Regional Geology and Tectonis Of Papua

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).

BASEMENT AGE AND COMPOSITION

Drilling at four Leg 180 sites penetrated basement (Fig. F1) and recovered a suite of dolerite (Sites 1109, 1114, and 1118) and gabbro (Site 1117) plus basalt in conglomerates (also at Site 1115). Metadolerites were also recovered as pebbles in talus at Sites 1110-1112. As the contacts with the basement were either unconformable (Sites 1109 and 1118) or faulted (Sites 1114 and 1117), the age and nature of the intrusives were uncertain; they could be related to the Papuan ophiolite, Miocene forearc basin, or late Miocene rifting (Shipboard Scientific Party, 1999).

Ion microprobe analyses of zircons in the Site 1117 gabbro gave a ²³⁸U/²⁰⁶Pb age of 66.4 ± 1.5 Ma (Monteleone et al., this volume), whereas ⁴⁰Ar/³⁹Ar plagioclase apparent ages of basement samples from Sites 1109, 1117, and 1118 varied considerably with the extent of sample alteration (Monteleone et al.; Brooks and Tegner, both this volume). Some of the least-altered dolerites yielded ⁴⁰Ar/³⁹Ar plagioclase isochron ages of 58.9 ± 5.8 and 54 ± 1 Ma and plateau ages of 58.2 ± 1.0 and 55.6 ± 0.3 Ma (Monteleone et al.; Brooks and Tegner, both this volume). Variable alteration and associated Ar loss resulted in a suite of younger plagioclase apparent ages from 54 to 31 Ma.

Monteleone et al. (this volume) infer from these dates that the gabbro crystallized in the late Maastrichtian and, together with the dolerites, cooled into the Paleocene and was partially altered through the early Oligocene. They were not thermally reset by subsequent rifting events and so must have remained at shallow and cool (<250°C) levels in the crust (Monteleone et al., this volume).

Brooks and Tegner (this volume) conducted inductively coupled plasma-mass spectrometer (ICP-MS) analyses of dolerites from Sites 1109 and 1118, as well as of Papuan Ultramafic Belt (PUB) (Jaques and Chappell, 1980) and Woodlark Island (Luluai volcanics) (Ashley and Flood, 1981) basalts. From the conservative trace elements least mobile during alteration, they conclude that the dolerites from Sites 1109 and 1118 consist of material derived by partial melting of enriched mantle. The dolerites are geochemically similar to the Woodlark Island basement (and Ontong Java Plateau) but are unlike the basalts of the PUB, which came from a source similar to that of normal mid-ocean-ridge basalt (N-MORB).

The initial interpretation of multichannel seismic (MCS) data in the vicinity of Sites 1118, 1109, and 1115 needs correcting in light of the new dates of basement. The Shipboard Scientific Party (1999) inferred that the well-layered reflectors dipping ~10°N beneath the late Miocene-Quaternary sediments on this part of the Woodlark Rise represented Miocene forearc basin sediments similar to those imaged and drilled north of the D'Entrecasteaux Islands (Tjhin, 1976; Francis et al., 1987). Although such sediments were intersected at Site 1115, reinterpretation of the regional grid of MCS and gravity data (Goodliffe et al., 1999) shows that Site 1115 occurs near the eastern edge of the forearc basin (Fig. F3). Less than 500 m of Miocene sediments occurs beneath Site 1115 (Goodliffe et al., this volume), whereas the forearc basin is nearly 5 km thick only 15 km farther west (Fang, 2000).

The implication is that the >2.5 s two-way traveltime-thick prerift section of subparallel reflectors between Sites 1109 and 1115 represents >5 km of basement. Given the reflector geometry and the enriched mid-ocean-ridge basalt (E-MORB) character of the dolerites at Sites 1109 and 1118, this is apparently an extensive sill complex, possibly capped by basalts updip to the north of Site 1109. The alternative interpretation, that Sites 1109 and 1118 (and Site 1114) happened to intersect dikes within an otherwise thick lava sequence, is not supported by the lesser proportion of basalt vs. dolerite pebbles in the conglomerates derived from the erosion of basement. Although it is possible that the sequence has been thickened by thrust repetition, the simplest interpretation is that a thick igneous province was regionally tilted up on its southern edge, probably during the Paleogene orogeny, and subsequently extensively eroded. The presence of gabbro at ~1200 mbsl at Site 1117 in the same structural block of Moresby Seamount as dolerite at ~700 mbsl at Site 1114 is consistent with this interpretation.

It is instructive to compare what is now known about the ages and composition of basement in the Papuan region onshore and offshore. The PUB ocean tholeiites-gabbros-hartzburgites locally have a cover of Maastrichtian micrites (Davies, 1980). This age is correlative with the zircon age of crystallization for the Site 1117 gabbro. In other areas the foraminifer-bearing micrites overlying the PUB basalts are late Paleocene (P4 = 59-56 Ma) (Rogerson et al., 1993) in age. Other Paleocene ages include the cooling ages from Leg 180 dolerites and gabbros, as well as unpublished ⁴⁰Ar/³⁹Ar hornblende ages from PUB pegmatitic gabbros of 55.5 and 58.4 Ma (R. Duncan, pers. comm., 2001). K/Ar and ⁴⁰Ar/³⁹Ar hornblende ages of 66-56 Ma from granulites at the sole of the PUB in the Musa-Kumusi divide have been interpreted to date the PUB emplacement (Lus et al., 1998). Middle Eocene stratigraphic ages suggest that the large province of ocean tholeiites covering the southern part of the Papuan Peninsula (Milne Basic Complex and Sadowa Gabbro) (Smith, 1982) are younger than the PUB. The stratigraphically pre-late Oligocene Luluai volcanics on Woodlark Island have been considered to be equivalent to the Milne Basic Complex, but there are no radiometric ages to support this interpretation (Ashley and Flood, 1981; Davies et al., 1984).

Volcanics of island arc affinity also form part of the regional basement. Tholeiitic andesites and boninitic lavas from the Dabi Volcanics on Cape Vogel have ⁴⁰Ar/³⁹Ar total fusion and plateau ages of 58.9 ± 1 Ma, as well as 53.7 ± 1 Ma (Walker and McDougall, 1982). The Lokanu volcanics, intersected at the base of the Nubiam 1 well, are late Paleocene (Francis et al., 1987). Tonalite-diorite-dacite intrusions through the PUB are K/Ar dated at 57-47 Ma (Rogerson et al., 1993).

Thus, the upper crust of the Papuan Peninsula, D'Entrecasteaux Islands, and offshore regions of the Woodlark and Pocklington Rises is composed of a variety of basement types (dominantly ophiolitic N-MORB and E-MORB, but also arc tholeiites/boninites and dacites/tonalites) and ages (late Maastrichtian, Paleocene, and middle Eocene). The relations between many of its component parts remain unknown in detail. Furthermore, Archean zircons in Pliocene-Pleistocene conglomerates sourced from core complexes on Goodenough Island attest to the presence of Archean continental components in the lower crust (Baldwin and Ireland, 1995). These may derive from subducted parts of Australian continental fragments such as the Papuan Plateau.

This amalgam of oceanic, arc, and continental basement terranes superimposed by Neogene arc magmatism controls the rheology of the orogenic continent that is rifted in the late Miocene-Holocene. For example, Martinez et al. (2001) show that the prevalence of upper crustal ophiolites creates a density inversion capable of driving vertical extrusion of ductile lower crust in metamorphic core complexes such as that occurring in the D'Entrecasteaux Islands.