Structural And Geochronological Constraints On The Irtysh Shear Zone In Central Asia

Accretionary orogens are developed along convergent plate boundaries and are characterized by long-term lateral subduction and continental growth.Such orogens have operated throughout much of Earth history.The Central Asian Orogenic Belt（CAOB）,bounded by the Siberian Craton,Tarim Craton,and North China Craton,is a classic example of an accretionary orogen.Its accretionary mechanism has attracted extensive attention from international academia.Since 1990s,many projects（IGCP 662,592,480 and 420）have been approved by the International Geoscience Programme to study the tectonic evolution of the CAOB.A model of multiple arc systems has been proposed,which effectively explains the tectonic pattern that multiple arc systems had operated synchronously.However,the tectonic evolution of the CAOB is weak for the reconstruction of these terranes within the CAOB remains relatively poorly constrained.The Irtysh Shear Zone represent a major suture within the CAOB,which records the key information on the collision between the peri–Siberian orogenic system and the Kazakhstan orogenic system/East Junggar Terrane after the closure of the Ob–Zaisan Ocean.In this paper,we present new structural and geochronological data across the eastern segment（Qinghe area）,the western segment（Burjin area）,and the Kazakhstan segment（Char area）of the Irtysh Shear Zone,with the aims to constrain the closure time of the Ob–Zaisan Ocean and enriches the dynamic evolution model of arc–arc collisions in the accretionary orogenic environment.The detrital zircon U–Pb and Hf isotopic study was carried out across the Irtysh Shear Zone.It has been shown that during the Late Carboniferous,there was no exchange of detritus on both sides of the Irtysh Shear Zone,thus indicating that the Ob–Zaisan Ocean was closed after the deposition for these Late Carboniferous strata.The youngest constraint for the closure of the Ob–Zaisan Ocean can be determined by the active time of the sinistral Irtysh Shear Zone,which represents the tectonic boundary of the Siberian margin and the Kazakhstan collage/East Junggar Terrane.The ⁴⁰Ar/³⁹Ar dating on syn–shearing minerals along the Irtysh Shear Zone constrains an age range of～286–265 Ma for the sinistral strike–slip deformation,which together with our detrital zircon data suggests that the closure of the Ob–Zaisan Ocean occurred within the Late Carboniferous or Early Permian.Evidence for the collision between the Chinese Altai and the East Junggar Terrane was recorded in the eastern segment of the Irtysh Shear Zone.Field observations from the Qinghe area show that this segment of the Irtysh Shear Zone consists of four NW–SE sinistral mylonitic zones,and domains bounded by these mylonitic zones show variable fold patterns.In the southern Chinese Altai,three generations of structures(D_CA1–₃)are recognized.The earliest generation of foliation(S_CA1)is only recognized locally and is transposed to the orientation of the dominant S_CA2foliation.The latter is associated with a shallowly plunging stretching lineation,which is sub–parallel to the NW–SE hinge of D_CA3 folds.These third–generation folds(D_CA3)show a steeply–dipping axial plane trending NW–SE.We infer that the original sub–horizontal S_CA2foliation associated orogen–parallel stretching lineation might indicate that the southern Chinese Altai was subjected to orogen–parallel extension.The sinistral shearing along these mylonitic zones dated by ⁴⁰Ar/³⁹Ar geochronology（283–265 Ma）,which is simultaneous with the development of NW–SE D_CA3 folds.The combination of sinistral shearing and NW–SE D_CA3 folds likely represents an episode of transpressional deformation.The earlier collisional phases involved orogenic thickening(D_CA1)and orogen–parallel extension(D_CA2),with the latter being responsible for Permian high–temperature metamorphism caused by the mantle upwelling.The Irtysh Shear Zone that represents the tectonic boundary of the Chinese Altai and the West Junggar Terranes,lies along the stratigraphic boundary of the Devonian Burjin Formation and Carboniferous Nalinkala/Qiaqihai formations in the Burjin area.Field observations from the Buerjin area show that the Irtysh Shear Zone is dominated by sinistral strike–slip kinematics,and is characterized by an overall～WNE–ESE orientation.On both sides of the fault,the NW–SE folds are recognized,likely corresponding to an episode of initial orthogonal collision between the Chinese Altai and the West Junggar Terrane.The fold structures were cut by the Permian sinistral brittle strike-slip fault,indicating the oblique collision between the the Chinese Altai and the West Junggar Terrane occurred in Permian.The metamorphism and deformation style changed dramatically along the Irtysh Shear Zone in NW China.Along this shear zone,the metamorphic degree of rocks gradually increases from the Burjin area in the west to Fuyun/Qinghe area in the east of the southern Chinese Altai,indicating that the Irtysh Shear Zone may represent an inclined arc–arc collision zone.The eastern and western segments of the Irtysh Shear Zone may resulte from the deformation and metamorphism of middle–lower crust and upper crust respectively.In the Fuyun/Qinghe areas,the occurrence of original sub–horizontal foliation and associated orogen–parallel stretching lineation might indicate the orogen–parallel material flow in the middle–lower crust.This deformation was not recorded in the middle–upper crust level（Burjin and Altai areas）.The combination of sinistral shearing and NW–SE D_CA3folds likely represents an episode of transpressional deformation,which was driven by oblique collision between the Chinese Altai and the West/East Junggar terranes.Overall,three episodes of late Paleozoic deformation represent crustal thickening,orogen–parallel extension,and transpressional deformation during the convergence between the Chinese Altai and the East/West Junggar terranes.The deformation structure of orogen–parallel extension is mainly preserved in the middle–lower crustal rocks in the eastern segment of the Irtysh Shear Zone.