The Dashui Subduction Complex in the Eastern Tianshan-Beishan Orogen (NW China): Long-Lasting Subduction-Accretion Terminated by Unique Mid-Triassic Strike-Slip Juxtaposition of Arcs in the Southern Altaids

Comprehensive studies of the tectonic nature and the formation age of the Dashui complex in the Eastern Tianshan-Beishan Orogen may address and help to better understand the complicated accretionary tectonics of the southern Altaids. Based on our detailed geological mapping, structural analysis, and understanding of new geochemistry and detrital zircon U-Pb dating of critical samples, the Dashui complex can be subdivided into one melange (Central Zone) and two units of forearc basin and slope basin (Northern and Southern Zones). The Northern Zone is composed of debris flows and turbidites. The Central Zone has a "block-in-matrix" structure, and the blocks of basalts have E-MORB or N-MORB geochemical signatures. The Southern Zone is characterized by debris flows. Therefore, we name these rocks Dashui subduction complex. Detrital zircons from the three zones show a major single age peak at ca. 460-420 Ma and a minor age peak at ca. 280-260 Ma, and the maximum depositional ages from north to south are: 264, 244 and 246 Ma, respectively, indicate that the Dashui subduction complex developed independently within an overall intra-oceanic arc until ca. 244 Ma, and there must have been a large-scale tectonic separation between the Central Tianshan Arc to the north and the Shuangyingshan Arc to the south. The structural geometry and kinematics of the Dashui complex and adjacent areas indicate that this complex likely experienced syn-subduction thrusting and strike-slip deformation. In particular, the late Xingxingxia strike-slip faulting finally juxtaposed the Central Tianshan, Mazongshan, and Shuangyingshan arcs.