Geological history of Borneo
The base of rocks that underlie
Early amalgamation from Gondwana
The SE Asia region is made up of fragments of
The rifting, migrating northwards and accretion to form Sundaland took place during the Late Paleozoic and Early Mesozoic in three stages. The stages were controlled by the opening and closing of the Tethys Ocean separating Gondwana from Eurasia. Gondwana was in the south and Eurasia migrated north to its current location where it is relatively fixed today. These stages were during the Devonian, Late Permian and Late Triassic.[1]
In addition to a basement of crustal fragments rifted from Gondwana, SE Asia is also characterised by:
- Mesozoic ophiolite emplacement across much of SE Asia due to the subduction of the Palaeo-Pacific plate.
- A prolonged period of magmatism across much of SE Asia that formed extrusions and intrusions into the basement ophiolites and crustal blocks.[5]
- Sedimentation that dominated through the Late Mesozoic across the region following the end of the subduction.[6]
- Cenozoic extension and Rifting of the South China Sea to the north of Borneo during the Eocene where multiple models have been suggested to accommodate this spreading.[2]
Mesozoic subduction
During the Jurassic and Cretaceous time periods there was extensive volcanism and intrusions caused by the subduction of an oceanic plate. The western Pacific plate was subducted beneath SE China and SE Asia. The initiation of this subduction caused ophiolites to be emplaced and the development of the Pacific arc, a magmatic belt across SE Asia. Ophiolites are large blocks of oceanic crust that have been thrust onto continental crust due to compression that forms subduction type collisional plate boundaries.[6]
The early Pacific arc is a curved line of volcanism and intrusions where the Palaeo-Pacific Ocean subducted beneath Borneo, Vietnam and South China. The resulting long period of magmatism is evident from 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 magmatism continued until 79Ma, when the margin moved east.[6] The ages of magmatism in these locations are 186 Ma to 76 Ma for the Schwaner Mountains granites of Kalimantan; at 180 Ma to 79 Ma in South China; at 165 Ma to 140 Ma in Hong Kong and at 112 Ma to 88 Ma in Dalat in Vietnam.[6] 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 220,000 km2.[6]
Ophiolite emplacement
There are many crustal fragments in northeast Borneo that are derived from Mesozoic ophiolites placed during the initiation of the western Pacific Ocean plate subducting beneath southeast China and southeast Asia.[6] This ophiolite basement underlies regions from Sabah in northeast Borneo and the Meratus Mountains in southeast Borneo. The ophiolite crustal fragments were also emplaced in Central Java to the south of Borneo and Palawan and the Philippines in the North.[6]
An ophiolite is a sequence of oceanic lithosphere that has been thrust on top of continental lithosphere and therefore has
Late Cretaceous – Cenozoic sedimentation
A prolonged period of extension of the South China Sea took place during the Late Cretaceous to Miocene.[7] 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 kinematic processes.[5] Looking at the sedimentary record can help constrain this period of extension and its causes. An account of the late Mesozoic and Cenozoic sedimentation follows.
Rajang Group
Sedimentation took place predominantly during the Early Tertiary in the Crocker Basin which is found today in the northwest of Borneo and off the present-day coast. Deep marine turbidites and mudstones known as the Rajang Group were deposited.[6] Deposition is thought to coincide in time with when India began colliding with Asia.[2] Within the Rajang Group sedimentary record, the end of the South China Sea spreading is recorded showing a transition from active continental margin in the Late Cretaceous to a deep marine setting through the Tertiary.[4][6] The Rajang Group has subsequently undergone strong deformation and been uplifted to its current 1 km elevation to form the Interior Highlands of Borneo.[2][8]
Kinabatangan Group
In the
Serudong Group
The
Cenozoic South China Sea extension
Northeast Borneo's Cenozoic tectonics are still in hot debate, where there are three models explaining the crustal extension in the north of Borneo.[5] The Proto-South China Sea plate previously lay to the east of today's Dangerous Grounds off the coast of Sabah, northwest Borneo.[2] The Dangerous Grounds is a crustal 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 shoals and reefs scattered ground.[5] It is a basement of continental crust and forms the continental shelf of the South China Sea to the northwest of Sabah.[1] The spreading of the South China Sea is no longer active.
During the Eocene, extension of the Dangerous Grounds began, followed by further extension during the Oligocene.[5][10] This extension across the South China Sea was in the northwest-southeast direction and caused thinning and rifting. This led to the formation of the South China Sea basin to the northwest of the Mesozoic arc that once extended across much of SE Asia.[7] The extension that occurred formed a triangle-shaped rift area due to the kinematics of the rifting taking place like the blades of an opening pair of scissors.
The three competing models mentioned above describe this rifting and thinning during the Late Mesozoic and vary slightly between different authors. Here, the three models are illustrated as:[6]
- 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
- 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 NW Borneo
- 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.[6]
Extrusion model
The extrusion model links the
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. Three major faults are known to traverse Indochina and South China are the Three Pagodas, Mae Ping and the Red River Fault, but not where they continue off the coast. It is not confirmed how far they cut into the South China Sea.[2]
The West Baram fault runs northwest-southeast off the coast of Sarawak, Borneo and separates blocks of differing geological histories, one of which is the Dangerous Grounds to the northeast. This fault is important as the slip is thought to accommodate proposed subduction of the proto South China Sea. Therefore, the amount of displacement along the strike-slip faults in Borneo are key to understand which model is correct. Along the West Baram fault there is not extensive displacement, indicating very little passage of the proto-South China Sea and possibly no subduction. However these models cannot conclude that the lack of lateral displacement demonstrates that the Extrusion Model must be false. A narrow proto-South China Sea or other crustal deformation could potentially have accommodated the subducting proto South China Sea and account for the lack of displacement along strike-slip faults.[5]
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.[6] This is unlikely to have caused South China Sea spreading singlehandedly but probably contributed to the forces involved.
Subduction model
The subduction model proposes that the proto-South China Sea formed a
The subduction zone is generally thought to be about 400 km to the northwest of the suture.[5] This subduction period took place throughout the Eocene and ended in the Early Miocene.[5]
Late Cenozoic Magmatism and the Mount Kinabalu intrusion
Dating and defining the history of the Mount Kinabalu 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 southeast 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.[11]
Mount Kinabalu is in Sabah, northeast Borneo and is 4095 m high. It was intruded at the same time as other Late Cenozoic magmatism that occurred in the South China Sea. The Mount Kinabalu intrusion is a