EVOLUTION OF THE BAY AN HAR-WEST SICHUAN PERIPHERAL FORELAND BASIN IN SOUTHWESTERN CHINA

The consumption of the Bayan Har Ocean in the Palaeo-Tethys was reflected in the depositional records of transition from remnant oceanic basin to peripheral foreland basin. Since the Bayan Har foreland basin was separated from its extended part toward Yangtze craton (Sichuan Basin) by the present-day Longmenshan overthrust zone,the previous study of each sedimentary basin was carried out independently.In this paper,an integrated study has been conducted so as to give a viable explanation for the evolution of the two basins.
While the Yangtze plate as a subduction one wedged westwards during the Late Permian,there occurred bidirectional subduction toward the Kunlun-Qaidam landmass in the north and toward the Qiangtang-Qamdo landmass in the south.The authors contend that the initiation of the transition from the remnant oceanic basin to peripheral foreland basin, when the bulk of the Bayan Har Ocean was closed,dates back to the middle-late Ladinian (T₂²)rather than the Late Triassicas is interpreted by most geologists at home.This new idea is supported by the formation of the foreland bulge in the vast area of central Sichuan basin in correspondence with the flexural subsidence of the foreland basin during the middle and late Ladinian.The subsidence of the foreland basin led to the termination of the starved sedimentary basin(T₁-T₂¹)on the passive margin,where more than 2000- to 10000 m thick flyschoid deposits were laid down and overlapped onto the Yangtze cratonic margin.The littoral coal-bearing molasse deposits and continental coal-bearing molasse deposits were accumulated in response to the progradation of the overthrust zone from north to south during the Norian and Rhaetian.The foreland basin was migrated into the area of western Sichuan Basin during the middle and late Late Triassic when the evolutionary stage of the postorogenic foreland basin with the continental molasse commenced under the influence of intracontinental convergence.Finally,the later uplifting of the Sichuan Basin permitted the termination of the foreland basin during the Paleogene.