Basement structures from satellite‐derived gravity field: South China Sea ridge

TLDR

The authors construct a crustal model of the South China Sea using constrained forward and inverse gravity modeling. They evaluate sediment effects with NOAA’s global sediment thickness and compaction models, integrate GEBCO bathymetry and seismic crustal thickness data with a flexural isostasy framework, and invert the gravity field to derive gravity‑equivalent basement topography. The gravity analysis uncovers high‑frequency signals, including a ~1000‑km linear feature parallel to the extinct spreading center, crustal thicknesses of 8–12 km oceanic and 10–20 km continental, elastic thickness below 6 km, a continuous midbasin ridge that varies from steep to gentle valleys with seamounts, and a spreading center that bends from an older east‑west to a younger southwest‑northeast orientation.

Abstract

The satellite‐derived gravity over the South China Sea reveals high spatial frequency signals not present in the bathymetric models. The most evident is a linear feature of near 1000 km length, which parallels the South China Sea extinct spreading center. We formulate a crustal model of the South China Sea by constrained forward and inverse gravity modeling. The effect of the sediment layer is evaluated using the global sediment thickness model of NOAA and fitting the sediment compaction model to observed density values. The General Bathymetric Chart of the Oceans (GEBCO) and constraining data from seismology on crustal thickness are integrated in the inversion process, which includes the flexural isostasy model. The model crustal thickness values vary between 8 and 12 km and between 10 and 20 km over the oceanic and continental crustal part of the South China Sea, respectively. The elastic thickness is below 6 km over most of the sea, with lowest values in the margin (between 2 and 4 km). Finally, we invert the gravity data for the gravity‐equivalent basement topography, which in sediment‐covered areas corresponds to the basement and in sediment‐free areas coincides with the bathymetry. We find a continuous ridge structure along the entire midbasin: It changes from a steep valley in the younger SW basin to a less pronounced valley in the NE basin, lined with seamounts. Our model shows the spreading center to be continuous and to bend from the older EW orientation to the younger SW‐NE orientation rather than being made up of two separate axis segments.