Basin formation and Neogene sedimentation in a backarc setting, Halmahera, eastern Indonesia Gary J, Nichols Department of Geology, Royal Holloway and Bedford New College, University of London, Egham, Surrey TW20 0ET, UK and Robert Hall Department of Geological Sciences, University College London, Gower Street, London WCIE 6BT, UK Received27 October 1989; revised 15June 1990; accepted 6July 1990 It has been proposed that basins in backarc settings form in association with subduction by thinning of continental crust, by backarc spreading in oceanic crust, by compression (retro-arc basins) or by trapping of pieces of oceanic plate behind an arc. The Halmahera Basin in eastern Indonesia developed in a backarc setting but does not fall into any of these categories; it formed by subsidence of thickened crust made up of imbricated Mesozoic-Palaeogene arc and ophiolite rocks. Halmahera lies at the western edge of the Philippine Sea Plate in a complex zone of convergence between the Eurasian margin, the oceanic plates of the West Pacific and the Australian-Indian Plate to the south. The basement is an imbricated complex of Mesozoic to Palaeogene ophiolite, arc and arc-related rocks. During the Miocene this basement complex formed an area of thickened crust on which carbonate reef and reef-associated sediments were deposited. We interpret this shallow marine region of thickened crust to be similar to many of the oceanic plateaux and ridges found within the Philippine Sea Plate today. In the Late Miocene, convergence between the Philippine Sea Plate and the Eurasian margin resulted in the formation of the Halmahera Trench to the west of this region of thickened crust. Subduction of the Molucca Sea Plate at the trench caused the development of a volcanic island arc. Subsidence in the backarc area produced a broad sedimentary basin filled by clastics eroded from the arc and from uplifted basement and cover rocks. The basin was asymmetric with the thickest sedimentary fill on the western side, against the volcanic arc. The Halmahera Basin was modified in the Plio-Pleistocene by east-west compression as the Molucca Sea Plate was eliminated by subduction. Keywords: Halmahera, eastern Indonesia; backarc basin; basin formation Introduction Backarc marginal basins are thought to form typically by trench rollback causing backarc spreading (Dewey, 1980) and the generation of new crust (Karig, 1971). Basins in this setting may also form by the rifting of continental crust, for example the Sea of Japan and the Okinawa Trough (Kobayashi, 1985), or by the trapping of oceanic crust behind an arc (for example, the Bering Sea, where no backarc spreading has occurred (e.g. Scholl et al., 1986)). The crust underlying marginal basins is normally thicker than typical oceanic crust, but has an oceanic character (Mitchell and Reading, 1986). Retro-arc basins (Dickinson, 1974) form as compressive foreland basins in continental crust behind a continental margin volcanic arc. The Halmahera Arc in eastern Indonesia (Figure 1) is a volcanic arc resulting from the eastward subduction of the Molucca Sea Plate. Unlike many other arcs in the western Pacific, the Halmahera Arc is a relatively young feature which was initiated in the Late Miocene. It is therefore possible to document the evolution of an arc and backarc region without the complications of a long history of arc-trench tectonics. A thick sedimentary sequence accumulated in a sedimentary basin behind this volcanic arc during the Late Miocene 0264-8172/91/010050-12 ©1991 Butterworth-Heinemann Ltd and Pliocene and this sequence is now partly exposed on land as a result of Pliocene or younger uplift and deformation. We term this sedimentary basin the Halmahera Basin (Figure 2). The Halmahera Basin was and is situated in a backarc setting, but is not underlain by newly formed oceanic crust and does not show the characteristics of a retro-arc, foreland-type basin. It formed in the late Neogene in an area underlain by anomalously thick crust composed of ophiolitic and volcanic arc rocks and associated sedimentary rocks. The Halmahera Basin formed in such a position because the thickened crust influenced the Neogene development of subduction in this region (Hall and Nichols, 1990a). Although not previously described, backarc basins formed in this type of setting may not be unusual. The island of Halmahera is an excellent place to examine the entire evolution of a young, short-lived arc-trench system. In this paper we document the Neogene evolution of Halmahera and the adjacent offshore areas using new stratigraphic and structural data from the region. These data are integrated into the regional tectonic framework to illustrate the evolution of the Halmahera backarc basin. 50 Marine and Petroleum Geology, 1991, Vol 8, February