Geological history of Borneo

Overview

Borneo, an island in South East Asia, has been formed by the arc-continent collisions, continent–continent collisions and subduction–accretion due to convergence between the Asian, India–Australia, and Philippine Sea-Pacific Plates over the last 400 million years. . All Borneo’s volcanoes are extinct and there is no strong evidence of any current seismic activity .

The South East Asia region is made up of fragments of crust that drifted northwards after being separated from Gondwana . Some of these crustal fragments have accreted to Eurasia forming Sundaland which is places such as peninsular Malaysia and Thailand. Fragments that did not reach so far north are what we see today as southwest Borneo, Sumatra and Java . This accretion of Sundaland took place during the Late Palaeozoic and Early Mesozoic. During the Mesozoic ophilites were emplaced due to the subduction of the Pacific plate followed by a prolonged period of magmatism across much of SE Asia that formed extrusions and intrusions into the basement ophiolites and crustal blocks . On the ceasing of the subducation, sedimentation dominated through the Late Mesozoic and Cenozic with extension across the region . The processes causing the spreading of the South China Sea to the north of Borneo that took place during the Eocene are not fully understood and there are multiple models have been suggested to describe this spreading .

File:Western Pacific subduction during the Mesozoic - active margin.png

Ophiolite emplacement caused by the Mesozoic subduction of the western Pacific plate.

Mesozoic subduction

Initiation of subducation of the western Pacific plate beneath SE China and SE Asia caused ophiolite emplacement and the development of the early Pacific arc. The long period of magmatism it induced is evident as intrusions and extrusions reaching as far north as south China, through Hong Kong, the South China Sea continental shelf, Vietnam and to Kalimantan in southwest Borneo. This period of magnetism continued until 79Ma when the margin moved east . The ages of magmatism in these locations in order of earliest magnetism are proposed 186 to 76 Ma for the Schwaner Mountains granites of Kalimantan; 180 to 79 Ma in South China; 165 to 140 Ma in Hong Kong and 112 to 88Ma in Dalat in Vietnam . Granites in Sabah have been explained by this arc related magmatism forming the arc tonalities that intruded into the ophiolites . The magmatic belt of arc granites and volcanism across SE Asia produced by this subduction was hundreds of kilometres wide with a total area of 220,000km2 . This took place during the Jurassic and Cretaceous Periods . In southwest Borneo, the Schwaner Mountains in Kalimantan also have granitic continental fragments of Australian continental crust. These continental fragments make up the basement that formed prior to the magmatic activity and ophilote emplacement. The fragments provide evidence for the drifting, collision and amalgamation of continental fragments from Gondwana. The rifting from Gondwana and migrating northwards of the fragments took place in three stages due to the opening and closure of the Tethys Ocean which separated Gondwana in the south, from Eurasia in the north. These stages were during the Devonian, Late Permian and Late Triassic ..

Inferred location of subduction zone and trench responsible for Mesozoic magmatism and ophiolite emplacement

Ophiolite emplacement

Much of the northeastern part of Borneo is Mesozoic ophiolite that was emplaced during the initiation of the western Pacific subducting beneath SE China and SE Asia . This ophiolite basement underlies regions from Sabah, in northeast Borneo, across Borneo and in southeast at the Meratus Mountains to Central Java in the south and Palawan and the Philippines in the North . An ophilotie is a sequence of oceanic lithosphere that has been thrust on top of continental lithosphere therefore has peridotites, gabbros and basaltic units overlain by cherts and submarine volcanic and sedimentary rocks . Due to its origin as oceanic crust, it is ultramafic or mafic in composition and has a high concentration of pyroxene and amphibole. Exposures of the ophiolite basement can be seen in the Segama Valley, Darvel Bay, Telupid and Kudat . K-Ar dating of the ophiolites has indicated they are Cretaceous in age indicating the subduction beneath Borneo took place at this time .

Late Cretaceous – Cenozoic Sedimentation

After ceasing of subduction, subsequent to the ophiolite emplacement and magmatism of the South China Sea related to the Mesozoic western Pacific subduction, there was a period of sedimentation. During the Late Cretaceous to Miocene there was a prolonged period of extension of the South China Sea . The cause of the northwest-southeast spreading of the South China Sea is not yet defined however there are various potential models that represent different processes . Predominantly sedimentation took place during the Early Tertiary in the Crocker Basin of northwest of Borneo and off the present day coast. Deep marine turbidites and mudstones known as the Rajang Group were deposited . This is thought to be approximately the same time as India started collision with Asia . Within the Rajang group the end of the South China Sea spreading is recorded in the sedimentary record showing transition from active continental margin in the Late Cretaceous to a deep marine setting through the Tertiary . The Rajang Group has subsequently undergone strong deformation and been uplifted to its current 1km elevation to form the Interior Highlands of Borneo .

In the Late Eocene, above the Rajang Group, there is an unconformity marking the inversion event of the Sarawak Orogeny . Following this Oligocene deposition of the Kinabatangan Group occurred, which also consisted of turbidites and marine mudstones indicating transition to a deep marine environment . Part of this group is the Crocker Fan that throughout SE Asia is the largest volume of deep marine Paleogene sediment in a single basin . Renewed extension during the Miocene caused uplift in Sabah. This uplift began to change sedimentary deposits from deep marine turbidites and cherts to shallowing coastal marine sediments during the Miocene and into the Pliocene . The latter saw deposition of carbonates and show transition form marine to deltaic fluvial deposits which together form the Serudong Group . Early Miocene sedimentation of up to 14 m is seen in basins to the east and northeast of Borneo with accommodation space formed by Eocene rifting, and erosion material derived from the Borneo highlands and uplift of older parts of the basin margins . Along with the regional uplift, the renewed extension also potentially caused Late Cenozoic A-type magmatism responsible for intrusions such as at Mt. Kinabalu .

Cenozoic South China Sea Extension

Northeastern Borneo’s Cenozoic tectonics are still in hot debate, where there are 3 models for the extension in the north of Borneo in the Baram Basin . The Proto South China Sea plate previously lay to the east of today’s Dangerous Grounds off the coast of Sabah, northwest Borneo . The Dangerous Grounds is a block that forms part of the South China Sea and lies to the northwest of Borneo gaining its name by sailor’s description of the shoal and reef scattered ground . It is a basement of continental crust and forms the continental shelf of the South China Sea to the northwest of Sabah . During the Eocene, extension of the Dangerous Grounds began, followed by further extension during the Oligocene . The extension of the South China Sea was in the northwest-southeast direction and caused thinning and rifting leading to the formation of the South China Sea basin to the northwest of the Mesozoic arc that once extended across much of SE Asia .

The 3 competing models mentioned above each describe this rifting and thinning of the Late Mesozoic and vary slightly between different authors. Here, Burton-Johnson (2013), illustrates the three models as 1) The extrusion model where lateral strike slip faults are projected through the South China Sea and lateral displacement caused by the India-Asia collision forces the extension, 2) The Subduction Model which coincides well with evidence on the formation of the Dangerous Grounds where slab pull caused extension as the Proto South China Sea was subducted beneath northwest Boreno and 3) The continental rift basin model where distant processes of slab roll back south of Borneo near Java and Sumatra caused magmatism leading to the crustal extension northwest of Borneo and formation of A-type granites.

File:3 models for spreading of the south china sea.png

The three models for the spreading of the South China Sea northwest of Borneo

Extrusion Model

The extrusion model links the India-Asia collision to the extension of the South China Sea. The collision would cause the lateral extension of strike slip faults in the South China Sea. The Northward movement of India to the east pushes the south east Asia block to away from the collision resulting in the clockwise rotation of Borneo forming a large extensional basin . In defining whether this model is correct it is important to understand if the strike slip faults of south China extend through the South China Sea as far as Borneo such as the Red River Fault. The West Baram fault runs northwest-southeast off the coast of Sarawak and separates blocks of differing geological histories one of which is the Dangerous Grounds to the northeast. This fault is important as the amount of slip is thought to accommodate proposed subduction of the photo South China Sea therefore the strike slip faults in Borneo are key to understanding which model is correct. Along the West Baram fault there is no major displacement evidence suggesting this fault did not have to accommodate for the passage of the subduction of the proto South China Sea. However models cannot conclude that the lack of lateral displacement demonstrates that the extortion model must be true because the proto South China Sea was so narrow that there the lateral displacement may not have had to be extensive.

Continental Rift Basin Model

One of the three models for the thinning and rifting of the continental crust of the South China Sea is initiated by Java and Sumatra subduction causing extrusion and slab roll back along the margin. The stresses from these were experienced in the South China Sea and together they caused the southward extension of the South China Sea. This model indicates subduction to the south of Borneo near Java and Sumatra but not subduction of Borneo or beneath Borneo.

Late Ceneozoic Magmatism and the Mount Kinabalu Intrusion

Dating and defining the history of the Mount Kinibalu granitic intrusion faces many challenges due to the high erosion, difficult access through relief and rainforests, difficult to define boundaries and alteration. The general geology of SW Asia during the Cenozoic is not well defined making the reconstruction of geological events leading to Borneo’s current setting an interesting point of study. Mount Kinibalu is in norhteast Borneo in Sabah at 1500 m . It was intruded at the same time as other Late Cenozoic magnetism that occurred in the South China Sea. The Mount Kinibalu intrusion is a laccolith style pluton that caused the deformation of sediments overlying the intrusion. It intruded between 7.85 and 7.22 Ma into shallow depths of less than 12 km splitting the overlying sediments from the underlying ultramafic basement rocks. Other magmatism in Borneo during the Late Cenozoic include intrusive and extrusive in Sarawak, the Semporna volcanics of Eastern Sabah and in Kalimantan .

Subduction Model for South China Sea Spreading

The subduction model proposes that the proto South China Sea formed a subduction zone to its east, which subsequently, by the process of slab pull, subducted the proto South China Sea beneath northwest Borneo. When these Dangerous Grounds collided with the Mesozoic Granites of the Schwaner Mountains in Borneo, it brought an end to the subduction . This is due to the less dense Dangerous Grounds not having a big enough density contrast to the overlying plate. Therefore, with the end to subduction a suture was formed where the proto South China Sea had subducted. The proposed suture lies beneath Mout Kinibalu in northestern Borneo. The subduction zone is generally thought to be about 400 km to the northwest of the suture . The overlying plate today forms parts of southeast Sabah, Palawan and Mindoro and contains terranes of continental and island arc crust . This subduction period took place throughout the Eocene and ended in the Early Miocene.

Thin section from Mt. Kinabalu showing amphibole rich granite.