A Back-arc Basin of Neo-Tethys in Southeastern Tibet

The northward subduction of the Neo-Tethys oceanic plate beneath Eurasia has led to the development of a substantial continental margin arc belt with a length of > 10,000 km along the southern margin of Eurasia. The indentation of India Continent into Eurasia resulted in the widespread segmentation of this substantial Andean-type subduction system. The Gangdése and the Myanmar-Tengchong arc belts are two such segments. A comparison of the subduction process of the Myanmar-Tengchong magmatic belt with that of the Gangdése arc belt reveals a high degree of similarity. However, in the Southeastern Tibet, the formation of the Myanmar-Tengchong magmatic belt from the Jurassic to the late Cretaceous period is debated due to the incomplete tectonic facies succession. Thus, our study focus on the spatial distribution of tectonic facies within the Myanmar-Tengchong belt. The detailed field studies, zircon U-Pb and bulk ⁴⁰Ar/³⁹Ar dating results, bulk geochemistry, and Sr–Nd isotopic data contributed to the identification of a late Early Cretaceous volcanic succession in a region to the east of the Myanmar-Tengchong magmatic belt. This volcanic succession consists of basalt, basaltic andesite, andesite with minor amount of dacite, intercalated with clastic and limestone rocks. This volcaniclastic succession had previously been interpreted as being of Carboniferous or Triassic age. However, the dating of zircons from five andesite samples and the ⁴⁰Ar/³⁹Ar dating results of groundmass from two basalt samples suggest that the volcanic succession likely erupted at about 106 Ma, during the Late Cretaceous Albian Age. The basalts from the volcanic succession exhibit characteristics of back-arc basin basalt, while the sedimentary rocks demonstrate features of a shallow marine face. These findings suggest that this succession represents the remnant of a back-arc basin spanning a period from 120 to 106 Ma, which is the northward extension of a Late Cretaceous back-arc basin in the southern part of the southeastern Tibet. The subduction process of the Neo-Tethys well is constrained by the north-south extending back-arc basin and the Myanmar-Tengchong magmatic belt. In the early stage, the Neo-Tethys oceanic plate subducted beneath Eurasia at a normal angle, resulted in a continental margin magmatic arc. Since ca.120 Ma, the rolling back of the subducted oceanic slab produced a back-arc basin behind the continental arc. From ca.106-77 Ma, the hinter segment of the subducted oceanic slab initiated a process of flat subduction, likely due to the break-off of the rolled segment of the subducted oceanic slab. Meanwhile, the arc-magmatism was very weak, and the extensional back-arc basin inverted to a compressional retro-arc foreland basin. After ca.77 Ma, the subduction angle became normal again, marking the onset of another period of intense magmatism started. This tectonic model likely shed lights on the subduction process that defined the Gangdése magmatic arc. It is possible that the back-arc basin, situated east of the Myanmar-Tengchong magmatic belt, is comparable to the northern Lhasa, where the Early Cretaceous marine sequences intercalated with bimodal volcanic rocks are common and have been interpreted as back-arc rift basins by some studies.