| The Tongbai-Dabie orogenic belt is an important part of Qinling-Tongbai-Dabie orogenic belt.It can be divided into Tongbai,West Dabie and East Dabie.In this paper,structural geology,metamorphic petrology and isotope thermochronology have been used to study the three sections and two boundary faults in detail,and we have obtained the following main understandings.The Tongbai-Huwan shear zone is located on the northern side of the Tongbai-Dabie orogenic belt,and it's a sinistral ductile shear belt which developed strong deformable rocks such as mylonite,ultramylonite and straight gneiss.What's more,the shear zone developed rodding structure with layer structure and there is a special detachment surface binding between layers.The finite strain measurement value Rxz is between 1.80?2.04,and the average is 1.95,the vorticity value Wk is between 0.82?0.90,the average is 0.86,which is greater than 0.75 and indicates that the Tongbai-Huwan shear zone is mainly simple sheared.The displacement of the Tongbai-Huwan shear zone is from 38.3km to 41.1km.The fractal dimension is from 1.207 to 1.302,and from the core to the edge of the shear zone,the change of particle size and fractal dimension is gradually becoming larger and larger.The differential stress value of the shear zone is from 23.583MPa to 46.983MPa,and the strain rate is from 1.70×10-11 to 9.74×10-11s-1,the differential stress and strain rate showed a positive correlation,both becoming gradually smaller from the core to the edge in the shear zone.The formation condition of Tongbai-Huwan shear zone is about T=560?684?,P=0.6?0.9GPa,belonging to the high amphibolite facies,which is in a crustal rheological layer position.By the comparison with the Shangdan fault zone and the Xiaotian-Mozitan shear zone we can know that Tongbai-Huwan shear zone belongs to Xiaotian-Mozitan shear zone,rather than the Shangdan fault zone.The Yindian-Malong shear zone is located on the southern side of the Tongbai-Dabie orogenic belt,which is a dextral ductile shear zone.The finite strain measurement value Rxz is between 1.75?2.03,and the average is 1.89,the vorticity value Wk is between 0.88?0.93,the average is 0.90,which is greater than 0.75.Using symbiotic amphibole and plagioclase,the formation condition of Yindian-Malong shear zone is about T=581?680?,P?0.66GPa,belonging to the high greenschist facies to the low amphibolite facies,and the formation depth is about 15?30km,which is the product of the rheology of the middle and lower crust.The two shear zones in the northern and southern boundaries of the Tongbai-Dabie orogenic belt are originally the same shear zone,which is equivalent to a strong tectonic layer.But with the back-shaped structure of the orogenic belt formed due to the late compression and uplift,only the north and south shear boundaries of the Tongbai-Dabie complex were preserved after the later weathering and denudation.The Tongbai orogenic belt is a giant antiform structure,and the antiform structure has layered structure in vertical direction.Besides,a large number of L tectonites and rodding structure are also developed in the Tongbai orogenic belt.The rodding structure has layer structure,and the layers are connected by detachment surface,The detachment surface has mineral stretching lineation and thermal scratch.These phenomena indicate that the Tongbai complex in the back core of the Tongbai orogenic belt shows strong rheological characteristics.The kinematics direction of Tongbai-Huwan ductile shear zone in the northern boundary of Tongbai complex is opposite to that of Yindian-Malong ductile shear zone in the southern boundary,whereas the rheological direction of Tongbai complex is from west to east.The two shear zones both develop rodding structure,and the rodding structure has layer structure.The rodding structure and L tectonites were formed under the rheological action parallel to the Tongbai orogenic belt.The Yindian-Malong shear zone and Tongbai-Huwan ductile shear zone are the upper boundaries of the east-facing rheological parallel orogenic belt of the lower crust of Tongbai orogenic belt under the compressive background,but the overlying part of the orogenic belt was stripped off.Only the Tongbai-Huwan ductile shear zone in the north,the Yindian-Malong ductile shear zone in the south and the Taibaiding shear zone in the top were preserved.To be exact,the eastward extrusion of the Tongbai orogenic belt under the compression background from late Indosinian to early Yanshanian was not a whole extrusion,but a hierarchical differential rheology.While rheology occurred,sliding occurred between layers,and the detachment surface is a sliding lubricant.The west Dabie orogenic belt is a giant antiformal structure in the form with west plunging hinge and there is a detachment surface between layer structure.Structural analysis indicated that there are at least three stages of deformation in west Dabie orogenic belt.The first phase is the formation of the boundary shear zones.The second phase is the formation of detachment surface and eclogite lens.The third period is intrusion of late rock mass.The statistical results of stress field suggested the west Dabie orogenic belt is mainly affected by NS direction compressional stress,which truthfully recorded the ancient stress state of the west Dabie oro genic belt.Finite strain measurement results show that from north to south,the deformation intensity of the rock has the variation law with strong deformation zone?weak deformation zone?strong deformation zone?weak deformation zone?strong deformation zone.The metamorphic conditions of the west Dabie orogenic belt are T=450?620?,P=1.0?1.6GPa,which belongs to the blue schist facies to the eclogite facies.The zircon LA-ICP-MS U-Pb isotope dating results show that the west Dabie area at least suffered two periods of magmatic intrusion,the first stage is around 750Ma,and this age(about 0.8Ga)recorded the characteristic events of the Yangtze plate.The second stage is around 143Ma,this expressed that there was already magmatic activity in the late Jurassic in the west Dabie area,which was consistent with the results obtained in the Tongbai mountain area.The east Dabie orogenic belt is a giant antiformal structure,which widely developed tectonic gneiss and mylonite,and these have recorded the rheological characteristics of the Dabie complex.The Xiaotian-Mozitan shear zone in the northern boundary of the Dabie complex in the eastern Dabie orogenic belt is characterized by sinistral strike-slip kinematics,and the Yindian-Malong shear zone in the southern boundary is dextral strike-slip.Moreover,the rocks near the North-South shear zone in the eastern Dabie orogenic belt show the same kinematic characteristics as the corresponding shear zone,which is consistent with the results obtained in the west Dabie orogenic belt.Thus,the Dabie complex is limited to flow from west to east in the kinematics.The statistical results of the stress field showed that the east Dabie orogenic belt was mainly suffered the extrusion pressure in two directions of NE-SW and NW-SE and the stretching along the orogenic belt.This east Dabie orogenic belt was affected by the two systems of the Tethyan tectonic system and the Pacific tectonic system.these two stress states should be the products of intracontinental deformation in eastern China under the background of the continuous compression stress after the collision between the two plates of Yangtze plate and the North China plate in Indosinian period.Flinn diagram expressed strain ellipsoid belong to the uniaxial cigar type,which indicated that the east Dabie orogenic belt was dominated by shear tension deformation.The horizontal extension of the strain ellipsoid is stronger than vertical coaxial component,and it is suggested that the formation of the rocks in the east Dabie orogenic belt is mainly affected by the tension parallel to the orogenic belt and the compression perpendicular to the orogenic belt.From the north to south in the east Dabie orogenic belt,the strain intensity appeared strengthening?weakening?strengthening?weakening?trengthening.That is to say the rocks between the Xiaotian-Mozitan shear zone and the Yindian-Malong shear zone existed the variation law with strong deformation zone?weak deformation zone?strong deformation zone?weak deformation zone?strong deformation zone.The corresponding strong deformation zones on both sides are the Xiaotian-Mozitan shear zone and the Yindian-Malong shear zone,and the middle strong deformation zone corresponded to the strong rheological zone,which located in the core of the east Dabie Mountain.The rheological environment of tectonic gneiss in the east Dabie orogenic belt is about T=720?740?,P=0.75?0.90GPa,belonging to high amphibolite facies-low pressure granulite facies metamorphism.The Dabie Mountains experienced near horizontal east-west plastic rheology along the orogenic belt during the late collisional orogenic period,which lead to the NWW-SEE trend of gneiss and the lineation also along the orogenic belt in the Dabie Mountains.This paper considered that the time of plastic rheology was about 210±10Ma.Later activities of the Tan-Lu fault zone reformed the original NWW-SEE trending structure of the Dabie Mountains,resulting in the gneissic trend in the eastern end of the Dabie Mountains transforming to NE trending,especially in the northern margin of the Dabie Mountains.In conclusion,this article holds that the geometry form of Tongbai-Dabie orogenic belt is a large antiform structure.It is formed under the background of the collision and compression between the North China block and the Yangtze block after the Indosinian period,and it is mainly subjected from the nearly north-south trending compressional stress.What's more,the antiform structure has layered structure in vertical direction,and there is a special detachment surface linked between layers.According to the mineral stretching lineation and the heat scratches in the detachment surface,we can know that slide happens between the layers.Yindian-Malong shear zone and Xiaotian-Mozitan shear zone belong to the same tectonic layer of the antifonn structure,which formed as the formation of the antiform structure in the Tongbai-Dabie orogenic belt under the background of the collision between the North China block and the Yangtze craton.Because of the later weathering and denudation,the top part of the antiform structure has been eroded,which made the Tongbai complex and Dabie complex exposed,so only the two shear zones in two sides of the antiform structure are preserved.According to the information provided by the chronology,we can know that the plastic rheology events in the Tongbai-Dabie orogenic belt started at about 210Ma.The rheology at this-time is the deep plastic rheology,which has reached the peak at about 180Ma.In the middle stage of tubular rheology,the shear belt(?170Ma)was formed due to the existence of differential,rheological boundaries,and the Tongbai-Dabie complex continued tubular rheology from west to east.The small-scale magmatic activity was most likely to start at around 150Ma in the Tongbai-Dabie orogenic belt.The activity of the shear zone and magmatic activity could occur at the same time after 150Ma,and before the peak of magmatic activity in the Early Cretaceous(140Ma),the shear zone activity and tubular rheology slowly decayed and stopped.Then the magmatic activity began to intensify,after 130Ma,the magmatic activity began to be weaken.However,the extensional structure began to be strong.The Tongbai-Dabie orogenic belt is basically stereotyped until about 90Ma.Under the weathering and denudation of the late Cenozoic and the small-scale tectonic transformation,the Tongbai-Dabie orogenic belt has formed the present tectonic framework. |
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