Tectonic Evolution For The Yunmengshan Metamorphic Core Complex And The Relationship With The Destruction Of The North China Craton

Craton destruction and lithospheric thinning are hot geodynamic topics in recent years. Theyare mainly characterized by lithospheric thinning, geothermal increasing, large-scale magmaactivities, and development of metamorphic core complex (MCC) and extensional basins. As oneof the responses of lithospheric thinning, MCC preserves the deformation characteristic in thedeeper crust. The Yunmengshan MCC (YMCC) is situated in the central segment of the Yanshantectonic belt north of the North China Craton (NCC). Because of its special tectonic location, it isregarded as the perfect object to research the craton destruction and Yanshanian tectonic belt.Based on detailed field investigation, two important ductile shear zones (the Sihetang shearzone and Dashuiyu shear zone) and a series of NE-NNE brittle normal faults (e.g. Hefangkou faultand Qifengcha-Liulimiao fault) are recognized in the study area. Because of isostatic rising, thearched Sihetang shear zone appears along the western, northern and northeastern margins of theYunmengshan Batholith (YMSB). It is developed along the contact zone of the YMSB, Archeanbasement and Proterozoic cover. Exposure structures, microstructures and quartz C-axis fabrics allindicate top-to-SSW sense of shear for the Sihetang shear zone. The overturned strata at the northof the YMSB show the Sihetang shear zone to be a thrust belt.The Dashuiyu shear zone occurs along the southeastern margin of the YMSB. It is a NE-toNNE-striking detachment shear zone with a top-to-the-SE shear sense. The shear zone always dipsSE and becomes steeper SW-wards with a preferred dip angle of30°in the north and60°in thesouth. Mineral elongation lineation in the shear zone plunges SE consistently. The ductile shearzone is exposed along the southern and middle segments, while its northern segment shows abrittle normal faults indicating decreasing exhumation NE-wards. A series of NE-NNE brittlenormal faults occur in east and west of the YMSB. The Hefangkou normal fault occurs mostlyalong the eastern margin of the Dashuiyu shear zone. It dips20°–70°toward SE and controls thedevelopment of the Huairou Basin filled with volcanic rocks of the Early Cretaceous ZhangjiakouFormation. The Qifengcha-Liulimaio fault occurs in the basement rocks west of the YMSB. Itstrikes NE and dips ca.60°toward NW.Microscopic observation reveals that quartz in the Sihetang shear zone and Dashuiyu shearzone mostly has a grain boundary migration (GBM) recrystallization. Feldspar in the Siheatangshear zone is widely recrystallized with a bulging (BLG) type as well as coexistence of GBM andSR (subgrain rotation). However, feldspar in the Dashuiyu shear zone commonly performs asbrittle fragments and BLG type recrystallization. Combining the characteristics of quartz C-fabrics,it is eastimated that deformation temperature of the Sihetang shear zone is400℃-600℃, and that of the Dashuiyu shear zone is300℃-520℃. Kinematic vorticity values are calculated by themethod based on relation between quartz C-fabrics and strain ratio, shape factor method, andporphyroclast aspect ratio method. The vorticity values of the Sihetang shear zone lies between0.61and0.8, indicating dominant pure shear component of the deformation. The values of theDashuiyu shear zone are between0.75and0.98, showing dominant simple shear.A series of measurements for brittle fault planes and striations in the YMCC have beenconducted for calculating principle stress orientations in this work. The results show that the thesenormal faults were developed under the regional NW–SE extension. It is demonstrated bycomparison that the extension orientation related to the formation of the normal faults is parallel tothat shown by elongation lineation in the Dashuiyu shear zone. It is inferred, therefore, that boththe normal faults and Dashuiyu shear zone resulted from the same extensional dynamic and theYMCC experienced the extensional deformation during uplifting.Zircon U–Pb dating of dykes and plutons as well as mineral40Ar/39Ar dating demonstrate timesequences for tectonic activities in the stusy area:1) Intensive magma activities took place fromthe Late Jurassic to the Early Cretaceous, leading to intrusion of the159Ma Shatuozi pluton, the156-153Ma Changyuan pluton, the158-151Ma Shicheng pluton and the148-142Ma YMSB.2)The Sihetang shear zone occurred at the late stage of the YMSB intrusion, during143-138Ma.3)During135-126Ma, the YMCC experienced intensive shearing of the Dashuiyu shear zone andsynkinematic intrusion.4) The MCC was subjected to rapid exhumation during125–114Ma dueto isostatic rising.5) During113-100Ma, the MCC changed into slow uplifting under the sameextensional condition.Within the central segment of the Yanshan tectonic belt, the Sihetang thrust-ductile shear zoneis identified as having developed during Phase B of the Yanshanian Movement. The time gap isrepresented by the angular unconformity marking Yanshanian Phase B. The age for volcanic rocksof the Zhangjiakou Formation above the unconformity is between143Ma and126Ma. The age ofthe Tuchengzi Formation below the unconformity lies between145Ma and130Ma. By datingsyntectonic dykes, the formation time of the Sihetang ductile shear zone is suggested to be143-138Ma, which accurately limits the time period of Phase B of the Yanshanian Orogeny.The Phase B of the Yanshanian Orogeny is recognized as the compressive activity caused bythe closure of the Okhotsk Ocean in the north. In the Yunmengshan area, it is presented by theSihetang thrust-ductile shear zone with a shear sense of top-to-SSW. The regional tectonic ofeastern China change into the paleo-pacific tectonic at this time. The NCC destruction reaches thepeak, with a large amount of MCC development in the northern margin of NCC. Under theconsistent NW-SE extension, the YMCC experienced shearing of the Dashuiyu shear zone, followed by isostatic rising. Then, the NE-to NNE-striking normal faults occur in the shallowcrust. It is suggested that the YMCC formed by the rolling-hinge model under the NW-SEextension. This formation mechanism is common for the intraplate MCCs in the NCC.Micafish with various shapes are excellent shear sense indicators in the mylonites in the studyarea. To understand its formation mechanism and reliability as shear sense indicator, thenumerical simulations are conducted for micafish. Micafish is regarded as anisotropic power-lawellipsoidal inhomogeneities embedded in a power-law viscous material. Naturally observedgeometries of micafish are reproduced in the modeling, and their formation mechanisms arefurther analyzed. It is demonstrated that their deformability by slip along cleavage and theiranisotropy lead to the commonly observed micafish geometries in mylonites. Regardless of theinitial condition of micafish, the shape long axis and the cleavage trace observed on the vorticity-normal section will most commonly end up at a small antithetic angle relative to the shear plane orparallel to it.