Journal of Southeast Asian Earth Sciences, Vol. 6, No. 3/4, pp. 249-258, 1991 0743-9547/91 $3.00 + 0.00 Printed in Great Britain PergamonPressLtd The character and significance of basement rocks of the southern Molucca Sea region ROBERT HALL,* GARY NICHOLS,t PAUL BALLANTYNE,* TIM CHARLTON* a n d JASON ALI~ *Department of Geological Sciences, University College London, Gower Street, London, WCIE 6BT, tDepartment of Geology, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, and ~Department of Oceanography,University of Southampton,UniversityRoad, Southampton,SO9 5NH, U.K. (Receit, ed 22 August 1990; accepted for publication 5 May 1991) Abstract--Pre-Neogene basement rocks in the southern Molucca Sea region include ophiolitic rocks, arc volcanic rocks and continental rocks. The ophiolitic complexes are associated with arc and forearc igneous and sedimentary rocks. They are interpreted as the oldest parts of the Philippine Sea Plate with equivalents in the ridges and plateaux of the northern Philippine Sea. In the Molucca Sea region igneous components include rocks with a "supra-subduction zone" character, boninitic volcanic rocks and basic volcanic rocks with a "within-plate" character; "MORB-type" rocks are rare or absent. The ophiolitic rocks are overlain by Upper Cretaceous and Eocene sedimentary and volcanic rocks. Plutonic rocks of island arc origin which intrude the ophiolites yield Late Cretaceous radiometric ages and amphibolites with ophiolitic protoliths yield Eocene ages. The "supra-subduction zone" ophiolites are speculated to have originated during a mid-Cretaceous plate reorganization event. For the Late Cretaceous and Eocene the present-day Marianas arc and forearc provides an attractive model. Volcanic rocks form the basement of Morotai, western Halmahera and much of Bacan. These also have an island arc character and are probably of Late Cretaceous-Paleogene age. Both the arc volcanic rocks and the ophiolitic complexes are overlain by shallow water Eocene limestones and an Oligocene rift sequence including basaltic pillow lavas and volcaniclastic turbidites. The distribution of the Eocene~)ligocene sequences indicate pre-Mid/Late Eocene amalgamation of the ophiolitic and arc terranes. Mid Eocene-Oligocene extension appears to be synchronous with opening of the central West Philippine Basin. Continental crust probably arrived in this region in the Late Paleogene-Early Neogene, either due to collision of the Australian margin with Pacific arc-ophiolite terranes or by terrane movement along the Sorong Fault Zone. INTRODUCTION THE MOLUCCA SEA (Fig. 1) is one of several small ocean basins in the western Pacific margin and is situated in the tectonically complex triple junction of the Australian, Eurasian and Philippine Sea plates. The age of the Molucca Sea crust is unknown. To the east and west are the active Sangihe and Halmahera volcanic arcs, to the north is the Philippines archipelago, and to the south are numerous small, non-volcanic islands extending between the larger islands of Sulawesi and New Guinea. The Molucca Sea will soon disappear; the Sangihe and Halmahera arcs are colliding and have over-ridden the Molucca Sea Plate (Hatherton and Dickinson 1969, Hamilton 1979). Collision has created a central ridge which is being thrust outwards onto the two colliding forearcs. The central zone, marked by intense shallow seismicity and a gravity low, is the Molucca Sea "col- lision complex" (Silver and Moore 1978, Hamilton 1979) and is exposed on the island of Talaud (Moore et aL 1981). The connection northwards between the collision zone and the southern Philippines is uncertain (e.g. Moore and Silver 1983, Hall and Nichols 1990, Pubellier et al. 1991). East of the Molucca Sea the Philippine Sea Plate is rotating clockwise with respect to Eurasia about an Euler pole near its northern edge (Ranken et aL 1984, Seno et al. 1987, Barrier et al. 1991) and the rate of convergence between Eurasia and the Philippine Sea Plate increases southward. North of 2°50'N the principal expression of this convergence is westward subduction at the Philippine Trench; the trench is very young (Hamilton 1979, Cardwell et al. 1980) with less than 150 km of subducted lithosphere. South of 2°50'N an accretionary prism of deformed sediments becomes a less pronounced feature and dies out, together with bathy- metric expression of the trench and seismic activity, at I°20'N (Nichols et al. 1990). There is no physiographic evidence or seismicity indicating a link between the Philippine Trench and plate boundaries to the southeast, and the southern end of the Philippine Trench is inter- preted by Nichols et al. (1990) to be linked to the Molucca Sea Collision Zone via a NE-SW dextral strike-slip zone. The area between eastern Halmahera and Waigeo therefore forms part of the Philippine Sea Plate. South of the Molucca Sea is a complex fault zone which broadly separates the Australian Plate from the Eurasian and Philippine Sea Plates. In the northern Bird's Head region of New Guinea (Visser and Hermes 1962) the northern boundary of the Australian Plate is the left-lateral Sorong Fault (Tija 1973, Dow and Sukamto 1984). West of the Bird's Head, this fault splays out into the Molucca-Sorong Fault, the North Sula-Sorong Fault (Hamilton 1979) and the Buru Fracture (Tjokrosapoetro and Budhitrisna 1982). Linea- ments with broadly E-W orientations, interpreted as left-lateral splays, can be identified on bathymetric maps of the region (Mammerickx et al. 1976), observed on seismic lines (Letouzey et al. 1983) and are seen on GLORIA records crossing the fault zone (Masson 1988). We use the term Sorong Fault Zone for the entire zone of faulting between the Bird's Head and east Sulawesi. 249