Tectonic Evolution And Crustal Rheology Of Yiwulushan In Western Liaoning Province During Late Mesozoic

Two stages of shear events happened in the Yiwulushan area during late mesozoic forming the Yiwulushan metamorphic complex firstly(middle-late jurassic)by ductile shearing from nearly south to north in deep crust and then the Waziyu metamorphic core complex secondly(early cretaceous)by extentional crustal detachement from NWW to SEE in upper-middle crust.The asymmetric Waziyu metamorphic core complex is composed of the supradetachment basin in the hanging wall(Fuxin-Yixian basin),the Waziyu main detachment fault zone,the Waziyu Extensional Shear Zone(WESZ)in the footwall.The detachment fault surface of Waziyu metamorphic core complex tends to NWW,dips about 30°.The whole tectonic rock series in the detachment fault zone reflects that the footwall rocks experienced a progressive evolutionary process from ductile deformation(ductile to brittile deformation)in deep crust to brittile deformation in shallow crust.A group of mineral stretching lineations developed in Waziyu metamorphic core complex shows NWW to SEE.Macro-and micro-structure features indicate foot-to-the ESE sense of shear.Yiwulushan metamorphic complex is a arched roof stretching NNE and consists of Yiwulushan Ductile Shearing Zone(YDSZ)and Yiwulushan rock mass.There develops another group of stretching lineations tending to nearly south or north.Kinematics symbols indiacate top-to-the SSW sense of shear.Base on micro-structure analysis,quartz crystallographic preferred orientation analysis,quartz dynamic recrystallization thermometer,differential stress piezometer,we worked out that WDSZ developed in middle-upper crust at greenschist facies situation and lifted gradually during shearing,while YDSZ in lower crust at amphibole facies situation with no remarkable lifting and the rocks entirely undergo ductile deforemation.Finite strain measurement and kinematic vorticity analysis demonstrates that the average kinematic vorticity values(Wk)of early YDSZ is 0.63 which indicates the fair contribution between pure shear and simple shear,while simple shear is dominate in WESD of which the average Wk is 0.86.In all the pole mole circle diagrams,we have?1>?2,indicating both of the WESZ and YDSZ are general shear zone and thinning during shearing.The Waziyu metamorphic core complex and Yiwulushan core complex together proves that the destruction of the north china craton and lithosphere thinning in late mesozoic started in middle-late Jurassic and peaked in early cretaceous.Extentional crustal detachment in upper-middle crust and ductile shearing in lower crust are the two factors of lithosphere thinning.