Strain, Kinematic Vorticity and Ductile Thinning along the Detachment Zone of the Yunmeng Shan Metamorphic Core Complex, Beijing, China

The Yunmeng Shan metamorphic core complex （MCC） is composed of the lower plate, the upper plate and the detachment zone. The detachment zone consists of ductile shear zone （mylonite zone）, chloritized microbreccias zone and the brittle fault plane. The ductile shear zone contains mylonitic rocks, protomylonites, and mylonites. Finite strain measurements of feldspar porphyroclasts from those rocks using the Rf/φ method show that the strain intensities increase from mylonitic rocks （Es=0.66-0.72） to protomylonites （Es=0.66-0.83）, and to mylonites （Es=0.71-1.2）. The strain type is close to flatten strain. Kinematic vorticity estimated by Polar Mohr diagrams suggest that foliations and lineation of mylonite （0.47〈Wk〈0.85） record a bulk simple-dominated general shearing at the initial evolution stage of the Yunmeng Shan MCC＇s detachment zone; and the extensional crenulation cleavage（ecc） （0.34〈Wk〈0.77） recorded a bulk pure-dominated general shearing at the later stage of the evolution. Kinematic vorticity measurements also show that the Yunmeng Shan MCC detachment zone is a result of a combination of simple-dominated general shearing caused by crustal extension at the early stage and pure-dominated general shearing caused by MCC uplifting at the late stage. The ductile thinning estimated by finite strain measurements and estimation of Kinematic vorticity ranges from 52% to 82%, which is the minimum thining estimation. Our studies provide new evidence for mechanisms of the Yunmeng Shan MCC detachement zone.

Deformation characteristics of granitic rocks in Erguna ductile shear zone,NE China

The Erguna ductile shear zone is situated in the Erguna Massif,which has been exposed along the eastern bank of the Erguna River in northeastern China.The authors present comprehensive study results on the macro-and micro-structures,finite strain and kinematic vorticity,quartz electron backscatter diffraction(EBSD)fabrics,and geochronology of granitic rocks in the Erguna ductile shear zone.The deformed granitic rocks have experienced significant SE-trending dextral strike-slip shearing.Finite strain and kinematic vorticity in all deformed granitic rocks indicate that the deformation is characterized by simple sheardominated general shearing with S-L tectonites.Mineral deformation behaviors and quartz C-axis textures demonstrate that the deformed granitic rocks developed under greenschist to amphibolite facies conditions at deformation temperatures ranging from 450 to 550℃.New LA-ICP-MS zircon U-Pb ages indicate that these granitic rocks were formed in Early Triassic(~248.6 Ma)and Early Cretaceous(~136.7 Ma).All the evidence indicates that this deformation may have occurred in Early Cretaceous and was related to the compression resulting from the final closure of the Mongol-Okhotsk Ocean.

Deformation Kinematics of Main Central Thrust Zone(MCTZ)in the Western Himalayas

The main central thrust(MCT)is one of the major thrusts in Himalayas.In central Himalaya,MCT was defined as a contact between underlying Lesser Himalayan Sequence(LHS)and overlying higher Himalayan crystallines(HHC).However,in the Kashmir Himalayas,the main central thrust zone(MCTZ),shear zone associated with MCT,is overlain by Kashmir Tethyan Sequence suggesting that the MCTZ has been deformed through a mechanism different than the mechanism responsible for MCTZ evolution in other parts of the Himalayas.In the present study we used structural,microfabric and kinematic analyses to investigate the deformation kinematics of MCTZ.Microstructural investigation revealed that the quartz in orthogneiss mylonites of MCTZ was dynamically recrystallized by grain boundary migration(GBM)and sub-grain rotation recrystallisation(SGR)with top-toSW sense of shear.The mean kinematic vorticity number(W;)just above the thrust ranges from 0.72to 0.84(40%–52%pure shear component)decreasing upwards to 0.65–0.71(35%–50%pure shear component).Deformation in the MCTZ is characterized by Rxzstrain ratio varying from 2.7 to 8.The present study suggested that the MCTZ suffered 3%–40%vertical shortening and 3%–66%transportparallel elongation.The results suggested that the HHC’s were not completely exhumed to the topographic surfaces in the Kashmir Himalayas.Along the basal decollement,i.e.,the main Himalayan thrust(MHT),the deformation continued until MCTZ reached the brittle-ductile transition where deformation mechanism changed to the brittle and the MCTZ rocks were transported to the surface through slip on brittle MCT.