Tectonics and Magmatism of Western Junggar and the Tien Shan Range, Xinjiang Province, NW China

Two late Palaeozoic collisions produced the ancestral Tien Shan range. The older, southern one accreted a north-facing passive continental margin on the north side of the Tarim Block to an active continental margin on the south side of an elongate continental tract, the Central Tien Shan. Collision occurred in the late Devonian - early Carboniferous along the Gunbeizi Fault (Southern Tien Shan suture). A younger, probably late Carboniferous collision along the Ilinhabir Fault (Northern Tien Shan suture) accreted the northern side of the Central Tien Shan to the Northern Tien Shan island arc. Northward-vergent thrusts generated by this collision are proposed as the tectonic load responsible for generating a Lower Permian foreland basin over the Northern Tien Shan. Late Palaeozoic basic igneous rocks from all of the lithospheric blocks in the Tien Shan possess characteristics associated with generation in a suprasubduction zone environment. Rocks from each block also possess distinctive trace element signatures, supporting the proposed three-fold structural division of the range. Part of this magmatism is post-collisional, contemporary with transtensional faulting. Accretion of island arcs, mélanges and ophiolites in Western Junggar was completed by the late Carboniferous, but the original orientations of collision zones and the locations of sutures remain obscure. Basic rocks from volcaniclastic sequences and ophiolites indicate contributions from both high Nb, LREE and low Nb, ΣREE sources. Mesozoic clastics were deposited over both ranges and neighbouring basins. Thermal subsidence following late Palaeozoic extension is suggested as the subsidence mechanism. The Cenozoic India/Asia collision has produced dramatic reverse faulting and uplift in the Tien Shan; deformation in Western Junggar is milder, dominated by transpressive movement on major faults. The V- shaped structural profile of the Cenozoic Tien Shan is inherited from the two Palaeozoic orogenies. The intensity of deformation varies inversely with the distance from the Indian indenter.