Structural evolution of the Irtysh Shear Zone (northwestern China) and implications for the amalgamation of arc systems in the Central Asian Orogenic Belt

The NW-SE Irtysh Shear Zone is a major tectonic boundary in the Central Asian Orogenic Belt (CAOB), which supposedly records the amalgamation history between the pen-Siberian orogenic system and the Kazakhstan/south Mongolia orogenic system. However, the tectonic evolution of the Irtysh Shear Zone is not fully understood. Here we present new structural and geochronological data, which together with other constraints on the timing of deformation suggests that the Irtysh Shear Zone was subjected to three phases of deformation in the late Paleozoic. D-1 is locally recognized as folded foliations in low strain areas and as an internal fabric within garnet porphyroblasts. D-2 is represented by a shallowly dipping fabric and related similar to NW-SE stretching lineations oriented sub-parallel to the strike of the orogen. D2 foliations are folded by similar to NW-SE folds (F-3) that are bounded by a series of mylonite zones with evidence for sinistral/reverse kinematics. These fold and shear structures are kinematically compatible, and thus interpreted to result from a transpressional deformation phase (D-3). Two samples of mica schists yielded youngest detrital zircon peaks at similar to 322 Ma, placing a maximum constraint on the timing of D-1-D-3 deformation. A similar to NE-SW granitic dyke swarm (similar to 252 Ma) crosscuts D-3 fold structures and mylonitic fabrics in the central part of the shear zone, but is displaced by a mylonite zone that represents the southern boundary of the Irtysh Shear Zone. This observation indicates that the major phase of D-3 transpressional deformation took place prior to similar to 252 Ma, although later phases of reactivation in the Mesozoic and Cenozoic are likely. The late Paleozoic deformation (D-1-D-3 at similar to 322-252 Ma) overlaps in time with the collision between the Chinese Altai and the intra-oceanic arc system of the East Junggar. We therefore interpret that three episodes of late Paleozoic deformation represent orogenic thickening (D-1), collapse (D-2), and transpressional deformation (D3) during the convergence between the Chinese Altai and the East Junggar. On a larger scale, late Paleozoic sinistral shearing (D3), together with dextral shearing farther south, accommodated the eastward migration of internal segments of the western CAOB, possibly associated with the amalgamation of multiple arc systems and continental blocks during the late Paleozoic. (C) 2015 Elsevier Ltd. All rights reserved