Seismic constraints on a remnant Mesozoic forearc basin in the northeastern South China Sea

Abstract

The northeastern of the South China Sea (SCS) has undergone tectonic inversion from a Mesozoic convergent continental margin to a Cenozoic divergent continental margin. The remnant subduction-related structures, such as volcanic arc and forearc basin, likely preserve key clues to reveal the dynamic mechanism of this tectonic inversion. We applied ocean bottom seismometers (OBS) data, combining with multi-channel seismic data, gravity data, drilling data, and petrologic observations, to reveal geophysical and structural features of the Dongsha Basin, a Mesozoic basin in the northeastern SCS. The Mesozoic strata are characterized by large thickness (~1–8 km), relatively high P-wave velocities (3.5–5.5 km/s), high densities (2.55–2.65 g/cm3), alternating folds, and thrust faults, which are significantly different from the features of Cenozoic strata. The Dongsha Basin is flanked by a Mesozoic granitic arc to the northwest and an inferred Palaeo-Pacific subduction zone to the southeast, indicating that it is geographically located in the Mesozoic forearc region. Considering the similar Mesozoic strata detected in the southern margin of the SCS and the contemporaneous accretionary complex outcrops observed in the Palawan Continental Terrane, we propose that the Mesozoic strata in these two regions originated from a common Mesozoic forearc basin, which was broken by the forearc rifting and subsequently separated by the spreading of Cenozoic SCS. Accordingly, a continuous Palaeo-Pacific subduction system is reconstructed, including the “arc-forearc-accretionary wedge” tectonic sequences. Reviewing the mechanisms of forearc rifting globally and the retreat of volcanic arcs onshore South China, we speculate that rollback of the Palaeo-Pacific subduction slab might be one of the causes for the forearc rifting and the initial extension of the SCS.