Strain、 Kinematic Vorticity Analysis And Ductile Thinning Of The Ductile Detachment Zone Of Metamorphic Core Complex In North China Craton

A series of metamorphic core complex are located at the northern margin of the North China Craton, and regarded as a window to understand the crust extension. We choose the Louzidian, Yunmengshan, Yiwulvshan and Liaonan as the research objects; and select strain measurement and kinematic vorticity as research methods; and determine the strain types, shear types and ductile thinning of metamorphic core complex and its detachment fault zone. Based on these, we can investigate the formation mechanism, ductile thinning of these metamorphic core complex, and further provide the evidences of mechanism of crust extension and thinning, quantification of thinning and geodynamic background of the North China Craton.The Louzidian low-angle ductile shear detachment zone at the south of Chifeng is a SE-dipped, low angle normal fault system. It is composed mainly of ductile shear zone, ductile-brittle shear zone and brittle fault zone. The ductile shear zone consists of, from bottom to top, mylonitic rocks, protomylonites and mylonites. Finite strain measurement of feldspar strain markers from those rocks using the Rf/φmethod shows strain intensities (Es) increase from 0.47-0.56 to 0.59-0.62 and to 0.67-0.96, respectively, and the strain types of the protomylonites and mylonties are flattening strain (lod parameter of protomylonites v=-0.228——0.535; lod parameter of mylonites v=0.14-0.87), respectively. The kinematic vorticity values (Wk) estimated by the Polar Mohr diagram and the Rigid Grain Net range from 0.81 to 0.90 with an average of 0.85 for the protomylonites, and from 0.51 to 0.80 with 0.66 on average for the mylonites; W(?) values of the extensional crenulation cleavage (i.e. C’) estimated by C method range from 0.63 to 0.37 with an average of 0.50. Dip angles of the maximum principle stress determined using the Maximum effective moment criterion evolved from 61°to 69°and to 75°, and finally normal to shear direction.The Yunmengshan metamorphic core complex is a NE trend, SE-dipped, low-angle normal fault system. It is composed mainly of, from bottom to top, mylonitic rocks, protomylonites and mylonites. Finite strain measurement of feldspar strain markers f using the Rf/φmethod suggests strain intensities increase from mylonitic rocks (Es=0.66-0.70), proto mylonite (Es=0.70-0.86) and mylonite (Es=0.72-1.0). Fulin and Hossack diagrams indicate strain types is close to flattening strain(lod parameter v=0.29-0.91). Kinematic vorticity measurements using Mohr diagram method suggest that foliations and lineation of mylonite (0.63<Wk<0.85) record a bulk simple-dominated general shearing at the beginning of evaluation of Hefangkou detachment fault; and Extensional crenulation cleavage (0.34<Wk<0.77) recorded a bulk pure-dominated general shearing at the end of evaluation of Hefangkou detachment fault. Based on the strain measurement and kinematic vorticity, we estimate the quantification of thinning ranging from 330 to 430m.The Yiwulvshan metamorphic core complex is a NE trend, NW-NNW-dipped, low-angle normal fault system. It is composed mainly of, from bottom to top, ductile shear zone (mylonite zone), ductile-brittle shear zone and brittle detachment fault plane. Ductile shear zone consists of mylonitic gneiss which lies at the bottom, and mylonite which locates at the top. Ductile-brittle shear zone performs as brittle structure overprinting the pre-existed ductile shear zone. Brittle detachment fault plane registers as fault gouge and fault breccia. Finite strain measurement of quartz strain markers using the Rf/φmethod suggests strain intensities of mylonite chang from 0.51 to 1.12. Fulin and Hossack diagrams indicate strain types is close to flattening strain (lod parameter v=0.47-0.90). Kinematic vorticity measurements using Mohr diagram method, oblique foliation in quartz ribbons and finite strain method suggest that foliations and lineation of mylonite record a bulk simple-dominated general shearing, the Kinematic vorticity ranges from 0.61 to 0.96 with an average of 0.80; and Extensional crenulation cleavage recorded a bulk pure-dominated general shearing, and its Kinematic vorticity changes from 0.24 to 0.53 with 0.37 on average. Based on the strain measurement and kinematic vorticity, we estimate the quantification of thinning ranging from 320 to 520m.The Liaonan metamorphic core complex is a NE trend, NNW-dipped, low-angle normal fault system. It is composed mainly of, from bottom to top, mylonitic granitic gneiss, ductile shear zone, chloritized zone, brittle detachment fault plane and the non-metamorphosed rocks within upper plate ranging from Proterozoic to Paleozoic, and to Mesozoic. Ductile shear zone consists of mylonitic granitic gneiss and mylonites. Finite strain measurement of feldspar strain markers using the Rf/φmethod suggests strain intensities of mylonite changing from 0.36 to 1.0 with an average of 0.75. Fulin and Hossack diagrams indicate strain types is close to flattening strain(lod parameter v=0.11-0.98). Kinematic vorticity measurements using Mohr diagram method, and finite strain method suggest that foliations and lineation of mylonite record a bulk simple-dominated general shearing, the Kinematic vorticity ranges from 0.74 to 0.96 with an average of 0.85; and Extensional crenulation cleavage recorded a bulk pure-dominated general shearing, and its Kinematic vorticity changes from 0.10 to 0.84 with 0.58 on average. Based on the strain measurement and kinematic vorticity, we estimate the quantification of thinning of Shuiyuandi, Dongjiagou, Dalijiatun, Sanshilipu, Pulandian and Yuantai, and their quantification of thinning are 395m,705m,212m, 387m,437m and 130m, respectively.Based on the field observations, we find there are two types of fabrics in the studying mylonites, one is S/C fabric defined by orientational quartz and feldspar; the other is extensional crenulation cleavage (ecc) or C’, and it is characterized by discrete sub-parallel minor shear bands; they transect and displace the mylonitic foliation and stretching lineation at a small angle and cause previous foliations rotation. These suggest that the ecc formed during relatively later increments of the same progressive deformation.The research of kinematic vorticity indicate that foliation and lineation of mylonites record a bulk simple-dominated general shearing, and extensional crenulation cleavage recorded a bulk pure-dominated general shearing; together with the sequence of foliation and extensional crenulation cleavage of mylonites, we consider that the mylonites in study area experience simple-dominated at early period, and pure-dominated at later period during mylonites formation. This result depicts subtly the mechanism of deformation of detachment fault zone, and further demonstrates that simple shear at the early stage and pure shear at the late stage in the formation of metamorphic core complex has probably general significance. Crust extension represented by simple shear and magma intrusion represented by pure shear are dominated factors that control the formation of metamorphic core complex and its detachment fault zone. Strain and kinematic vorticity researches are the effective methods to reveal the dynamics of tectonic regime transition and lithospheric thinning in North China during Late Mesozoic. The quantification of ductile thinning estimated by strain and kinematic vorticity indicate that there really existed ductile thinning during the formation and evolution of detachment fault zone; although the quantification of ductile thinning is a little, it helps us understand preferably the extension and thinning of the North China Craton. There are some problems need to express clearly that the strain of feldspar is lower than the whole rock strain, whereas the strain of quartz can represent the whole rock strain; the veins which are characterized by locality and strong deformation are not consideration; so, the quantification of ductile thinning estimated by us is the minimum.This paper further confirm that there is a tectonic regime transition from crust thickening to crust extension thinning in the North China Craton at Mesozoic, and demonstrate the Maximum effective moment criterion is an effective method to interpret the low angle normal fault system.This conclusion coincides with other classical studies, such as the Yagan detachment fault in the Sino-Mongolian border area and the Chelmos shear zone of External Hellenides Greece. These shear zone are characterized by a transition of tectonic regime from simple-dominated shear at early stage to pure-dominated shear at a late stage which produced by magma intrusion.