Meso-Cenozoic sediment transport and tectonic transition in the western Sichuan foreland basin

Abstract：The authors analyzed the residual thicknesses of various Meso-Cenozoic structural layers and sedimentary characteristics in the western Sichuan foreland basin and found the following: The foreland basin at peripheries of the Sichuan craton occurred in the front of the Longmen Mountains during the Late Triassic, evidently belonging to a foredeep formed under the structural loads of the Longmen fold-thrust; in the Early Jurassic sedimentary strata were of areal distribution with no markedly flexural subsidence, indicating a tectonically quiet stage. In the early Middle Jurassic, the foredeeps migrated to the front of the Micang Mountains and the northern segment of the Longmen Mountains and the direction of foredeep sediments changed from northeast in the Late Triassic to nearly E-W then. Widespread lacustrine sediments imply a hungry basin. During the middle-late Middle Jurassic, the subsidence center of the western Sichuan basin migrated to the front of the Daba Mountains, with the direction of the corresponding flexural deformation changing from nearly E-W to NW, forming the Daba foredeep. During the Late Jurassic to Early Cretaceous, the subsidence center again returned to the front the Micang Mountain and very thick foredeep sediments indicate the main period of activities of the Micang thrust belt. Then during the Late Cretaceous to Paleogene, the foredeep jumped to the Ya'an-Mingshan area, southwestern Sichuan. The syn-orogenic subsidence center of the western Sichuan foreland basin migrated in an arcuate form in the western and northern parts with the Sichuan basin as the center, and the sedimentary sequences alternated and were stacked continuously. The contour maps of the bottoms of various Mesozoic structural layers show that the present structure low is located in northwestern and southwestern Sichuan, with E-W-striking contours distributed in northwestern Sichuan and NE-SW-striking contours in southwestern Sichuan. Therefore this paper suggests that Late Jurassic to Early Cretaceous structural deformation might control the present deformation of strata in the western Sichuan basin, forming the N-S tectonic compressional structure in northwestern Sichuan, and that the Cenozoic structural deformation is mainly manifested in the southwestern part of the western Sichuan basin, forming a NE-SW-trending stratigraphic distribution pattern.