Syncollisional trench deposits can provide important information for reconstructing the tectonic evolution of the suture zone and constraining the time of initial collision of land masses. In this paper, sedimentological, petrographic, detrital zircon geochronological, and heavy mineral studies were carried out on the Yaduo Formation of Gaize area and the Duoren Formation of Ritu area in the western Bangong Lake- Nujiang suture zone. Sedimentological analysis shows that the Duoren and Yaduo formations were deposited in a submarine fan environment. The youngest detrital zircon age of the Duoren Formation constrains the depositional age to the Late Jurassic. The sandstone of the Duoren and Yaduo formations have QFL ratio of 52444 and 32860, respectively, and are characterized by abundant sedimentary and acidic volcanic rock fragments. The heavy minerals of the sandstones of the Duoren and Yaduo formations are dominated by stable heavy minerals such as apatite, zircon and tourmaline. Both these formations have similar detrital zircon age distribution patterns, with the main peaks mainly distributed in the ranges of 350~200 Ma, 550~450 Ma, 900~750 Ma, 1900~1800 Ma, and 2550~2450 Ma. These data suggest that the detritus of the Yaduo and Duoren formations originated from the accretionary complex in the Bangong Lake- Nujiang suture zone and the magmatic rocks in South Qiangtang north of the depositional area. The large amount of sedimentary detritus occurring in the Duoren and Yaduo formations indicates that the source area experienced extensive structural deformation, leading to the denudation of sedimentary rocks and contemporaneous magmatic rocks, and which is considered as a product of the Lhasa- Qiangtang syncollision. On this basis, the timing of initial Lhasa- Qiangtang collision occurred prior to the deposition of the Duoren and Yaudo formations (Late Jurassic).