Basin-orogen patterns and the late Triassic foreland basin conversion process in the western Yangtze Block, China

Due to the formation of the Qinling and Sanjiang Orogenic Belts along the northern and southwestern margins, the western Yangtze Block experienced a complex basin-orogen tectonic transformation, forming a unique pattern for a basin-orogen system in the late Triassic that should be recorded by sediments in the Sichuan Basin. This paper combines with regional tectonics, field observations, and borehole and seismic exploration data to establish the prototypes of the basins and tectonic frameworks of basin-orogen from Member 1 (M-1) to 6 (M-6) of the Xujiahe Formation during the late Triassic in the western Yangtze Block. From stage M-1 to M-2, a set of deltaic deposits formed and extended parallel to the Qinling Orogenic Belt, indicating that the start of the scissor-type collision in the northern margin of the western Yangtze Block led to the development of the northern Sichuan Basin and Songpan-Ganzi peripheral foreland basin. Meanwhile, a set of deltaic deposits, which occur in the Southeastern Sichuan Basin, were controlled by the uplift of the Jiangnan-Xuefeng Tectonic Belt, transforming into a denuded zone at that time. A set of deltaic deposits developed in the southwestern part of the western Yangtze Block, indicating the beginning of the initial stage of the related foreland basin in this area at stage M-2. From stage M-3, the intensive activity of orogenic belts surrounding the western Yangtze Block shaped the tectono-sedimentary features along the margin of the western Yangtze Block. The successive development of delta plain and alluvial fan deposits and their westward and eastward expansions respectively in the north and southwest margins of the western Yangtze Block reflect the uplift of the Qinling and Sanjiang Orogenic Belts; while the formation of a set of deltaic deposits in the southeastern part of the western Yangtze Block results from the persistent uplift of the Jiangnan-Xuefeng Tectonic Belt. During stages M-5 to M-6, the lacustrine-swamp deposits began expansion and became the main sedimentary facies in the western Yangtze Block. Southward and northward migrations of the aforementioned two separated foreland basins, respectively controlled by Qinling and Sanjiang Orogenic Belts, caused these two separated foreland basins to connect with each other and to form a single unified complex foreland basin that was controlled by these two orogenic belts. These results suggest a two-stage foreland basin evolution mode for the western Yangtze Block during the late Triassic. At stage I (M-1 to M-4), two separated foreland basins developed in the northern and southwestern margins of the western Yangtze Block, controlled respectively by the Qinling and Sanjiang Orogenic Belts. During Stage II (M-5 to M-6), these two separated foreland basins connected with each other to form a single composite foreland basin related to those two orogenic belts.