Crustal structure of the NE continental margin of the South China Sea

The wide rifting mode that preceded the opening of the South China Sea in the Cenozoic generated a complex network of sedimentary basins, whose structure is currently being investigated. Until now, most studies focused on the Pearl River Mouth segment. Comparatively, towards Taiwan, the crustal structure of the north-easternmost part of the South China Sea margin (Tainan-Taixinan Basin sensu lato) is less explored.

To investigate the crustal structure of this segment, an extensive open access data set was used, including (a) 07 offshore well logs with biostratigraphic information, (b) over 15,000-line km of two-dimensional reflection seismic (c) over 4,100-line km of refraction seismic, (d) satellite free-air gravity anomaly data, and (e) bathymetry (GEBCO 15 seconds grid in meters). We interpreted seismic data together with the results of a gravity inversion scheme that provides three-dimensional variations of Moho depth and crustal thickness. The joint inversion of interpreted seismic and gravity-inverted Moho enabled the determination of crustal basement density variations along a set of 2D profiles.

This integrated approach enables us to distinguish at least five crustal domains from the continental shelf towards the ocean (i.e., north to south) showing contrasted stratigraphic and structural style, crustal thicknesses, and basement densities. (a) The proximal margin is characterized by a continental basement between 19 and 37 km thick, likely including thick Mesozoic to Paleozoic sediments and numerous intrusive rocks. (b) The necking zone is associated with the deepening of the top basement and increasing crustal thinning. This domain widens toward the northeast and is controlled by counter-regional faults that created half grabens filled by polyphasic syn-rift sediments. (c) To the south, the hyper-thinned crust (<~10 km thick) is controlled by regional low-angle normal faulting related to rifting prior to the South China Sea opening in the Oligocene. These rift structures seem to control the formation of NE trending wedge-shaped basins infilled by thin syn-rift deposits, possibly of Eocene and younger age. (d) Seawards, a domain of thicker crust is observed (10 to 16 km thick), characterized by an average high-density crust (>2900 kg/m^-3), the scarceness or absence of faulting, and the onlap of Miocene sediments. The transition towards the unambiguous oceanic domain is characterized by an array of outer highs of likely dominantly magmatic origin. (e) Unambiguous oceanic crust is characterized by chaotic high-amplitude crust with an average thickness of ~6 km, passively draped by post-Oligocene sediments.

This segment of the South China Sea margin is characterized by the presence of a failed rift axis, underlain by hyper-thinned crust. The age of rifting is not directly constrained, but this basin likely preserves the oldest rift phase preceding the opening of the South China Sea. Further south, the peculiar high-average density crustal domain appears most likely of magmatic origin, where Mesozoic to Cenozoic basalts have been dredged.

These new results on the crustal structure of the north-easternmost part of the South China Sea margin point toward a polyphase magmatic activity and more complex tectonic history than previously assumed.