Abstract:

Orogenic belts can be divided into three types: accretionary, collisional and intraplate-type. Accretionary orogenic belts are of great significance for understanding the subduction and closure history of the ancient oceanic basin, as they can preserve the most geological information during oceanic subduction. Whereas to clarify the tectonic significance of accretionary orogens, it is critical to identify the basic units of the ancient subduction zone, which include trenches, accretionary complexes, forearc basins, magmatic arcs, and back-arc basins, where information on the accretionary complexes, the main product of oceanic subduction, such as their composition and structure as well as their recorded structural deformation and metamorphic history, can directly reveal the dynamic evolutionary process and accretionary orogenesis during plate convergence. The South Qiangtang accretionary complex is a recently identified geological unit of central Tibet. It has the potential to answer such scientific questions as the formation and evolution of the Paleo-Tethys Ocean and the origin of the Central Qiangtang uplift, which is of importance for resource and energy exploration. However, there are many unknowns about the South Qiangtang accretionary complex, including its tectonic attribute, dynamic process, subduction zone structure, and deep geological process. In this paper, we aim to clarify the composition, structure, and geological evolution of the South Qiangtang accretionary complex through detailed studies on the ancient-subduction-zone identification, composition and structure of accretionary complexes, and exhumation mechanism of high-pressure metamorphic rocks, in order to provide a theoretical basis for understanding the regional metallogenic evolution and carbon mineralization pathway and guiding resources prospecting. In the central Qiangtang, we identified a relatively complete trench-arc-basin system consisting of continental margin arc, fore-arc basin, trench slope basin, and high-pressure metamorphic rock-bearing accretionary complex, evidencing an ancient subduction zone. On the plane, the South Qiangtang accretionary orogenic belt presents “mylonitic structures” at different scales, reflecting the extensive and strong ductile shear rheology during plate subduction; vertically, it exhibits an obvious double-layer structure, where the upper layer is a fold-thrust belt formed from deformation of the South Qiangtang terrane, while the lower layer comprises multi-stage deformations, reflecting the important role of subduction-at-continental-margin in the orogenic process. In addition, differential metamorphic evolution of high-pressure metamorphic rocks is obvious across the orogenic belt, and the exhumation mechanism is complex and diverse, reflecting the complex deep process of the subduction zone. Therefore, according to our research and previous data, the South Qiangtang accretionary orogenic belt is the product of Permian-Early Triassic subduction and accretion of the Longmu Co-Shuanghu Paleo-Tethys Ocean that expanded during the Late Devonian-Early Carboniferous period, and formed in the Middle-Late Triassic during the collision of the North and South Qiangtang terranes.

Key words: accretionary complex, subduction zone, Paleo-Tethys Ocean, South Qiangtang