Origin and History of the South China Sea Basin

Abstract Reconstructions of SE Asia at 5 Ma intervals for the past 50 Ma are presented. They are constrained by new data from the Philippine Sea plate, which forms the eastern boundary of the region, by recent interpretations of the South China Sea and Eurasian continental margin, forming the western boundary, and by the known motions of the Indian-Australian plate to the south. An attempt is made to satisfy geological and palaeomagnetic data from the region. The implications of these reconstructions for the Tertiary evolution of SE Asia are discussed in the light of other new data from the region. There are two regionally important periods of change during the past 50 Ma. Both appear to be the expression of arc-continent collision and resulted in major changes in the configuration of the region and in the character of plate boundaries. At c. 25 Ma the collision of the Australian continent with the Philippine Sea plate arc caused major effects which propagated westwards through the region. At c. 5 Ma collision of the Philippine arc and the Eurasian continental margin occurred in Taiwan. This appears to be a key to the recent tectonics of the region. Principal features of the model include the following interpretations. Middle Tertiary counter-clockwise rotation of Borneo closed a large proto-South China Sea and led to the development and destruction of marginal basins north of the Celebes Sea. The rotation implies that much of the north Borneo margin was not a subduction, but a strike-slip boundary for most of this period. It also suggests that the central West Philippine Sea, the Celebes Sea and the Makassar Strait formed part of a single marginal basin which opened between late Eocene and mid Oligocene, and narrowed westwards like the present South China Sea. Luzon is suggested to have formed in an arc on the north side of the Celebes Sea-West Philippine Basin, whereas most of the other Philippine islands probably formed part of an arc at the southern edge of the Philippine Sea Plate before the Early Miocene. Arc-continent collision in the early Miocene caused plate boundaries to change and initiated the clockwise rotation of the Philippine Sea plate. Since then the Philippine fragments have moved in a very narrow zone, mainly as part of the Philippine Sea plate, with significant strike-slip motion of fragments at the plate margin. Most subduction under the Philippines was oblique, mainly at the western edge, and north of Mindanao. The Molucca Sea was a very wide area which formed part of the Philippine Sea plate before c. 15 Ma and originated as trapped Indian ocean lithosphere. It has been eliminated by subduction on its east and west sides. The present-day double subduction system never extended north of the present Molucca Sea into the Philippines. The Sulawesi ophiolite has an Indian ocean origin and was emplaced on the west Sulawesi continental margin at the end of the Oligocene. The major change in plate boundaries at the beginning of the Miocene following arc-continent collision of the Australian margin with the Philippine Sea plate arc caused initiation of the Sorong Fault system and led to westward movement of continental fragments which were accreted to Sulawesi during the late Neogene. The Sula platform and Tukang Besi platform formed part of a single large microcontinent with the Bird’s Head before c. 15 Ma. They moved to their present positions after slicing of fragments from this microcontinent at different times and each was attached to the Philippine Sea plate for a few million years before collision. Most of the Banda Sea is interpreted to have an extensional origin and to have opened during the late Neogene. The reconstructions imply that there has been little convergence at the north Australian margin in Irian Jaya since the early Miocene and most convergence has occurred during the last c. 5 Ma. Movement of Philippine Sea arc fragments within the northern New Guinea margin along strikeslip zones probably accounts for the terrane character of this orogenic belt.

Reconstructing Cenozoic SE Asia

Cenozoic plate reconstruction of Southeast Asia

Paleomagnetic data from the African plate from the Early Jurassic onward are critically re-evaluated and updated, resulting in a set of 30 paleopoles, from which an apparent polar wander path is derived using 20 Ma intervals. This path is then used, in conjunction with kinematic models of the Indian and Atlantic oceans, to place other continents bordering the Indian Ocean (Eurasia, India), islands and plateaus (Madagascar, Seychelles-Mascarene), and blocks involved in the collision of India with Eurasia (south Tibet, Indochina) in an “absolute” paleogeographic frame fixed with respect to Africa. Paleogeographic maps are presented for the Early Jurassic (pre-breakup), the Jurassic-Cretaceous boundary (anomaly Ml5), the Early Cretaceous (M2), the Late Cretaceous (A 34), the Cretaceous-Tertiary boundary (A 29), the Early Eocene (A 24), the middle Eocene (A 21) and the Oligocene-Miocene boundary (A 6). Observed and predicted paleomagnetic directions for landmasses other than Africa (i.e., for western Europe, China, Korea, India, and Australia, which are not used in deriving the maps) are found to be in very satisfactory agreement. The quality of reconstructions, as measured by rms discrepancies between observed and predicted paleolatitudes, is of the order of 5°. The main features of the paleogeographic maps are discussed, with the aim of relating oceanic and on-land geodynamic events. The constraints provided by Tibetan data on the India-Asia collision, and the 20 Ma reorganization of geology and plate boundaries in a band extending from the south China Sea, through Tibet, to the Red Sea system are discussed in more detail. We emphasize the episodic nature of the breakup of Gondwanaland and formation of the Indian Ocean, and subsequent interaction with Eurasia, in which six major discrete events are identified, separated by ∼30 Ma phases of rather uniform plate motion.

Paleogeographic maps of the continents bordering the Indian Ocean since the Early Jurassic

Late Cretaceous-Early Palaeogene tectonic development of SE Asia

Potassium-argon ages of biotite and hornblende from sixteen samples of mainly granitic plutonic rocks from West Kalimantan indicate a complex history of magmatism and cooling from the Middle Jurassic to late Cretaceous, with most ages in the range 75 to 112 Ma. Samples from the Schwaner Zone in the northern part of the area give measured ages ranging from 76 to 116 Ma. In the southern region most samples are in the range 79 to 86 Ma, but two are older (127 Ma, early Cretaceous, and 153 Ma, Middle Jurassic). In both areas, concordant ages from biotite and hornblende indicate a well-defined event at about 80 to 85 Ma, which has also been recognized to the north in Sarawak, on islands of the South China Sea, in the Malay Peninsula, south Sumatra, and in the Java Sea. Palaeomagnetic measurements on thirty-nine samples of dyke rocks, lavas, tuffs, granites, hornfels and metatuff, all believed to be late Cretaceous, yield a mean palaeomagnetic pole at 21°E 41°N, and a palaeolatitude for West Kalimantan of 0°. The palaeomagnetic pole does not differ significantly from the Cretaceous pole estimated for the Malay Peninsula. It is concluded that since the Middle Cretaceous West Kalimantan and the Malay Peninsula have behaved as a unit, have remained in much their present latitude, but have rotated anticlockwise about 50 .

I. McDOUGALL

Papers

Palaeomagnetic data and radiometric ages from the Cretaceous of West Kalimantan (Borneo), and their significance in interpreting regional structure