Early-Middle Jurassic evolution of the northern Yangtze foreland basin: a record of uplift following Triassic continent-continent collision to form the Qinling-Dabieshan orogenic belt

摘要

The northern Yangtze foreland basin system was formed during the Mesozoic continental collision between the North and South China plates along the Mianlue suture. In response to the later phase of intra-continental thrust deformation, an extensive E-W-trending molasse basin with river, deltaic, and lake deposits was produced in front of the southern Qinling-Dabieshan foreland fold-and-thrust belt during the Early-Middle Jurassic (201-163Ma). The basin originated during the Early Jurassic (201-174Ma) and substantially subsided during the Middle Jurassic (174-163Ma). A gravelly alluvial fan depositional system developed in the lower part of the Baitianba Formation (Lower Jurassic) and progressively evolved into a meandering river fluvial plain and lake systems to the south. The alluvial fan conglomerates responded to the initial uplift of the southern Qinling-Dabieshan foreland fold-and-thrust belt after the oblique collision between the Yangtze and North China plates during the Late Triassic. The Qianfoya Formation (lower Middle Jurassic) mainly developed from shore-shallow lacustrine depositional systems. The Shaximiao Formation (upper Middle Jurassic) predominantly consists of thick-bedded braided river delta successions that serve as the main body of the basin-filling sequences. The upward-coarsening succession of the Shaximiao Formation was controlled by intense thrusting in the southern Qinling-Dabieshan fold-and-thrust belt. Palaeogeographic reconstructions indicated an extensive E-W foredeep depozone along the fold-and-thrust belt during the Middle Jurassic (174-163Ma) that was nearly 150km wide. The depozone extended westward to the Longmenshan and further east to the northern middle Yangtze plate. The northern Yangtze foreland basin was almost completely buried or modified by the subsequent differential thrusting of Dabashan and its eastern regions (Late Jurassic to Cenozoic).