Evolution Of An Extensional Detachment Shear Zone: Microfabric, Kinematic And Rheological Study Of The Shuiyu Ductile Shear Zone, Yunmeng Mountains, Beijing

During Early Cretaceous the Yunmengshan metamorphic core complex underwent pure shear dominated to simple shear dominated deformation under amphibolite facies to low greenschist environment.With decreasing distance to the Hefangkou detachment fault, the Yunmengshan granites show ductile to brittle deformation characteristics. Microstructures change from subgrain rotation (SGR) recrystallized feldspar plus grain boundary migration (GBM) recrystallized quartz, to bulging (BLG) recrystallized feldspar plus SGR recrystallized quartz, then to fractured feldspar plus SGR recrystallized quartz, and finally to fractured feldspar plus BLG quartz.The crystallized preferred orientation of quartz in Yunmengshan granite changes from crossed girdle made of Y-maxima and X-maxima to asymmetric single girdle made of Y-maxima, and then to Z-maxima with decreasing distance to Hefangkou detachment fault, indicating that the deformation temperature decreases while the strain rate increases.The kinematic vorticity numbers calculated from porphyroclast method ranges from 0.70 to 0.78 and that from oblique foliation ranges from 0.85 to 0.94, which indicates that the total deformation is general shear while the later stage of the deformation is simple shear dominated, inferring that the earlier stage of deformation is pure shear dominated. Therefore, it can be inferred that the rock underwent mainly pure shear when the extension started due to the overlying rocks while the simple shear elements increased when the extension continued as a result of loading off of overlying rocks.The strength profile of mid crust was set up and the peak strength of the mid crust is 98.7 MPa, indicating that the mid crust of this area during late Mesozoic was weak.