Jinsha River Suture Research Articles

The structure and tectonics of western Yunnan

In Yunnan, the region west of the Red River fracture has a particular NS, NNW-SSE zonal structural pattern, which is formed mainly by four suture zones of different ages. The easternmost is the Red River suture zone, along which Proterozoic oceanic crust thrust eastwards under the Yangtze continental plate. In the Indosinian-Yanshanian period (T-J), it reactivated and formed a large-scale nappe zone, the early Proterozoic metamorphic rocks of the Yangtze continent thrust westwards onto late Palaeozoic strata. The Lancang River suture zone is a plate suture zone formed in late Hercynian-early Indosinian (P-T), along which the ocean plate thrust westwards under the Zang-Dian micro-continent (i.e. Tibet-Yunnan) of Gondwana. It represents the collision zone between the Yangtze continent and the Zange-Dian micro-continent. In the suture zone, there are mainly Carboniferous-Permian volcanic island arcs, and its west margin is mostly occupied by the Lincang Granite which is a collisional-type granite with 212–258 Ma ages. The Jinsha River suture zone extends northwestwards from Jianchuan, and is similar to the Lancang River suture zone in nature. In the north, Carboniferous sediments are of ocean basin type. To the south of Jianchuan, however, it is connected with the Carboniferous-Permian fracture trough west of the Red River suture zone. Therefore, the Jinsha River suture zone is probably a transform fault. In the early Palaeozoic, there were elastic rocks between the Red River and Lancang River suture zones, and the sedimentary ocean basin migrated westwards to the area along the Lancang River. It became a platform until late Paleozoic and then became a graben basin called the Lanping-Simao Mesozoic depression. In the Lancang River suture zone, there was no basin behind the arc, but it uplifted and formed a late Paleaozoic rift. To the west, the Baoshan basin is a Palaeozoic basin formed by long-lived depression on land. The Nu River suture zone exhibits different characteristics in different regions. In the northern part, in Tibet, there is a Jurassic ocean basin, which was closed in the early Cretaceous. Ophiolite crops out widely, and other features of the suture zone are evident. The southern part of the suture in Yunnan is poorly developed and may be a shear zone. In the Tengchong-Yingjian area, west of the Nu River suture zone, the metamorphic basement occurs widely, and there is a Yanshanian-Himalayan granite belt formed by crustal anatexis, which is the acid magmatic rock belt on the uplifted side of the Yaluzangbu River suture zone extending south into Burma.

The ophiolites of the Tibetan Geotraverses, Lhasa to Golmud (1985) and Lhasa to Kathmandu (1986)

Ophiolite belts are found in Tibet along the Zangbo, Banggong and Jinsha River Sutures and in the Anyemaqen mountains, the eastern extension of the Kunlun mountains. Where studied, the Zangbo Suture ophiolites are characterized by: apparently thin crustal sequences (3-3.5 k m ); an abundance of sills and dykes throughout the crustal and uppermost mantle sequences; common intraoceanic melanges and unconformities; and an N-MORB petrological and geochemical composition. The ophiolites probably formed within the main neo-Tethyan ocean and the unusual features may be due to proximity to ridge-transform intersections, rather than to genesis at very slow -spreading ridges as the current consensus suggests. The Banggong Suture ophiolites have a supra-subduction zone petrological and geochemical composition — although at least one locality in the Ado Massif shows MORB characteristics. However, it is also apparent that the dykes and lavas show a regional chemical zonation, from boninites and primitive island arc tholeiites in the south of the ophiolite belt, through normal island arc tholeiites in the central belt to island arc tholeiites transitional to N-MORB in the north. The ophiolites could represent fragments of a fore-arc, island arc, back-arc complex developed above a Jurassic, northward-dipping subduction zone and emplaced in several stages during convergence of the Lhasa and Qiangtang terranes. The ophiolites of the Jinsha River Suture have a N-MORB composition where analysed, but more information is needed for a proper characterization. The Anyemaqen ophiolites, where studied, have a within-plate tholeiite composition and may have originated at a passive margin: it is not, however, certain whether true oceanic lithosphere, as opposed to strongly attenuated continental lithosphere, existed in this region.