Most of Sabah in northern Borneo is covered with Paleogene to Lower Miocene deep marine turbidite sequences that were deposited along the southern side of the Proto-South China Sea (PSCS). They include the Sapulut and Trusmadi formations of central-south Sabah, the Labang and Kulapis formations of eastern Sabah, the Kudat Formation of NW Sabah and the Crocker Formation of western Sabah. Sandstone petrography, heavy mineral analysis and detrital zircon U-Pb geochronology reveals changing sources associated with the evolution of the PSCS. Volcanic lithic fragments in some Labang Formation samples and Middle Eocene zircons in a lower Crocker Formation sample, as well as unstable heavy minerals such as apatite and epidote indicate input from contemporaneous volcanism, likely derived from the PSCS subduction arc to the north. By contrast, abundant ultra-stable heavy minerals and Mesozoic zircons indicate multi-recycling from southern sources. Changes in provenance are seen across key stratigraphies. The lower part of the Crocker Formation has similar provenance as the Rajang Group in Sarawak and is interpreted as a more distal equivalent. While the upper Crocker Formation has a similar provenance as the Nyalau Formation in Sarawak and is interpreted as its deeper marine continuation. Parts of the Labang and Kulapis formations suggest an extension of this depositional system into eastern Sabah. In the Early Miocene the Palawan microcontinental fragment collided with the Cagayan Arc, resulting in uplift of a forearc high and formation of melanges in eastern Sabah. The uplifted forearc was most likely the provenance source for the Temburong Formation in western Sabah.

The Sabah province of northern Borneo records a long-lived achieve of deep marine turbidite deposition ranging in age from the Mesozoic to the Early Miocene. Whilst the Mesozoic is poorly preserved, the Paleogene to Early Miocene deep water deposits form a relatively complete succession that was deposited along the southern margin of the Proto-South China Sea (PSCS). This comprises the Sapulut and Trusmadi Formations of central-south Sabah, the Labang and Kulapis Formations of eastern Sabah, the Kudat Formation of NW Sabah and the Crocker Formation of western Sabah. Sandstone petrography, heavy mineral analysis and detrital zircon U-Pb geochronology reveal the changing provenance sources associated with the evolution of the PSCS across key stratigraphies and can be used to reconstruct its subduction history. Most analysed formations contain abundant ultra-stable heavy minerals and Mesozoic zircons which indicate multi-recycling from southern sources. However, volcanic lithic fragments in some Labang Formation samples and Middle Eocene zircons in a lower Crocker Formation sample, as well as unstable heavy minerals such as apatite and epidote, indicate input from contemporaneous volcanism, likely derived from the PSCS subduction arc to the north. The thickest and most extensively exposed deep water sequence, the Crocker Formation in western Sabah, was deposited by two different drainage systems. The lower part of the Crocker Formation has a provenance similar to the Rajang Group in Sarawak and is interpreted as a more distal equivalent sourced by multi-recycling of Borneo and Malay Peninsula sources with some input from the Cagayan/PSCS arc. In contrast, the upper Crocker Formation has a provenance similar to the Nyalau Formation in Sarawak and is interpreted as its deeper marine continuation, sourced by sediments from the Sunda Shelf-Malay Peninsula transported in a drainage system which by-passed SW Borneo. The detrital mineralogy of parts of the Labang and Kulapis Formations suggest an extension of this Nyalau-Upper Crocker depositional system into eastern Sabah. In the Early Miocene the Palawan microcontinental fragment collided with the Cagayan Arc and finally closed the PSCS. The forearc was uplifted and mélanges preserved across eastern Sabah document this collision. The uplifted forearc was most likely the source of sediments in the Lower Miocene Temburong Formation in western Sabah, which marks the end of deep marine deposition in Sabah.

The Paleogene of Sabah is characterized by deepmarine sedimentation on the south side of the Proto-South China Sea (PSCS), which was subducted beneath the Cagayan Arc and North Sabah. With the onset of collision and elimination of the PSCS in the Early Miocene several melanges formed, including the Kuamut Formation. Zircons from this formation indicate magmatism associated with the PSCS subduction continued after c. 21Ma. The Temburong Formation in western Sabah indicates a continuation of magmatism until c. 19.6Ma. Deep water turbidite deposition and melange formation ended, marked by the Top Crocker Unconformity (TCU). The enigmatic Tajau Member of the Kudat Formation, likely deposited during and shortly after collision, suggests a northern, Palawan, source based on immature mineral assemblages and detrital zircon spectra. The post-TCU Neogene sedimentary rocks were deposited in fluvio-deltaic to shallow marine environments, after the Early Miocene Sabah Orogeny. This event uplifted, deformed and exposed pre-Neogene rocks. Sandstones are compositionally mature, have a mainly quartzose recycled orogenic source, and ultra-stable heavy mineral assemblages dominated by zircon, tourmaline and rutile (ZTR). The lower Neogene formations (Tanjong, Kalabakan) have substantial amounts of chrome spinel, while upper Neogene formations (Sandakan, Bongaya, Umas Umas) are almost devoid of chrome spinel and have higher contents of ZTR. This suggests a major unconformity between two Neogene sedimentation cycles. The Sikuati Member of the Kudat Formation has characteristics similar to the Tanjong Formation, suggesting it represents an equivalent to the first sedimentation cycle in north-western Sabah. Based on sandstone petrography, heavy mineral assemblages and detrital zircon ages, sediment sources were recycled Paleogene turbidite sequences with input of chrome spinel from uplifted peridotites for the first sedimentation cycle, and recycled Paleogene turbidite sequences and older Neogene fluvio-deltaic formations for the sedimentation cycle 2.

The Schwaner Mountains in southwestern Borneo form a large igneous province with a complex magmatic history and poorly known tectonic history. Previously it was known that Cretaceous granitoids intruded metamorphic rocks of the Pinoh Metamorphic Group assumed to be of Paleozoic age. Jurassic granitoids had been reported from the southern Schwaner Mountains. Most ages were based on K-Ar dating. We present new geochemistry, zircon U-Pb and 40 Ar/ 39 Ar age data from igneous and metamorphic rocks from the Schwaner Mountains to investigate their tectono-magmatic histories. We subdivide the Schwaner Mountains into three different zones which record rifting, subduction-related and post-collisional magmatism. The Northwest Schwaner Zone (NWSZ) is part of the West Borneo Block which in the Triassic was within the Sundaland margin. It records Triassic to Jurassic magmatism during early Paleo-Pacific subduction. In contrast, the North Schwaner Zone (NSZ) and South Schwaner Zone (SSZ) are part of the SW Borneo (Banda) Block that separated from NW Australia in the Jurassic. Jurassic granitoids in the SSZ are within-plate (A-type) granites interpreted to have formed during rifting. The SW Borneo (Banda) Block collided with eastern Sundaland at c. 135 Ma. Following this, large I-type granitoid plutons and arc volcanics formed in the NWSZ and NSZ between c. 90 and 132 Ma, associated with Cretaceous Paleo-Pacific subduction. The largest intrusion is the c. 110 to 120 Ma Sepauk Tonalite. After collision of the East Java-West Sulawesi (Argo) Block, subduction ceased and post-collisional magmatism produced the c. 78 to 85 Ma Sukadana Granite and the A-type 72 Ma Sangiyang Granite in the SSZ. Rocks of the Pinoh Metamorphic Group mainly exposed in the NSZ, previously assumed to represent Paleozoic basement, contain abundant Early Cretaceous (110 to 135 Ma) zircons. They are interpreted as volcaniclastic sediments that formed contemporaneously with subduction-related volcanic rocks of the NSZ subsequently metamorphosed during intrusion of Cretaceous granitoids. There are no igneous rocks older than Cretaceous in the NSZ and older than Jurassic in the SSZ and there is no evidence for a continuation of a Triassic volcanic arc crossing Borneo from Sundaland to the east.

Borneo occupies a central position in the Sundaland promontory of SE Asia. It has a complex Cenozoic geological history of sedimentation and deformation which began at about the same time that India is commonly suggested to have started to collide with Asia. Some tectonic reconstructions of east and SE Asia interpret a large SE Asian block with Borneo at its centre which has been rotated clockwise and displaced southwards along major strike–slip faults during the Cenozoic due to the indentation of Asia by India. However, the geological history of Borneo is not consistent with the island simply forming part of a large block extruded from Asia. The large clockwise rotations and displacements predicted by the indentor model for Borneo are incompatible with palaeomagnetic evidence and there is no evidence that the major strike–slip faults of the Asian mainland reach Borneo. Seismic tomography shows there is a deep high velocity anomaly in the lower mantle beneath SE Asia interpreted as subducted lithosphere but it can be explained just as well by alternative tectonic models as by the indentor model. Very great thicknesses of Cenozoic sediments are present in Borneo and circum-Borneo basins, and large amounts of sediment were transported to the Crocker turbidite fan of north Borneo from the Eocene to the Early Miocene, but all evidence indicates that these sediments were derived from local sources and not from distant sources in Asia elevated by India–Asia collision. The Cenozoic geological history of Borneo records subduction of the proto-South China Sea and Miocene collision after this ocean lithosphere was eliminated, and a variety of effects resulting from long-term subduction beneath SE Asia. There is little to indicate that India–Asia collision has influenced the Cenozoic geological record in Borneo.

Eocene–lower Miocene sandstones of the Crocker turbidite fan of north Borneo were derived from nearby Borneo and southeastern Asian sources, rather than distant Asian sources eroded after India-Eurasia collision. They are compositionally mature due to tropical weathering, but are mostly first-cycle sandstones derived from granitic rocks and subordinate metamorphic, sedimentary, and ophiolitic rocks. Detrital zircon ages range from Archean to Eocene, and the majority are Mesozoic. The most important source areas were Cretaceous granites of the Schwaner Mountains in southwest Borneo during the Eocene, and Permian–Triassic granites and Proterozoic basement of the Malay-Thai Tin Belt during the Oligocene.

Borneo, located in equatorial SE Asia, is the third largest island in the world, although it is topographically low. One of the unusual features of Borneo is the large amount of Cenozoic clastic sediments that have accumulated in several basins on and around the island. The ultimate source areas of the Cenozoic sequences have been suggested to be either mainland SE Asia/Indochina or Borneo itself. During the Paleogene until the earliest Miocene, deep marine turbidites were deposited in an accretionary wedge setting. Provenance studies on these Paleogene sediments based on detrital modes of sandstones and heavy mineral studies show that they have all been derived from a recycled orogenic source. Upper Cretaceous to Eocene sediments, the Sapulut and Trusmadi Formations, are compositionally mature (quartzose recycled), and may have been derived from mainland SE Asia/Indochina. During the Eocene there was an important change in sediment maturity and a provenance shift from quartzose to intermediate recycled sandstones. Heavy mineral studies show that the Eocene-Oligocene sediments of the Crocker Formation are mainly derived from granite, and that granite debris has been derived directly from its source or has been only slightly recycled/transported. The ultimate source area for these sediments was probably the Schwaner Mountains of southern Borneo, although a minor component of ophiolite debris suggests derivation from nearby basement of northern Borneo. The relative immaturity and heavy mineral suites of the Crocker Formation indicate it was derived from Borneo itself rather than SE Asia/Indochina, but primarily from basement sources rather than by recycling of older sediments. * SE Asia Research Group, Royal Holloway, University of London INTRODUCTION Borneo, the third largest island in the world with an area of almost 750,000 km, is located in equatorial SE Asia. It consists of the Indonesian state of Kalimantan, the Malaysian states Sabah and Sarawak, and the Sultanate of Brunei. Borneo has a low topography, and surface elevation rarely reaches more than 1000 m. Only in Sarawak and Sabah are there peaks of more than 2500 m, and the isolated granite of Mount Kinabalu (4095 m) in Sabah is the highest mountain in SE Asia. Borneo has a tropical wet climate, and until the mid 20th century most of Borneo was covered by thick lowland rainforest. Sabah lies within the wide plate boundary zone between the Eurasian, Pacific, Philippine, and Australasian plates, and is bounded by three marginal basins, the South China Sea, Celebes Sea and Sulu Sea (Figure 1). The South China Sea is relatively shallow in the west, deepening towards the east, and formed as a result of extension, including the creation of new seafloor in the east, within a large continental region. Extension has been active since at least the Early Cenozoic, and ocean floor formation began in the Oligocene and ended in the Middle Miocene. Subduction of the proto-South China Sea occurred along the western Sabah margin from at least Eocene times, and ceased in the Early Miocene. The Celebes Sea is a marginal basin, located between the Sulu arc and the north arm of Sulawesi. Magnetic anomalies from the Celebes Sea have been identified as Eocene in age. The Sulu Sea is a small (about 400 kilometres wide), NE-SW elongated and deep basin, and formed in the Middle Miocene in a back-arc setting. Borneo has a complex history of Cenozoic convergence, and is surrounded by several onshore and offshore sedimentary basins. Thicknesses of up to twelve kilometres of clastic sediments have been