Contraction and Extension in Northern Borneo: Subduction Rollback-Driven
Earth Sciences, Royal Holloway University of London, Egham, United Kingdom.
During the Paleogene the Proto-South China Sea was subducted beneath Borneo. Subduction ended in the Early Miocene with collision of the Dangerous Grounds block and the Sabah-Cagayan Arc. After collision much of northern Borneo became emergent, but subsidence resumed and during the Miocene most of the area was submerged and thick sediments were deposited above the Top Crocker Unconformity.
The NW Borneo-Palawan Trough developed after the Middle Miocene and is not the site of subduction nor the position of the former trench. Between the trough and land is a wide fold and thrust belt with structures broadly parallel to the trough. To the SW the trough ends abruptly at the West Baram Line which is not an active fault. In the offshore region from the largely undeformed Sarawak Shelf to offshore Sabah fold belt are sedimentary successions with several regional unconformities, including the Deep Regional Unconformity, Middle Miocene Unconformity, Shallow Regional Unconformity and several others. Their cause is uncertain.
The offshore fold and thrust belt is often interpreted as a collision-related feature resulting from convergence between Borneo and the Dangerous Grounds block. However, there is no seismicity, dipping slab or volcanicity. The onshore geology has some unexpected features. The mountains of the Crocker Ranges parallel to the trough expose the Early Miocene collisional fold belt. They have widened significantly since the Middle Miocene and the shelf edge has migrated northwards. The older fold belt changes orientation abruptly through 90° at the position of Mt Kinabalu, a 7-8 Ma granite pluton. To the south of the mountain belt there are ‘circular basins' which are remnants of a much larger basin filled with sediment during the Middle and Late Miocene. Further south is an extinct volcanic arc that can be traced from Sabah into the Sulu Arc, active during the Middle and Late Miocene, which was the product of northward subduction of the Celebes Sea.
The connections between the early subduction and collision history, the rising Crocker Mountains, granite magmatism, offshore deformation, trough development, regional unconformities and Neogene subduction are complex. I suggest that subduction rollback of the Celebes Sea initiated Neogene extension in northern Borneo, leading to formation of the Sulu Sea, extensional unroofing of the Crocker Ranges, granite intrusion, movement of the deep crust, and gravity-driven fold and thrust deformation offshore.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012