Sedimentary data from field and borehole investigations allow reconstruction of the southwesterly variations of proximal sedimentary processes along the Longmenshan thrust belt during Late Triassic and Early Jurassic times and relating them to the development of a transpression basin. Conglomerates, which are the early indicators of the tectonic activity and orogeny appear for the first time in the second member of the Upper Triassic Xujiahe Formation and crop out only in the northern segment of the Longmenshan thrust belt. They are also present in the central segment in the fourth member of the Xujiahe Formation. In contrast the Early Jurassic Baitianba conglomerates were deposited all along the front of the Longmenshan thrust belt. The contact between the Upper Triassic and Lower Jurassic units is an angular unconformity and or a disconformity in the northern and central segments of the Longmenshan thrust belt, but becomes conformable to the southwest, along the strike. The isopach maps of different stratigraphic units show the along-strike shift of the depocenters with time, the result of southwestward propagating contraction and deformation of the Longmenshan thrust belt. In conjunction with the regional structural analysis and paleocurrent reconstructions, the southwesterly variations of sedimentary processes demonstrate an along-strike kinematic change of tectonic process in the western Sichuan basin from the northeast to the southwest. These data indicates that a transpressional deformation occurred along the Longmenshan thrust belt during the Late Triassic, and was synchronous with the development of the western Sichuan basin, which behaved as a transpressional foreland basin.