Back-arc suture zone along the north margin of the eastern Cimmerian continent

The Cimmerian continent is the north piece of the Gondwana super-continent collaged to the south edge of the Laurasia super-continent during the closure of the Paleo-Tehyan Ocean. Researchers recently have identified the Rushan-Pshart-Longmu Co-Shuanghu-Bitu-Changning-Menglian-Inthanon suture zone as the northern boundary of the eastern Cimmerian continent. However, a detailed correlation of this complicated tectonic zone from Pamir to Sumatra remains unclear. For example, while a back-arc suture is developed in southeast Tibet and Southeast Asia, no evidence of the same suture has been reported in eastern and central Tibet. In this study, we investigate an ophiolite me & PRIME;lange suite exposed in the Deqin back-arc suture of this tectonic zone in eastern Tibet, and interpret it as the northward extension of the Jinghong-Nan River back-arc suture. The me & PRIME;lange includes serpentinites, gabbros, basalts, and minor abysmal radiolarian-bearing cherts, and these rocks appear as isolated blocks in fault contact with Permian strata. Geochemical data of the serpentinites from the Deqin suture and Bitu suture indicate that they are MOR-type and SSZ-type sutures, respectively. Zircon U-Pb dating of the gabbro and diorite from the Deqin suture yields crystallization ages ranging from ca. 246 Ma to ca. 276 Ma, and a zircon U-Pb age of ca. 285 Ma is obtained from the Bitu suture. Geochemical analysis shows that the rocks from the Deqin suture have OIB or MORB characteristics, suggesting southwest subduction of the oceanic lithosphere beneath the North Lancangjiang tectonic belt since the Middle Permian. Two Meso-Tethyan magmatic rock samples from the Bitu and the Deqin sutures crystalized at 114 Ma and 111 Ma, respectively, and their geochemical data indicate OIB-type signature. We conclude that the Bangong Co-Nujiang Ocean lithosphere subducted northeastward beneath the Qamdo terrane at a low angle. Based on the correlation between the Rushan-Pshart suture and the Longmu Co-Shuanghu suture, our pre-Cenozoic plate reconstruction suggests that the Karakorum fault right-laterally offset the northern boundary of the Cimmerian continent by 140 & PLUSMN; 10 km. The offset further implies that the Cenozoic deformation of the Tibetan plateau is accommodated mainly by distributed structures rather than by several discrete fault systems.