The Makassar Strait has long been recognised as a zone of major geological importance. It is situated between major mountain belts of western Sulawesi and fold belts of east Borneo and is the site of the original Wallace’s Line separating Australian and Eurasian faunas and floras. Lower crust (and possibly sub-crustal mantle) basement is exposed in west Sulawesi and there has been major late Tertiary contraction. The main depocentre contains up to 17 km of sediment.
Regional Tectonic Setting
Sulawesi is one of the most geologically complex regions in Southeast Asia. The island is composed of microcontinental fragments, magmatic arcs, ophiolites, and metamorphic complexes that amalgamated during the Late Mesozoic and Cenozoic. It lies at the junction of the Eurasian, Australian and Philippine Sea plates and has experienced subduction, collision, strike-slip faulting, crustal extension, and rapid uplift and subsidence.
Despite decades of research, many aspects of Sulawesi’s tectonic evolution remain poorly constrained due to limited geochronological data. Research in the region aims to establish the timing of deformation, magmatism, and basin development, and to understand how these processes shaped the island and its surrounding basins.
Examples of Our Research in Sulawesi:
Tectonic Evolution of Sulawesi
Much of Sulawesi’s geological complexity results from interactions between continental fragments, oceanic crust and volcanic arcs. Several studies focus on constraining the timing and mechanisms of deformation, uplift, and extension across the island.
Research has examined metamorphic core complexes, strike-slip fault systems, and extensional basins to better understand the processes that shaped the island. Field observations, structural analysis, and radiometric dating have revealed that crustal extension played a far more significant role in the Neogene evolution of Sulawesi than previously recognised.
These studies help refine regional tectonic models and provide new insights into the development of Southeast Asia’s plate boundary zone.
Strike-Slip Fault Systems
Sulawesi hosts several major active strike-slip faults, most notably the Palu-Koro Fault and the Matano Fault system.
The Palu-Koro Fault trends north–south across central Sulawesi and divides the island into eastern and western structural domains. The Matano Fault trends east–west and forms part of a complex network of fault systems across central and southeastern Sulawesi.
Field studies of these faults have identified:
- segmented fault strands
- ductile shear zones and mylonitic fabrics
- pseudotachylite veins indicating seismic slip
- serpentinite-rich fault zones acting as tectonic lubricants
Understanding the geometry and evolution of these faults provides insights into regional plate motions and the tectonic development of central Indonesia.
Exhumation and Mountain Building
Several projects investigate how deep crustal rocks have been exhumed to form Sulawesi’s mountain ranges.
Low-temperature thermochronology and radiometric dating have been used to reconstruct the thermal and uplift histories of granitic intrusions and metamorphic rocks. These studies show that uplift and exhumation are closely linked to regional tectonic processes such as subduction rollback, crustal extension, and strike-slip deformation.
Rapid uplift on land is often accompanied by subsidence in adjacent offshore basins, suggesting strong coupling between onshore tectonics and basin development.
Basin Development and Sedimentation
Sedimentary basins across Sulawesi and Borneo record the tectonic evolution of the region. Studies of stratigraphy, sedimentology and provenance provide important constraints on basin formation and deformation.
Key research areas include:
Gorontalo Bay
Gorontalo Bay is a deep semi-enclosed basin between the North and East Arms of Sulawesi. Multibeam bathymetry and seismic data reveal water depths exceeding 2000 m and thick sediment accumulations.
Evidence of rapid subsidence, crustal thinning, and active volcanism suggests that the basin developed through Neogene extension linked to regional tectonic processes.
Celebes Molasse
The Celebes Molasse comprises widespread conglomerates and sandstones deposited unconformably above older rocks across Sulawesi. Detailed stratigraphic studies aim to better constrain its age and depositional environments, providing important constraints on the timing of regional collision and uplift.
Lariang and Karama Basins
Field mapping and sedimentological studies in western Sulawesi have revealed a complex Cenozoic history involving rifting, marine transgression, carbonate platform development, and later basin inversion associated with regional tectonics.
Barito and Asem-Asem Basins
In southeast Kalimantan, thick Cenozoic sedimentary successions record the evolution of terrestrial and marine environments through time. These basins provide rare exposures of Eocene sediments in Southeast Asia and allow detailed study of early basin development and sediment routing.
Volcanism and Island Formation
Volcanic islands and seamounts in the region also record important tectonic events.
The Togian Islands in Gorontalo Bay form a volcanic ridge of probable Late Miocene age. Their origin remains uncertain, with possibilities including brief subduction-related volcanism or extension-related magmatism. Dating and geochemical analysis of volcanic rocks help determine the origin of these islands and their contribution to surrounding sedimentary basins.
The isolated volcano Una-Una, which erupted violently in 1983, has unusual potassium-rich chemistry and does not fit typical subduction-zone volcanic models, highlighting the complexity of the region’s tectonic setting.
Numerical Modelling of Lithospheric Deformation
Understanding deformation processes requires integrating both brittle and ductile behaviour of the lithosphere. Numerical modelling projects have developed hybrid models combining discrete element techniques (for brittle faulting) and finite element methods (for ductile flow).
These models are used to simulate extension and basin formation in Indonesian back-arc regions and to compare results with analogue experiments and field observations.
Geomorphology and Neotectonics
Remote sensing and digital elevation models have been used to analyse active tectonics in Sulawesi.
Geomorphic indices such as:
- stream length gradient index (SL)
- mountain front sinuosity (Smf)
- valley floor width-to-height ratio (Vf)
have been applied to river systems along the Palu-Koro Fault to assess tectonic activity. These studies demonstrate that satellite-derived DEM data can successfully identify active structures and variations in tectonic uplift.
Regional Significance
Research in Sulawesi and adjacent basins provides important insights into:
- plate interactions in Southeast Asia
- development of strike-slip fault systems
- basin formation and inversion
- crustal extension and exhumation
- sedimentary basin evolution and provenance
Because the region lies at the intersection of multiple tectonic plates, it provides a natural laboratory for understanding how continental fragments, oceanic crust and volcanic arcs interact during plate convergence.
Southeast Asia Research Group 