The region around the sites of Leg 180 drilling includes the Papuan Peninsula, D'Entrecasteaux Islands, and the conjugate rifted margins and islands of the Woodlark and Pocklington Rises (Fig. F3). As reviewed in Davies et al. (1984) and Taylor (1999), the tectonic history of interest begins in the Late Cretaceous with a passive margin and ocean bordering northeast Australia. Northward subduction within the oceanic lithosphere developed an island arc and pulled continental fragments, including the Papuan Plateau, from Australia by opening the Coral Sea Basin (62-52 Ma) (Weissel and Watts, 1979; Rogerson et al., 1993). Former Australian margin sediments (now Owen Stanley Metamorphics) were accreted at the trench until the Papuan Plateau was partially subducted and an arc-continent collision ensued (Davies and Jaques, 1984). This Paleogene orogeny formed the core of the paleo-Papuan Peninsula, much of which today is 1-3 km above sea level and is underlain by a crust 30-45 km thick (Ferris et al., 2000).
Southward (reversed direction) subduction along the Trobriand Trough produced arc magmatism from the early Miocene (possibly late Oligocene) through the Holocene (Davies and Smith, 1971; Davies et al., 1984; Hegner and Smith, 1992; Stolz et al., 1993). The modern volcanic front extends from Mt. Lamington (which erupted in 1951) through Mt. Victory to Fergusson Island and in the Pliocene continued to the Amphlett Islands and Egum Atoll (4.4- to 3.5- and 2.9-Ma andesite, respectively) (Smith and Milsom, 1984). Numerous Miocene-Holocene andesitic and lesser shoshonitic volcanic centers occur behind the volcanic front (Fig. F3). Trenchward, there is a forearc basin with depocenters up to 5-7 km thick bounded by an outer forearc basement high, capped by the Lusancay-Trobriand-Woodlark Islands (Tjhin, 1976; Pinchin and Bembrick, 1985; Francis et al., 1987).
Metamorphic core complexes developed along (D'Entrecasteaux Islands) or just behind (Misima, Suckling-Dayman massif, and Emo metamorphics) the volcanic front in the latest Miocene to Holocene (Davies, 1980; Davies and Warren, 1988, 1992; Hill et al., 1992, 1995; Baldwin et al., 1993; Hill and Baldwin, 1993; Lister and Baldwin, 1993; Hill, 1994; Martinez et al., 2001). Core complex formation accompanied continental rifting, peralkaline rhyolite volcanism (Smith, 1976), and westward propagation of seafloor spreading since at least 6 Ma that formed the oceanic Woodlark Basin (Weissel et al., 1982; Taylor and Exon, 1987; Taylor et al., 1995, 1999). West of 153°E, spreading split the formerly contiguous Woodlark and Pocklington Rises approximately along the volcanic line, producing inherently asymmetric conjugate passive margins, with a Neogene forearc to the north and a Paleogene collision complex to the south. East of 153°E, the Neogene volcanic arc terminates and the boundary between the Woodlark Rise and the Solomon Sea is a transform margin (Figs. F1, F3). Thus, the (eastern) Woodlark Basin, where spreading initiated, did not originate as a backarc basin (Weissel et al., 1982).