BACKGROUND

Previous Basement Sampling
Although it has been sampled in only a few locations, the basement of the OJP is the best sampled of any Pacific plateau with drill holes (Fig. 1) at Deep Sea Drilling Project (DSDP) Site 289 (9 m basement penetration) and ODP Sites 803 (26 m) and 807 (149 m). Unlike any other Pacific plateau, slivers of the southern edge of the OJP are exposed above sea level in the eastern Solomon Islands, principally on the islands of Santa Isabel and Malaita, where ~1.2- and 3.5-km-thick basement crustal sections have been sampled recently (Tejada et al., 1996; Parkinson et al., 1996; Petterson et al., 1997).

Physical Features and Gross Structure of the OJP
The OJP covers an area of more than 1.5 x 10⁶ km² (roughly the size of Alaska) and consists of two parts: the main or high plateau in the west and north and the eastern lobe or salient (Fig. 1). The plateau surface rises to depths of about 1700 m below sea level (mbsl) in the central region of the high plateau but lies generally at depths between 2000 and 3000 mbsl. The plateau is bounded by the Lyra Basin on the northwest, by the East Mariana Basin to the north, by the Nauru Basin to the northeast, and by the Ellice Basin to the southeast. The southern and southwestern boundaries of the OJP have collided with the Solomon Islands arc and now sit at the junction of the Pacific and Australian plates. Much of the plateau's surface is relatively smooth, although it is punctuated by several large seamounts. In many areas, the basement crust is covered with pelagic sediments >1 km thick. Physiography around the margins of the plateau is complicated. In the north and northeast, numerous horst and graben structures appear to predate much of the sediment cover (e.g., Kroenke, 1972; Berger et al., 1992). Faulting and deformation along the OJP's southern and southwestern margins are associated with the plateau's collision with the Solomon arc (e.g., Petterson et al., 1997). An extensive fold belt, the Malaita Anticlinorium, embraces the island of Malaita and the northern half of Santa Isabel.

Crustal thickness on much of the high plateau is considerable, even in comparison to other plateaus. Seismic and combined seismic and gravity evidence indicate crustal thickness is generally in the 30- 43 km range (e.g., Furumoto et al., 1976; Hussong et al., 1979; Miura et al., 1996; Richardson and Okal, 1996; Gladczenko et al., 1997). Over much of the high plateau, the depth to the top of Layer 3A is 10-16 km (Neal et al., 1997). Lower crustal seismic wave velocities suggest a granulite-grade gabbroic lower crust, whereas sub-Moho P-wave velocities of 8.4-8.6 km/s detected in the northwest and southwest portions of the plateau may indicate the presence of eclogite at depth (Saunders et al., 1996; Neal et al., 1997). The maximum extent of OJP related volcanism may reach well beyond the plateau proper, as the Early Cretaceous lavas filling the Nauru Basin, and similar lavas in the East Mariana and Pigafetta basins to the north, may be closely related to the OJP (e.g., Castillo et al., 1994; Gladczenko et al., 1997; Neal et al., 1997).

Tectonic Setting and Age of Emplacement
The original plate-tectonic setting of the OJP is open to some question because well-defined magnetic lineations do not appear to be present on the plateau. However, block-faulting structures along the eastern margin of the high plateau, interpreted as roughly north northeast-trending fracture zones, led to proposals that the OJP formed at a west-northwest trending ridge (Hussong et al., 1979) and possibly at a triple junction (Winterer, 1976; Hilde et al., 1977). Preliminary isotopic study of OJP basement lavas suggested a hot-spot connection and that the plateau may have formed at a ridge-centered or near-ridge hot spot (Mahoney, 1987). Subsequent major and trace element data were found to be consistent with plateau formation on thin lithosphere by large fractions of partial melting of a hot-spot-type source (Mahoney et al., 1993; Tejada et al., 1996; Neal et al., 1997). From bathymetry and satellite-derived gravity fabric, Winterer and Nakanishi (1995) inferred that a north-northeast-trending spreading axis ran through the OJP, whereas Neal et al. (1997) argued that the north-northeast-trending fabric represents fracture-zone orientation. M- series magnetic lineations adjacent to the plateau in the Nauru and Lyra basins run east-northeast to west-southwest. Coffin and Gahagan (1995) reviewed the available geophysical evidence and concluded that it weakly favors emplacement of most of the OJP in an off ridge location.

Richards et al. (1991), Tarduno et al. (1991), and Mahoney and Spencer (1991) all favored the starting plume head of the Louisville hot spot (now at ~50°S) as the source of the OJP, but a recent plate reconstruction suggests the plateau was formed well to the north of this hot spot's current location (Neal et al., 1997). As noted above, the plume-head model predicts that plateaus are emplaced in single massive eruptive events of short duration. Surprisingly, however, 40Ar-39Ar ages of OJP lavas in the Solomon Islands and at existing drill sites reveal a sharply bimodal distribution (Fig. 2), with ages of 122 ± 3 Ma and 90 ± 4 Ma (total ranges); thus, it is possible that most of the plateau may have formed in two relatively brief episodes (Mahoney et al., 1993; Tejada et al., 1996; Parkinson et al., 1996). Because sampling over the plateau's huge area has been very limited, the relative importance of these two episodes is unclear. However, Tejada et al. (1996) argued that the 122-Ma event was significantly larger than the 90-Ma event, which they hypothesized to have been largely focused on the eastern salient. Alternatively, further sampling may show that eruptions actually occurred over a span of 30 m.y. or more (e.g., Tejada et al., 1996; Ito and Clift, 1998).

Between 124 and 100 Ma, the OJP appears to have been positioned close to the Pacific-plate Euler pole, so that the plateau would have moved little relative to the inferred hot spot source (Neal et al., 1997). At ~100 Ma, plate motion changed from a northwestward to a more northward trajectory, and from that time until about 85 Ma the OJP moved northward, such that at ~90 Ma the southeastern corner of the plateau may have been situated rather close to the 120-Ma position of the central high plateau. Following the 90-Ma eruptive episode, postemplacement rifting and seafloor spreading may have occurred for up to several million years within the OJP's eastern salient, in conjunction with spreading in the Ellice Basin to the east (Neal et al., 1997).

After a long period of northward and northwestward motion, the OJP collided with the old Solomon arc during the early Neogene, initially in a diachronous "soft docking" without significant deformation. Following a reversal of subduction direction, the intense deformation of the Malaita Anticlinorium occurred in the late Miocene through Pliocene (Petterson et al., 1997). The bulk of the plateau appears to be more or less unsubductible (Cloos, 1993; Abbott and Mooney, 1995), but the post-Miocene removal of a portion of the lower OJP between Santa Isabel and San Cristobal (Makira) is evident from recent seismic surveys (Mann et al., 1996).

Results from Previous Sampling of Igneous Basement
OJP basement at all previously drilled sites, and in the islands of Malaita and Santa Isabel, consists of pillowed or massive flows of basalt averaging ~9 m in thickness. Dikes are rare in the island exposures; hence, the eruptive vents for most of the lavas may be rather distant. All of the basalts appear to have been emplaced well below sea level and are overlain by bathyal pelagic marine sediments (Neal et al., 1997, and references therein). However, all of the sites except Site 289 are located at the margins of the plateau; it is possible that the shallow central regions of the high plateau and eastern lobe were originally shallow or even subaerial. Basement of the 122-Ma age group comprises lavas from Sites 289 and 807, Malaita, and part of Santa Isabel, whereas the 90- Ma lavas are found at Site 803 and in Santa Isabel (Fig. 2; Mahoney et al., 1993; Tejada et al., 1996; Parkinson et al., 1996). Also, volcanic ash layers of Cenomanian to Coniacian age (i.e., in roughly the ~95- to 87-Ma range) are present at DSDP Site 288 (which did not reach basement) on the western end of the plateau's eastern salient (Andrews, Packham, et al., 1975). At Site 289, several late Aptian ash layers lie above basement (Andrews, Packham, et al., 1975) and may indicate fairly prolonged volcanism in some areas following eruptions at 122 Ma (early Aptian).

The basalts at all sites are unmetamorphosed, moderately evolved (Fig. 3), low-K tholeiites with relatively flat primitive-mantle-normalized incompatible element patterns (intermediate between those of normal mid-ocean ridge basalts and most oceanic island or continental tholeiites; Fig. 4) and a narrow range of ocean-island-like Nd-Sr-Pb isotopic ratios (Fig. 5). Major and trace element modeling indicates the lavas represent high-degree partial melts (Mahoney et al., 1993; Tejada et al., 1996; Neal et al., 1997). Two geochemically and stratigraphically distinct groups of lavas are apparent in the basement section at Site 807 and in the much thicker section on Malaita. The upper 46 m of lavas at Site 807 (Unit A) are isotopically and chemically very similar to the upper 750 m of lavas in central Malaita, termed the Singgalo Formation. The lower basalt units at Site 807 (Units C-G) and the single flow encountered at Site 289 resemble the lower 2.7 km of the volcanic pile on Malaita, termed the Kwaimbaita Formation (Tejada, 1998). The 90-Ma lavas of Site 803 and Santa Isabel are isotopically similar to the stratigraphically lower 122-Ma Kwaimbaita and Units C-G basalts (Mahoney et al., 1993; Tejada et al., 1996). Thus, an isotopically ocean-island-like mantle source containing (at least) two distinct components was important at the northern and southern margins of the plateau, and the source of the 90 Ma lavas was similar to the source of the stratigraphically lower lavas erupted at ~122 Ma.