MULTI-SUBDUCTION AND TECTONIC EVOLUTION OF THE BEISHAN OROGEN, CENTRAL ASIA

The Beishan orogen is located in the central southern margin of the Central Asian Orogenic System, which recorded long-term and multi-cycle tectonic processes associated with continental rifting, block drifting, oceanic subduction, arc- or continent-continent collision, and intracontinental deformation. Due to the superposition of multi-stage tectonic events, the tectonic configuration of the Beishan Orogen from late Neoproterozoic to early Mesozoic is extremely complex and confusing. This issue is particularly important for the Beishan orogen and its larger tectonic domain, which recorded the growth processes of the accretionary orogen and the closing time and position of the Paleo-Asian Ocean. To clarify this point and to better constrain the tectonic evolution of the Beishan Orogen, we examined the geology of the blocks/magmatic belts and mélange belts in the Beishan region by combining geological mapping, geochronology, and geochemistry analyses, and existing tectonostratigraphic, magmatism, sedimentation, and metamorphic records. This work provides a comprehensive tectonic model of the Beishan Orogen, which emphasizes the temporal and spatial relationship of rock assemblages formed in various tectonic settings. We propose the Beishan region as a unified continent that was separated by an intracontinental oceanic basin along the Hongliuhe-Niujuanzi-Xichangjing mélange belt in the late Neoproterozoic. During the early Paleozoic, abundant magmatic rocks and a back-arc extensional basin were created during the north-dipping subduction and closure of the intracontinental oceanic basin. The subduction and northward roll-back of the Paleozoic south-dipping Paleo-Asian Ocean lithospheric slab resulted in the formation of extensional arc magmatic rocks and sedimentary basins distributed in the whole Beishan region. We suggest that the slab roll-back may be one of the main dynamic mechanisms for the continental growth of the accretionary orogen. Late Carboniferous–early Permian intracontinental rift was also developed in the southern Beishan orogen along the Liuyuan-Zahngfangshan belt. Finally, the late Paleozoic–early Mesozoic closing of the Paleo-Asian Ocean on the north side of the Beishan Orogen indicates the beginning of intracontinental evolution.