Abstract

Initially, the partly submerged southeastern outgrowth of Asia known as Sundaland was described as the most coherent shelf in the world with an "old central land mass" and more unstable marginal parts subjected to Tertiary tectonism. This led to the emergence of the Sunda Mountain System and an intervening complex of (oil) basins.

Brouwer's (1925) idea on the tectonic development of the region recognized the marked influence of the Alpine orogenic system. Subsequently, the outer zones were explained by Kuenen (1936) and Umbgrove (1947) on the basis of Vening Meinesz's crustal "downbuckling hypothesis" of the early thirties. Inherently, various ideas on Sumatra have develop. a.o. large nappe structures with a purported root zone in the offshore Riau islands and West Malaysia (Tobler-1917, Zwierzycki-1930, de Haan-1943, van Bemmelen-1949 and Osberger-1956) though not without criticism by Rutten (1927) and Klompe et al. (1957).

Of these, Bemmelen's (1948) "Undation Theory" stands out with the Anambas islands as a center of orogenic cycles from which "crustal waves" spread laterally.

This concept of several orogens in an arcuate pattern around the core of Sundaland was accepted without serious reservations until the development of plate tectonics in the late 1960's (Hatherton and Dickinson-1969, Fitch-1970, Hamilton-1970, 1973, and Katili-1971, 1973).

Katili's 1973 model utilizes the scheme of a double island arc with subduction zones moving systematically away from the core of Sundaland towards the Indian Ocean with lithospheric descent during Permian, Jurassic, Cretaceous, Miocene and Pliocene to present times.

More recent studies recognize the pre-Tertiary framework of Sundaland to consist of a complex mozaic of constantly moving fragments or microplates, initially accreted in the late Triassic. The zone of weakness between the rigid microplates in Sumatra was the locus of extensional tectonism, high heatflow and subsequent compression during the Tertiary back-arc basinal history which leads to optium conditions for the generation and trapping of Tertiary oils. (Cameron-1980, Eubank & Makki-1981, and Pulunggono & Cameron-1984).