Profiler‐sonobuoy measurements in the South China Sea Basin

Abstract

Analyses of single‐channel seismic reflection profiles, and wide‐angle reflection and refraction data from sonobuoys, indicate that the deep part of the South China Sea basin is typically oceanic in structure, except that layer 2 is about 1 km thicker than usual and layer 3 is only about half the usual thickness. The top of layer 2 has a rough upper surface and becomes deeper in the northern part of the basin, where it is overlain by sediments of velocity 1.7–3.8 km/s up to 3 km thick. A 6‐km‐thick sequence of sediments fills a basement depression just north of the outer subsurface (peripheral) ridge of the Sunda shelf that may be structurally part of the Palawan trough. Thick sediments were measured at the outer parts of the Sunda shelf, Palawan shelf, Northeast Borneo shelf, and in the Taiwan straits (Pescadores channel) south of the Penghu‐Peikang basement high. The Manila trench is divided into two sections, a sediment‐filled trench between southernmost Taiwan and Stewart bank off central northern Luzon and a topographic trench between Stewart bank southward into the Mindoro straits. Burial of part of the trench by sediments that thicken to the north and the thick sediment cover of the northern South China Sea basin indicate provenance from the north. Each of the margins of the South China Sea basin is interpreted to have a different structural style. The northern margin of the basin is predominantly a zone of tension (graben and horst block structures), the southern margin is predominantly a zone of compression (fold structures), the western margin may be a zone of shear, and the eastern margin is a subduction zone (Palawan trough and Manila trench). The geological and geophysical characteristics of the basin suggest a middle Tertiary to Recent history of development that involved spreading of the sea floor. It seems probable that the southern margin of the basin, called the Reed bank crustal block, was formerly attached to mainland Asia along the present eastern edge of Macclesfield bank. Sometime during the Paleogene the Reed bank crustal block broke away from the continent, and new sea floor was created between the southeastward migrating block and the Asian mainland. Old ocean crust in front of the advancing block was subducted along the northwestern sides of Borneo and Palawan. Subduction ceased in the early Miocene with the collision of the crustal block and the Borneo‐Palawan isthmian ridge.