Late Mesozoic compressional folds in Dongsha Waters, the northern margin of the South China Sea

•Dongsha Waters in the northern South China Sea hosts a unique relict Mesozoic Basin.•Blind fold trains are found in the basin.•These folds are correlated as growing with the Late Yanshanian magmatism.•Compression may be due to far-field impact of subduction of the Mesozoic Pacific.

Once considered as a component part of the Cenozoic Pearl River Mouth Basin (PRMB), Dongsha Waters was found with distinct tectono-stratigraphic style, in which the upper structural layer comprises only thin (usually 0.5-1 km) sediments of mid-Miocene to Quaternary, whereas the lower layer consists of Early Cretaceous terrestrial, Jurassic marine and possibly Triassic sedimentary rocks, totaled 4-9 km thick. The major sedimentary hiatus between them corresponds to the Late Cretaceous to mid-Miocene Epoch, well during the continental rifting to oceanic spreading stage. Faulting and magmatism during the opening of the South China Sea are basically weak over the central part of Dongsha Waters where the crustal thickness is mostly around 25 km thick, reflecting a relatively stable block. All these indicate less influence exerted by the Cenozoic rifting, strikingly contrasting to the neighboring areas where thick rift-to-drift sediments have accumulated. With the recognition of Mesozoic strata in this and the previous study, a relict Mesozoic basin is proposed to be existed over Dongsha Waters and named as Dongsha Basin.There are two generations, Late Neogene and Late Jurassic to Early Cretaceous, of compressions featuring seafloor and blind folds, respectively. The former presents only in few localities in east to Dongsha Island. The later distributes widely as individuals in north and as trains in south. They are characterized mostly by gentle growth folds inside which only a few minor upthrust faults with offsets in the order of a few tens to hundreds of meters. The upthrust faults dipped mostly southeastward against to the northwestward subduction of paleo-Pacific as suggested in a previous study. They featured more like back-thrust growth tectonics, formed a broad NNE-SSW trending belt oblique to the northern margin of the South China Sea, and thus are inferred to have been the far-field impact of the Late Mesozoic subduction of the proto-Pacific.