Research On Structural Deformation And Dynamics Of The North Qinling Tectonic Belt

As the main part of central orogenic belt of China, The Qinling orogenic belt is always the interested research area among geologists. The north Qinling tectonic belt is located in the core of Qinling orogenic belt, which is bounded by Luonan-Luanchuan fault (LLF) in the north and Shangnan-Danfeng fault (SDF) in the south, and have undergone strong metamorphism and deformation. Therefore, deformation in the orogenesis of the Qinling orogen since the Palaeozoic is very important to reconstructing the last assembly process of the North China Plate and South China Plate. Previous research work in the region is mainly concentrated in the tectonic attribute of blocks in the north Qinling tectonic belt, but lack of fine structure analysis of internal block and boundary faults. In this paper, we present original results of our field investigation, and structural geology, metamorphic petrology and geochronology studies on the north Qinling tectonic belt. Combining with regional geologic data, the tectonic evolution process in Palaeozoic and geodynamics of the north Qinling tectonic belt are discussed.The detailed field investigation and microstructure analysis reveals that the rocks in the north belt of Liuling group develop NNE-dipping foliation and dipping stretching lineation. The structural deformation of the north belt of Liuling group is characterized by rocks thrusting toward south, tectonic slices superimposed and forming N-verging tight folds. Metamorphism study on garnet-bearing mica quartz schist suggests the peak metamorphism reaches epidote-amphibolite facies to low amphibolite facies of560℃～588℃and0.59～0.65GPa,and it is similar to the environment of continental orogenic belt. Muscovites from one sample garnet-bearing mica quartz schist and two samples of phyllonite give40Ar/39Ar plateau agesrespectively247±1Ma,247±1Ma, and249±1Ma.Fine structural analysis suggests that different parts of Shangdan fault zone have different structural deformation characteristics. South belt develops faults thrusting towards south and N-verging tight homoclinal folds. However mylonites are exposed in the center of Shangdan fault belt with dipping foliations and nearly horizontal stretching lineations, and A-type folds whose hinges occurrence is parallel to the stretching lineation are well developed. To the north, the mineral stretching lineation pitch to SEE with plunge angle of20-50°, and also A-type folds whose hinges occurrence is parallel to the stretching lineation are common to see.Mineral deformation thermometer during mylonitization and quartz C axis fabric analysis are carried out to estimate the deformation environment of Shangdan fault zone. The result shows the deformation temperature is mainly between400℃～500℃, and indicates mylonites formed in the middle-high greenschist facies to low amphibolite facies metamorphic conditions. In generally the deformation temperature of west part is higher than theeast part.Differential stress calculation show that ancient stress difference of the Shangdan fault zone is up to140Ma. The kinematic vorticity analysissuggests that the deformation wasmainly dominate by the pure shear, but contains a simple shear component. Under the background of the finite strain of rocks shows the flattened ellipsoid.A detailed structural analysis indicates the Qinling Group has undergone two major tectonic cycles. The early sedimentary strata has been subjected metamorphismin Jinning period and formed a metamorphic rock association of schist-gneiss series. The deformation is characterized byformation oftight-recumbent folds and development of penetrativegneissosity and NNW-dipping mineral lineation.During the Caledonian-Hercynian the Qinling Group once again suffered strongly metamorphism and deformation, forming SEE-dipping stretching lineations and A-type folds in early deformed Qinling group. The peak metamorphism of Qinling group reaches amphibolite facies conditions of686℃and0.63Gpa. The p-T path of the Caledonian-Hercynian tectonometamorphiccycle shows clockwise, indicating decompression during rapid uplift.The above all geological phenomena demonstrate the Qinling Group has experienced orogen-parallel westward uplift withboth vertical and horizontal displacement components.The Zhuxia fault zone has experienced two tectonic events. Early thrust shearing made the Qinling group thrust over the top of the Erlangping group at about400Ma.Later tectonic event made the early structural deformation seldom preserved. The foliation of mylonite steeply dips to SWS, and stretching lineation plunges ESE at2-15°. Macro and micro deformation evidences strongly suggested that the later tectonic event hasthe kinematics characteristics of dextral shear. The deformation temperature is mainly between400-550℃. The40Ar-39Ar plateau age of muscovitegrains from mylonite suggests the dextral shear occurredno later than146Ma inWNW-ESE direction.The Waqiao fault zone is trending SEE-NWW, dips to NNE. Mineral stretching lineationsin mylonite are both the steeply NNE-dipping lineation, and nearly horizontal lineation. The Waqiao Fault zone has the kinematics characteristics of the thrusting from north to southand containing sinistral shear component. Quartz C-axis preferred orientation reflected the deformation temperature of the shear zone is between400-650℃, which belongs to mid-high temperature deformation. A40Ar-39Ar plateau age of muscovitegrains from the mica schistformed in syntectonic period reveals that the Waqiao ductile shearing occurred no later than387Ma.On thebasisof tectonic analysis,we proposed that the main component and thepresent tectonic framework of north Qinling tectonic belt were formed during the Palaeozoic orogeny. The main geodynamic mechanism is compression and collision tectonic background between the Yangtze block and the North China block. It is the results of northward subduction of the Qinling ocean. As the Qinling ocean closed, followed by two plates collision, all the blocks in the north Qinling are in place, at this time the regional metamorphism reached its peak. The Zhuxia fault zone and Waqiao fault zone are initiated by the regional metamorphism and thermal metamorphism.