Structural Feature And Chronology Study Of The Xiaergou-Wulasitai-Wulanmoren Ductile Shearing Zone In The Central Tianshan

The Central Tianshan Block is one of the major continental constituents of the Central Asian Orogenic Belt(CAOB)and contains vital information for understanding the amalgamation of the Eurasia continent.Two large-scale ductile shear zones occur along the northern and southern margins of the Central Tianshan Block(the Central Tianshan Northern Margin Fault and the Central Tianshan Southern Margin Fault,respectively),which have been extensively studied.However,there are still quite a lot debates on their deformation history and tectonic evolution.By contrast,several sub-shear zones also occur within the Central Tianshan Block,and their formation mechanism and structural relationships with the major boundary shear zones are poorly studied.In addition,although Silurian-Devonian marine sediments are widely distributed in the middle and east parts of the Central Tianshan Block,no detailed geochronology study has been conducted on them.Generally,in-situ detrital zircons from these Paleozoic sedimentary rocks could provide significant data for better reconstruction of the Paleozoic and even Precambrian crustal evolution process of the Central Tianshan Block.The Xiaergou ductile shear zone is the eastern segment of the Central Tianshan Northern Margin Fault and the Wulasitai-Wulanmoren ductile shear zone is the largest ductile shear zone occurring in the interior of the Central Tianshan Block,aiming at above issues,we carried out(1)field observations integrated with structural analysis and zircon U-Pb dating on the metamorphosed rocks along these two ductile shear zones,(2)detrital zircon U-Pb dating on the sedimentary rocks from the Silurian Ahebulake Group that are widespread in both ductile shear zones.As a section of the Central Tianshan Northern Margin Fault,Xiaergou ductile shear zone(XSZ)is exposed as a 4-to 5-km wide and 10-km-long high-strain zone in which Silurian to Devonian meta-sediments and deformed late Paleozoic intrusions outcropped.Mylonites in the XSZ show deformation fabrics of north-dipping and SE-dipping mylonitic foliations,and east-to-the-west sub-horizontal stretching lineation.Macro and micro kinematic analyses suggest that the XSZ is a dextral ductile strike-slip fault,and the zircon U-Pb dating results show ductile shear deformation at-315-294.6 Ma.The Wulasitai-Wulanmoren ductile shear zone(WWSZ)is a SE-NW-striking high-strain deformation zone which extends for more than 40 km along strike with variable widths of?1-5 km.The WWSZ is dominated by Proterozoic schists and gneisses,and Silurian to Early Devonian volcaniclastic rocks as well as Paleozoic granitic intrusions,all these rocks were subjected to ductile deformation.Mylonites in the WWSZ show deformation fabrics of south-dipping and NE-dipping mylonitic foliations,sub-horizontal stretching lineation and sub-S-N crenulation.Structural and kinematic observations indicate that the WWSZ is a sinistral ductile strike-slip shear zone,and our new zircon U-Pb data constraint the ductile shearing at-312-299 Ma.This study together with previous investigations reveal that the Central Tianshan Northern Margin Fault should have undergone three times post-collisional right-lateral ductile deformation at?360Ma,?310-290Ma and?275-245Ma,and the dextral ductile shearing of the Central Tianshan Southern Margin Fault mainly occurred at?310-290Ma.Taking into account the previous geodynamic studies,we suggest that the Central Tianshan-Yili-Junggar blocks emplaced the Tarim and Siberia eastward before Early Permian as a result of the post-orogenic strike-slip motion along the northern and southern sides of the Tianshan belt.In the meanwhile,due to the different movement rates interior the block,strike-slip motion occurred inside the Central Tianshan Block.Furthermore,mineral dynamic recrystallization characteristics of the metamorphosed rocks along the XSZ and WWSZ indicate a middle crust origin.The mechanism of their exposure is that such intra-continental ductile deformation and syntectonic extrusion weakened the crust and accelerated the exhumation of high-grade metamorphic rocks.The Ahebulake Group sedimentary sequences are dominantly composed of paragneiss,marbles,greenschists,meta-sandstones and meta-greywackes,whose main provenances are in-situ Paleozoic arc-related intrusions and Meso-Neoproterozoic basement rocks of the Central Tianshan Block.Detrital zircon U-Pb dating results show that sedimentary rocks from the Ahebulake Group were likely deposited later than about 400 Ma instead of in the Silurian,as previously assumed.In addition,the study of Precambrian detrital zircons from the Ahebulake Group further document the occurrence of in-situ Precambrian basements in the Central Tianshan Block,and a strong Neoproterozoic affinity between the Central Tianshan Block,Yili Block and Tarim Craton is suggested through zircon age distributions comparison,whereas a similar tectonic origin of the Chinese Central Tianshan and Kyrgyz Middle Tianshan is not suggested.