The North-West Borneo Trough

Abstract

The North-West Borneo Trough is bordered along its south-east margin by a mélange wedge that has been the subject of disagreement with insufficient discussion. Offshore Palawan it has been interpreted as an accretionary prism that has been preserved in place when subduction ceased in the Middle Miocene. It is unconformably overlain by undeformed Upper Miocene to Holocene draping strata. Farther south-west along the Trough, the seismically identical mélange wedge has been named a Major Thrust Sheet System, which was assumed to have been thrust as a nappe north-westwards over the autochthonous Dangerous Grounds terrane of attenuated continental crust of the South China Sea passive margin. The accretionary prism model is the simplest, resulting in interpretation of the North-West Borneo Trough as a fossil trench. Subduction was halted by the rifted and attenuated continental crust arriving at the Benioff Zone, choking subduction and causing isostatic uplift of the Western Cordillera of Sabah. The subduction system therefore became a collision zone resulting in the term ‘foredeep’, in which case the actual trench position remains obscure and somewhere towards the south-east, but shown in unrestrained geological cartoons. Enigmas remain in the Palawan area of the Philippines. The Trough position is bathymetrically obscure in places and its given position makes it impossible to derive the Calamian micro-continent from continental Asia as required from its stratigraphy. In the south-west, the Trough terminates abruptly at the West Baram Line. The Trough contains several spectacular edifices, formerly suggested to be volcanoes or mud volcanoes but herein shown to be carbonate build-ups that were drowned, indicating that the Trough subsided too fast for build-up continuation, whereas post-rift thermal subsidence of the Dangerous Grounds was slower, allowing the Spratly Islands build-ups to continue active.