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CONTENTS

Geographically the Indonesian archipelago is often divided into eastern and western parts, the boundary between them being placed in the Makassar Strait and passing from there south through the Lombok Strait. Wallace's line, defined in 1863 and 1910, divided the Indonesian region into two on the basis of differences of flora and fauna (Fig. 1). Nishimura et al. (1981) proposed that a major tectonic discontinuity separates Eastern Indonesia from Western Indonesia between Sumbawa and Flore, this view being based on investigations of the differences in geophysical, geochemical and submarine morphological features. In this paper a new subdivision is proposed, introducing a Central Indonesia Province which represents a transition between the largely Eurasian elements of Western Indonesia and the Pacific and Australasian related elements of Eastern Indonesia (Fig. 2). The boundary between the Western and Central Indonesian regions is thought to mark the location of a pre-Tertiary subduction zone at the southeastern Eurasian margin, whilst the boundary between the Central and Eastern Indonesian regions is at the location of a Paleogene subduction complex accreted to this margin. The latter is marked by the Selayar-Bonerate ridge, which separates the Flores Sea from the Banda Sea, the two areas being apparently different in their tectonic development. The tectonic development of Western Indonesia has been largely controlled by the development of the Indian Ocean following the break up of the Gondwana supercontinent and the consequent northward drift of the Indian continental mass. The heart of the region is formed by the Sundaland craton which is Late Mesozoic. The present Central Indonesian region is situated in a back-arc tectonic setting. However, from the Cretaceous to the Eocene this area was the site of complex subduction, fore arcs and magmatic arcs. Central Indonesia now also represents the transition from a largely continental province to the more oceanic Eastern Indonesia Province. Its tectonic development has been determined by eastward accretion at Cretaceous and Paleogene subduction zones and subsequently by the opening of the Makassar Strait. Most recently the structures have been modified as a result of the propagating collision of the Australian continent with the Banda Arc.