Deformation characteristics and formation mechanism of the Yunmengshan metamorphic core complex

Abstract

The Yunmengshan metamorphic core complex in the middle part of the Yanshan Fold and Thrust Belt records crust extension processes of the eastern North China Craton during its peak destruction. Development of the metamorphic core complex was controlled by the generally NNE-striking Dashuiyu Shear Zone. The shear zone dips SE and becomes shallower NE-wards, leading to exposures of a ductile shear zone in the southern and middle parts and brittle faults in the northern part. Exposure structures, microstructures, and quartz C- axis fabrics indicate that the ductile shear zone belongs to an extensional shear zone with a top-to-the-SE shear sense. Deformation temperatures of 300-520 ℃ suggest a midcrustal origin for the ductile shear zone. A ductile deformation belt in the footwall of the shear zone is only as wide as 1-3 km, indicating no widespread mid-crustal ductile flow in the region during the deformation. Zircon U-Pb dating of dykes and plutons as well as hornblende and biotite 40 Ar/ 39 Ar dating demonstrate that the metamorphic core complex originated at 135 Ma and experienced intense shearing of the Dashuiyu Shear Zone, development of the supradetachment basins, and synkinematic intrusion during 135-125 Ma. The metamorphic core complex was subjected to rapid exhumation during 125-114 Ma when the Dashuiyu Shear Zone suffered continuous activity and passive doming. The shear zone and its hanging wall were cut or replaced by a series of brittle faults when they were uplifted to a brittle regime, showing that exhumation took place in continuous extensional activities. The metamorphic core complex turned into slow exhumation in an extensional regime in the following latest Early Cretaceous. The evolution history suggests that the Yunmengshan metamorphic core complex was developed by the rolling-hinge model, a common formation mechanism for intraplate metamorphic core complexes in the North China Craton, under the continuous NW-SE extension during the Early Cretaceous (135-100 Ma).