Formation and inversion of a short-lived continental back-arc basin in Southeastern Tibet

The differences in the tectonic interpretation of ophiolite suites have become a major issue of the debate in the tectonic reconstruction of an ancient orogenic belt, especially when it comes to subduction polarity. In this regard, the Sanjiang Paleo-Tethyan Orogenic Belt in the southeastern Tibetan Plateau provides an excellent case study. The Sanjiang Paleo-Tethyan Orogenic Belt in the northern, eastern, and southeastern Tibet is bounded by the western Jinshajiang‒Garzê‒Litang suture to the north and the Shuanghu‒Changning‒Menglian suture to the south and west. The southern Jinshajiang Suture separates the Zhongzha Block to the east and the eastern Qiangtang Block to the west. The tectonic nature of the NNW-trending southern Jinshajiang ophiolitic mélange remaining controversial. A detailed linear traverse mapping was c across the southern Jinshajiang ophiolitic mélange, with a focus on pillow lavas and the structural relationship between the lavas and their country rock (Paleozoic sedimentary rocks). The results of a field study, in conjunction with new geochronological data and geochemical data, have enabled the identification of the Zhongdian continental back-arc basin. This basin was filled with a flysch succession and at least two horizons of pillow basalt from 267 to 254 Ma. The fining- and thinning-upward nature of the sedimentary succession, widespread syndepositional folds and syndepositional breccias, and submarine channel sediments, as well as intensive basaltic volcanism suggest that this back-arc basin generated in a typical extensional environment. The inversion of the back-arc basin was completed within a relatively short period of one million years (254~253 Ma), resulting in the development of overturned folds of the flysch succession and a latest Permian to Early Triassic back-arc foreland basin in front of the folded belt. Whole-rock geochemical data for the basalts and coeval gabbros suggest that the petrogenetic process of the basalts in the back-arc basin is likely comparable to that of basalt in a rift system as well, which is a lithospheric extension induced uplift of lithospheric mantle and asthenosphere and allowing decompression partial melting of the mantle peridotite. The late stage pillow basalts exhibit a stronger arc signature than the earlier massive basalt and diabase. The Zhongdian back-arc basin is considered to be an extinct continental and arc-type back-arc basins, which are characterized by thick crust, shallow bathymetry, and may not evolve into “normal” oceans. The formation and inversion of the Zhongdian back-arc basin are believed to have been caused by rollback and subsequent break-off of the subducted oceanic slab. During the inversion, the crust shortening occurred predominantly in the back-arc basin, while the southwestern shoulder of the back-arc basin, which was weakly deformed, was shifted north-eastward for ~30 km. The formation process of the Zhongdian back-arc basin is comparable to that of a typical continental rift system, the asymmetric architecture of which has mostly been inherited by the structures formed during the basin inversion. The southern Jinshajiang ophiolitic mélange is representative of the inverted Zhongdian back-arc basin, which is a short-lived, partially mature oceanic basin.