt, the margin between North China Craton and Northeast China Terren and the Huadian basin.As the important part of the Central Asian Orogenic Belt, the Tectonic deformation and dynamic characteristics of the Mongolia-Okhotsk collisional belt have attracted much attention. Along the middle part of the Mongolia-Okhotsk collisional belt,5ductile deformation sites and1brittile deformation site were analyzed to reveal its tectonic deformation and dynamic features. B style fold, crumple structure, A style fold, mineral stretching lineation and S-C fabric indicate a NW-SE shearing. This information reveal that maybe experienced one large collision or more steps collions with the same direction, the Mongolia-Okhotsk collisional belt finally was formed. The brittle deformation of Site mg6was analyzed and two paleo-stress field were restored: NW-SE compression stress field and E-W compression stress field. The NW-SE compression stress field maybe was the result of the distant effect of Yanshanian movement during the Jurassic and Cretaceous. The E-W compression stress field maybe was the result of the distant effect of the India-Asia collision during the early Cenozoic.Based on the tectonic deformation analysis along the margin between North China Craton and Northeast China Terren, A large ductile shear zone which dominantly dip to north with a dip angle about30°to50°was confirmed. Lineations on the shear planes of the most part of this shear zone are mainly dip to the north. According to the shape of S-C fabric, rotational porphyroclast systems and micro structure of the deformed rocks, we can infer that this kind of lineations indicate a north to the south thrust shear. We can get the original rock age of the ductile shear zone by the SHRIMP zircon U-Pb dating and MC-ICP-MS zircon U-Pb dating methods. The deformation age of this ductile shear zone can be gotten by the40Ar-39Ar dating of syntectonic minerals suck as muscovite, biotites and hornblendes. We found that this large ductile shear zone formed during the late middle Jurassic and early late Jurassic. The relationship between this shear zone and Dunmi fault was also be considered. According to some geologists the40Ar-39Ar syntectonic muscovite age of the Dunmi fault was161±3Ma which represents the age of the strong sinistral strike movement. This age is nearly the same with the forming age of the ductile shear zone along the margin between North China Craton and Northeast China Terren. This may mean at the same time with the formation of this shear zone, Dunmi fault cuted it. We think the dynamic setting to form this shear zone is related with East Asian multi-direction convergent event which happened during the middle to late Jurassic.The NE striking Huandian Basion, situated on Dunmi Fault of the north branch of TanLu Fault in the northeast of China with more than25Km width and100km length, is a key region in deciphering the Late Mesozoic-Early Cenozoic tectonic evolution of the northeast China. Based on a field analysis of fault-slip data collected from different rock units, polyphase tectonic stress fields were documented and the changes in orientation of the principal stress were addressed, and finally a four-phase strss evolution of the basin was established during the Late Mesozoic-Early Cenozoic. The earliest one with N-S compression and E-W extension occurred in the Mid-Late Jurassic, caused by the combined effect of the plaleo-pacific plate and the Mongolia-Okhotsk Oceanic crust subducting under the Northeast China. The subsequent NW-SE extensional regime was responsible for the initial opening and sedimentary infill of the basin during the earliest Cretaceous,which occurred coevallywith the extensive Early Cretaceous magmatism, volcanism and extensional doming in Northest China. The tectonic regime then changed, in the late Early Cretaceous, to a compressional one with NW-SE compression and NE-SW extension, causing the inversion of this extensional basin. This compression affected the whole Northeast China, resulted in folding of the Late Cretaceous strata which underlies unconformably Early Cenozoic formation, reacting of pre-existing faults and uplifting of the pre-existing granite in the basin. This compressional phase seems to have lasted through the Early Paleogene and ended in the Late Paleogene by a compressional regime with NE-SW compression and NW-SE extension, as attested by the stratigraphic unconformity at the base of the Neogene in the Huadian Basin. The temporary evolution of these tectonic stress fields documented in the Huadian Basin provides a new insight into the influences of different plate tectonics exerting on the Northeast China over the Late Mesozoic to Early Cenozoic time. We infer that the Huadian Basin was influenced, inthe Middle Jurassic-Early Cretaceous, by the process of southward subduction of Mongolia-Okhotsk Oceanic crust and westward subduction of paleo-Pacific plate; and in the Late Cretaceous-Early Paleogene mainly by the subduction along the west pacific margin; and finally in the Late Paleogene by the far-field effects of India-Asia collision.