The Structure,Evolution And Regional Tectonic Significance Of The Wulian Detachment Fault Zone,Jiaodong Peninsula,China

The large-scale lithospheric thinning of the North China Craton occurred in the Late Mesozoic accompanied by regional extensional processes at the crustal level.Under this extensional background,a series of extensional structures developed in the interior and its margins of the North China Craton.The Wulian detachment fault zone located in the Sulu orogenic belt is one of the examples.The Wulian detachment fault zone,developed in the Jiaodong peninsula,is an important tectonic unit of the Wulian metamorphic core complex,which separates the UHP metamorphic rocks in the Sulu orogenic belt as the lower plate from the Zhucheng basin and the basin basement in the upper wall of the Sulu orogenic belt.The strike of the fault zone is generally from NNE to NE from south to north,inclined to NNW,with gentle dip.The fault zone is characterized by "wave-tile" extension in plane.It is mainly composed of fault breccia and mylonite,and downward transition to mylonitized gneiss.The orientation of the stretching lineation in the fault zone shows the kinematic characteristics of the near EW extension.The detachment fault zone generally has shear action in the W or WNW direction,indicating that the upper wall extends and slides in the W or WNW direction.The results of finite strain calculation show that the Flinn index K of Wulian detachment fault zone ranges from 0.98 to 2.0 with an average value of 1.35,which indicates that the extension deformation process of the late detachment fault zone has the characteristics of uniaxial tension and plane strain.The kinematic vorticity values of mylonite and primary mylonite obtained by polar molar circle method,quartz C-axis fabric method,extensional fold cleavage method,Rs/? method are 0.64?0.93,0.63?0.98,0.71?0.93,0.6?0.92,with mean values of 0.83,0.86,0.80 and 0.78,respectively.The results of strain measurements and kinematic vorticity estimation show that the general ductile shear zone formed under extensional tension of the detachment fault zone.From NW to SE,there is a decreasing trend in general and a significant decrease in part,which indicates that although simple shear and pure shear exist simultaneously in the shear process,their compositions are different in different parts of the shear zone.In the upper and middle part of the shear zone,the simple shear action is more obvious,showing higher vorticity value(vorticity value>0.75,maximum up to 0.98),lower thinning rate and lower K value.However,from the direction of detachment fault zone down to the lower plate,the component of pure shear strain increases obviously,showing lower vorticity value(vorticity value<0.70,minimum up to 0.64),relatively higher thinning rate and higher K value.The dynamic recrystallized quartz grain boundaries in Wulian detachment fault zone have statistical self-similarity and obvious fractal characteristics.The fractal dimension of subgranular rotational recrystallized quartz particles ranges from 1.260 to 1.319,with an average value of 1.276,and that of bulging recrystallized quartz particles ranges from 1.217 to 1.297,with an average value of 1.256.In the process of ductile deformation,the paleo-differential stresses during the deformation stages of subgranular rotational recrystallization and bulging recrystallization are different,ranging from 7.84 MPa to 21.58 MPa and from 18.51 MPa to 56.65 MPa,respectively.Based on the formula for calculating strain rate by fractal dimension,the strain rates of subgranular rotational recrystallization and bulge recrystallization quartz particles are 10-8.4?10-7.7S-1 and 10-10.5?10-9.7S-1,respectively.Using Kronenberg et al.parameters,quartz strain rate of subgranular rotational recrystallization and bulge recrystallization are 10-12.88?10-11.73s-1 and 10-13.72?10-12.46S-1,respectively flow rate of ductile deformation in this area is higher than that of general ductile shear zone,which may be related to the rapid detachment and extension of Wulian detachment fault zone.The formation age of Wulian detachment fault zone is 128 Ma to 123 Ma.Since the lower plate beginning of rapid exhumation at 128 Ma,the exhumation rate of Wulian detachment fault zone may reach its peak at 126Ma.The ductile shear activity of the fault zone has stopped at 122Ma,and the uplift after that is dominated by brittle faults.Combined with regional tectonic background analysis,the formation and evolution of Wulian detachment fault zone is a reflection of lithospheric thinning and destruction in the eastern margin of North China Craton,a manifestation of widespread but inheterogeneous lithospheric thinning in the eastern part of North China Craton,and the result of simple shear of fault zone caused by crustal extension,rapid uplift of the lower plate and upwelling of magma,and pure shear applied vertically to fault zone.