Publications

This paper compares the structural commonalities and differences of key fold-thrust belts (FTB) that fringe the Mesozoic Eastern Sundaland around the North-Western and Eastern circum-Borneo margins, which form important exploration targets. In a clockwise direction: the Bunguran, West Baram Delta, East Baram Delta, Central and Northern NW Sabah Trough, Sandakan, Tarakan, and Kutai-West Sulawesi FTBs are reviewed. Being located along tectonic suture lines that have existed at least since the Oligocene, these FTBs have a complex structural history that repeatedly switches between extensional and compressional tectonics. It is noted that only the belts along Eastern Borneo’s active margin of Sulu, Tarakan and Kutai-West Sulawesi are induced by crustal subduction, whilst the compressive (“failed subduction”) north-western margin exhibits a stretched, ductile continental crust of the South China Sea. Focussing on the latter, these belts were formed by processes including strike-slip tectonics, rapid contractional block uplifting, gravitational gliding and thrusting, inversion tectonics and compression-induced hydraulic clay diapirism. Not surprisingly, the strike-slip component appears to be perhaps the most enduring, albeit tectonic uplift of hinterlands and the resulting massive erosion are credited for the sediment load that filled the fringe basins, which eventually transformed into the FTBs. This study investigates and compares the importance of various structural processes affecting each FTB. Overall structural deformation of these belts shows that they are formed step-by-step, in partly gravity-induced belts along the Borneo margin with the oldest thrusts being located inboard and progressively propagating more distally outboard. The Eastern Borneo margin, in addition to thin-skinned tectonics, is perhaps more influenced by thick-skinned deformation caused by thrusting within the basement fabrics. This forms basement highs and mountain building during which the crust is shortened horizontally and thickened vertically, such as the deformation experienced in the West Sulawesi FTB.