Research On Tectonic Process And Formation Age Of The Detachment Fault Of The Lushan Metamorphic Core Complex

Metamorphic core complex?MCC?is the shallow response of deep geological process of lithospheric thinning.The Lushan MCC located in the northern margin of South China Plate is the ideal area for research on tectonic transition,deep geological process and dynamic mechanism.The research on detachment fault can reveal the continuous deformation and evolution of MCC from deep to surface and also be helpful to deepen the cognition of regional extension.The Lushan MCC with NNE-SSW trending is exposed in the Lushan area and the eastern detachment fault was for the first time defined in this paper.It reflects regional extension of NW-SE.The SE dip eastern detachment fault indicate the kinematic direction of the hanging wall slipping down to the southeast.Only the mylonite zone and tectonic schist zone are preserved well,and the migmatite zone and a series of pegmatite veins and felsic veins are also the most important components.The western detachment fault shows multi-level,multi-strength and multi-scale feature,with a tendency change of NW-W-SW corresponding to the orientation of northwest-west-southwest.It contains the mylonite zone,tectonic schist zone,breccia zone and migmatite zone,and a solid rheological structural belt is exposed above the western detachment fault.The main detachment surface is characterized by the strongest metamorphic-deformation and the biggest distance and it reflects the feature of deformation layering vertically.A series of sinistral strike-slip ductile shear structures with NNE trending,exposed in this region,are represented by the Xingzi fault and the Lianhua fault.It reflects regional compression of S-N.The Xingzi fault is in the eastern margin of the Lushan MCC with strike of 30°and felsic veins are widely developed.Multiple places of shear zone are cut by late extension-detachment stratum.The Lianhua fault with strike of 40°located in the boundary of sedimentary cover and ductile rheological layer,is characterized by quartz veins.The brittle regime experienced strong ductile deformation and mylonitization.Others,many small strike-slip ductile shear zones and NE trending folds between the two main faulted zones formed at the same period.The metamorphic temperature and pressure of plagioclasite-hornblende schist within the western detachment fault calculated by Al-in-hornblende barometer,amphibole-plagioclase geothermometer and amphibole thermometer are 583⁷18?,0.58⁰.72Gpa respectively.The metamorphic temperature of meta-mud phyllonite within the western detachment fault calculated by Ti-in-Muscovite geothermometer is 636⁷13?.The metamorphic temperature of tectonic schist and felsic vein within the eastern detachment fault calculated by Ti-in-biotite geothermometer is 617⁷06?.It reveals that the detachment fault formed under the middle-high amphibolite facies and great burial depth.The western detachment fault highly chloritic rocks were believed to be the product of retrograde metamorphism.The metamorphic temperature of Xingzi fault is 490⁵49?which shows epidote amphibolites facies-amphibolite facies according to two feldspar thermometer.And the formation temperature of the Xingzi fault is higher than that of Lianhua fault by contrasting structural deformation of both.The deformation temperatures of the eastern detachment fault are 500-580?and580-700?according to microscopic deformation characteristics which reveals the process of detachment and uplift from deep crust to surface,while the mylonitic rock of the western detachment fault underwent strongly static recrystallization.The deformation temperature of the Xingzi fault is 500⁵30?while that of the Lianhua fault is about 400⁵00?.The fractal dimension of the detachment fault is 1.081¹.132which indicates the deformation temperature is 600⁷20?,while that of strike-slip fault are 1.143 and 1.147 which indicates the deformation temperature is 550⁶00?.Others,the palaeo-differential stress of rocks from the detachment fault and strike-slip fault is 30.8⁴9.3MPa.The strain rate of rocks from the detachment fault is in the range of 1.1×10^-103.1×10^-11 which belongs to low strain-rate condition,while that from the strike-slip fault reaches 10^-12.The Flinn index and strain intensity of rocks of different structural units in the northern Lushan MCC are 0.76⁵.44 and 1.52⁴.12 respectively,got from finite strain measurement.The strain ellipsoid belongs to uniaxial cigar type which means horizontal extension is stronger than vertical coaxial component.Moreover,the structural units in the descending order of strain intensity are the eastern detachment fault,the western detachment fault,the ductile rheological layer close to detachment fault,the sedimentary cover.The result is consistent with macroscopic deformation characteristics.The zircon U-Pb dating and Ar-Ar dating of single mineral grain of metamorphic rocks,veins and intrusive rocks from the detachment fault and strike-slip fault basically reveals the tectonic evolution sequence of Late Mesozoic Lushan area:a)The Lushan area underwent sinistral ductile strike-sliping controlled by compressional stress field within 162¹50Ma;b)The compressional stress transformed into extensional stress and the main detachment fault began acting within 150¹40Ma;c)The eastern detachment fault formed based on the western detachment fault within 140¹35M and the extension and uplift stage followed;d)The extensional time of the Lushan MCC ranged from140Ma to 114Ma,meanwhile,granitic magma intruded within 133¹23Ma,and it's a rapid uplifting period;e)In the late stage of the extension?⁹5Ma?,ductile extension transformed into brittle extension and it's a balanced uplifting period.Results from comprehensive research of kinematic vorticity,finite strain measurement and geochronology,show magmatic activity generally lagged behind extension-detachment,that is,regional extension controlled magmatism,induced rising and emplacement of hypomagma and then resulted in the uplift of the Lushan MCC.Morever,according to geochronology and kinematics research,it is speculated that the west-dipping western detachment fault is the original detachment fault?150¹40Ma?of the Lushan MCC,while the east-dipping eastern detachment fault is a secondary detachment fault?140¹35Ma?formed by diapirism and folding from gravitational equilibrium of large-scale crustal thinning,and the original detachment fault has been worn away by intense neotectonic movement in eastern Lushan.The basic form of the detachment fault was mainly controlled by active extension and folding,and strong magmatism played a positive role in advancing and promoting the final form.This paper is mainly based on Early Cretaceous extensional tectonic system,and partially based on Mid-Late Jurassic strike-slip tectonic system.They constitute the tectonic framework of Late Mesozoic Lushan area and the extensional structures modified and overlaid the strike-slip structures.The strike-slip structures could be controlled by compressional stress field resulted from the subduction of the Pacific Plate,and it's the product of Paleotethys tectonic regime transforming into Pacific tectonic regime in Early Mesozoic.The Lushan MCC correlated with back-arc extension setting is the product of South China lithospheric thinning.The compressional stress transformed into extensional stress under delamination result from gravitational collapse of thickened lithosphere root,and this resulted in the upwelling of asthenospheric mantle matter and induced MCC and magmatic activity of shallow crust.NW-SE trending extension of the Lushan MCC is coincident with Late Mesozoic general extensional direction of East China.The start time of extension?150¹40Ma?provides chronological constraint for lower limit of South China lithospheric thinning.Besides,it is speculated that the peak time of thinning could well be 140¹20Ma according to chronological statistics of extensional and magmatic activity in South China.