Control of Coral Reefs Formation by Boudin-Like Crustal Extension Processes in the South China Sea: New Insights From Wide-Angle Seismic Imaging

Abstract

Coral reefs with uniform NE-SW trends are a prominent geological feature along the southern margin of the South China Sea (SCS), closely linked to continental crustal extension. However, their interrelationship and evolution remain inadequately understood. In this study, we present a P-wave velocity model extending from the abyssal basin to the reef areas along the southern SCS margin, derived from forward modeling and travel-time tomography using a 315-km-long wide-angle seismic profile OBS2019ZX1. The results reveal that the crustal thickness beneath the reef areas ranges from 17 to 21 km, with a crustal stretching factor (β) between 1.5 and 1.8, indicating more primitive crustal characteristics. Between the thicker crustal segments beneath the reefs lies an extensional basin, tens of kilometers wide with the thinnest crustal thickness of approximately 13 km and the β exceeding 2.4. We observe strongly heterogeneous crustal extension along the southern SCS margin, where NE-SW trending extensional basins separate thicker crustal segments to form the boudin-like crust. The rifting of the western conjugated margins in the SCS persisted longer and resulted in more complex crustal thinning compared to the eastern region. Crustal extension primarily focused on banded rift basins parallel to the spreading center, leading to the formation of wide rifted margins and dispersed crustal blocks. Under tropical climates, the topographic highs of the blocks exhibit high carbonate productivity, accumulating to form coral reefs. We propose that boudin-like crustal extension is a key factor controlling the formation of elongated NE-SW trending coral reefs along the southern SCS margin.