Bulletin of the Geological Society of Malaysia, Volume 63, June 2017, pp. 61 – 76
SE Asia Research Group, Department of Earth Sciences,
Royal Holloway University of London, Egham, Surrey, TW20 0EX, United Kingdom
*Corresponding author: firstname.lastname@example.org
Abstract: The term Proto-South China Sea has been used in a number of different ways. It was originally introduced to describe oceanic crust that formerly occupied the region north of Borneo where the modern South China Sea is situated. This oceanic crust was inferred to have been Mesozoic, and to have been eliminated by subduction beneath Borneo. Subduction was interpreted to have begun in Early Cenozoic and terminated in the Miocene. Subsequently the term was also used for inferred oceanic crust, now disappeared, of quite different age, notably that interpreted to have been subducted during the Late Cretaceous below Sarawak. More recently, some authors have considered that southeast-directed subduction continued until much later in the Neogene than originally proposed, based on the supposition that the NW Borneo Trough and Palawan Trough are, or were recently, sites of subduction. Others have challenged the existence of the Proto-South China Sea completely, or suggested it was much smaller than envisaged when the term was introduced.
We review the different usage of the term and the evidence for subduction, particularly under Sabah. We suggest that the term Proto-South China Sea should be used only for the slab subducted beneath Sabah and Cagayan between the Eocene and Early Miocene. Oceanic crust subducted during earlier episodes of subduction in other areas should be named differently and we use the term Paleo-Pacific Ocean for lithosphere subducted under Borneo in the Cretaceous. There is good evidence for subduction between the Eocene and Early Miocene below Sabah, and the western limit of Proto-South China Sea subduction was the West Baram Line. The subducted slab can be imaged in the lower mantle using P-wave tomography. There was no subduction beneath Sarawak, SW of the West Baram Line, between the Eocene and Early Miocene where there was terrestrial to marginal marine deposition. The present-day NW Borneo Trough and Palawan Trough are not subduction trenches and these relatively shallow features have different origins. The NW Borneo Trough is largely a flexural response to gravity-driven deformation of the Neogene sediment wedge NW of Sabah whereas the Palawan Trough is the continent-ocean transition at the SE edge of the modern South China Sea.
Keywords: Subduction, Borneo, Sabah, Sarawak, tomography