Deformation Characteristics Of Nappe Structure And Ductile Shear Zone In The Mazongshan Area Of The Beishan Orogenic Belt

Beishan is located in the east of Xinjiang Uygur Autonomous region,northwest Gansu Province and western Inner Mongolia Autonomous region.In the tectonic position,it is mainly composed of the Kazakstan-Siberian plate in the north and the Tarim plate in the west,and the North China plate in the southeast.After the later evolution of collision collage,a unified continent was formed.As a combination of North China plate and Tarim plate,the study of geological structure in this area provides important support for understanding the evolution of the whole joint zone.At the same time,it is of great significance to study the large-scale tectonic-magmatic activity-mineralization age in this area.At present,the study shows that there are Mesozoic thrust nappe structures and ductile shear deformation in the whole area of Maemong Mountain,but there is a lack of study on the structural characteristics and regional tectonic evolution of the Mesozoic nappe structures.In order to solve this problem,this paper selects the nappe structure in the northwest,middle and southeast segments of Ma Mengshan Mountain and the key structural parts of the ductile shear zone in the middle section as the research objects,through field structural analysis and chronology.The regional tectonic evolution processes,such as the tectonic deformation characteristics of the Mesozoic and the deformation of the ductile shear zone in the middle section of the Maemongshan area,are basically determined,and the North China plate is discussed.The influence of oceanic crust subduction on the geological evolution of the study area since the late Paleoera of the Tarim plate and the Siberian plate.Through the above research,the main points are as follows:(1)the nappe structural characteristics of the northwest section,the middle section and the southeast section of Majongshan area are determined,in which the overall nappe structural characteristics of the northwest section and the southeast section are basically the same.For the north to the south of the thrust nappe.In the Jurassic strata in the mahongshan coal mine in the middle part of the mahongshan area,the strata are affected by the north-south compression structure and fold,such as the mahongshan coal kiln synclinal,its axial plane is near east-west,and the north wing is in the mahongshan coal kiln area.Under the influence of the east-west reverse fault on the north side,the occurrence of local inversion shows that the nappe structure found in Ma Mengshan area is a fold fault assemblage in the Mesozoic brittle domain.(2)the ductile shear zones of Nemequan and Leba Quan in the middle part of Ma Mengshan area have thrust from west to east south,and have the characteristics of right lateral shear movement.Based on the zircon U-Pb dating of undeformed granite veins in mylonite belt in ductile shear zone,it is determined that the upper limit of ductile shear deformation age in Nemequan and Lebaquan areas is 416Ma,combined with the previously reported age.The ductile shear deformation age is roughly limited to 423-416Ma,which may be related to the northward and eastward subduction of the Niu circle oceanic shell towards the wild horse spring arc.(3)after the late Paleozoic orogeny,the Majongshan area along the Sino-Mongolian boundary experienced a major tectonic event:the oversized nappe structure from north to south caused by the shortening of the crust during the orogenic period.