Structural evolution of the eastern segment of the Irtysh Shear Zone: Implications for the collision between the East Junggar Terrane and the Chinese Altai Orogen (northwestern China)

The tectonic evolution of the Central Asian Orogenic Belt (CAOB) involved multiple episodes of arc accretions/collisions, but the reconstruction of these tectonic processes remains relatively poorly constrained. Evidence for a collision between the intra-oceanic island arc system of the East Junggar Terrane and the active margin of the Siberian Craton (Chinese Altai Orogen) is recorded in the eastern segment of the Irtysh Shear Zone (northwestern China). 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 northern part of the Irtysh Shear Zone (southern Chinese Altai Orogen), three generations of structures (D-CA1-D-CA3) are recognized. The earliest generation of foliation (S-CA1) is only recognized locally and is transposed to the orientation of the dominant D-CA2 foliation (S-CA2). 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. In the southern part of the Irtysh Shear Zone (northern East Junggar Terrane), the structural pattern is simpler and only involves a single generation of penetrative foliation (S-EJ1). New U-Pb detrital zircon and 40Ar/39Ar data provide constraints on the timing of collision and deformation. These results suggest that the collision occurred after the Early Carboniferous, with the timing of sinistral shearing constrained to the Early-Middle Permian. The combination of sinistral shearing and NW-SE D-CA3 folds likely represents an episode of transpressional deformation, which was driven by oblique collision between the East Junggar Terrane and the Chinese Altai Orogen during the Early-Middle Permian. The occurrence of original sub-horizontal S-CA2 foliation and associated orogen-parallel stretching lineation might indicate that following the initial collision (after the Early Carboniferous), the southern Chinese Altai Orogen was subjected to orogenparallel extension. In a larger-scale context, sinistral kinematics along the Irtysh Shear Zone in northwestern China and northeastern Kazakhstan, together with coeval dextral strike-slip deformation farther south, might reflect an eastward escape of orogenic materials, possibly in response to the Permian convergence of the Siberian, Baltic, and Tarim cratons.