Marginal basin evolution: the southern South China Sea

Abstract

The southern South China Sea is divided into contrasting morphology by the West Baram Line. To the west is the Sundaland extinct passive margin in which rifting began in the Eocene (∼46 Ma) and ceased at 19–21 Ma (anomaly 6), where sea-floor spreading began much later than along the shelf of China. The break-up hiatus lasted ∼3–5 Ma marked by the Mid-Miocene unconformity, also preserved on land Sarawak. The post-rift strata date from ∼16 Ma and drape over the rifted topography. To the east is a convergent margin that became a collision zone in the Middle Miocene. The Sunda Shelf is of uniform ∼30 km thickness except in localised deep basins, and extends to a water depth of ∼200 m. The continental slope is narrow. The continental rise ( Dangerous Grounds), is covered by water ranging from 500 m to 3.5 km depth at the continent–ocean transition. Its width ranges from 170 to 330 km. The Rajang Delta extends over the shelf and continental slope. Its post-rift sediments drape over the rifted proximal topography of the Dangerous Grounds. To the east the drape is thinner and has not completely buried the rifted topography. The cuestas appear to have supported the carbonate build-up infrastructures of the Spratly Islands, whose slopes rise abruptly from a sea-floor of 2–3 km depth. The Sabah and Brunei margin is a collision zone that was formerly convergent. The on land geology indicates a Mesozoic ophiolitic basement. The main collision feature is the Western Cordillera, constructed mainly of sandy Eocene to Lower Miocene turbidites, predominantly Oligocene to Lower Miocene in the West Crocker (32–18 Ma), uplifted episodically throughout the Upper Miocene and Pliocene, 14–8 Ma. The 2 km deep Northwest Borneo Trough may be a relict of the convergence phase, but could also be a collisional foredeep. The oil-prolific Baram Delta, resulting from uplift and erosion of the Western Cordillera, has built out as far as the Northwest Borneo Trough. It is suggested that the passive margin continental rise (Dangerous Grounds) has been underthrust beneath Sabah to cause uplift of the Western Cordillera. The West Baram Line accordingly abruptly separates the collision zone from the western extinct passive margin; and is a now extinct major right-lateral transform fault.