Several authors (e.g., Richards et al., 1991; Tarduno et al., 1991; Mahoney and Spencer, 1991) favor the starting-plume head of the Louisville hotspot (now at ~52°S) as the source of the OJP. Kroenke et al. (2004) used a new model of Pacific absolute plate motion, based on the fixed-hotspot frame of reference, to track the paleogeographic positions of the OJP from its present location on the equator back to 43°S at the time of its formation (~120 Ma). This inferred original position is 9° north of the present location of the Louisville hotspot and suggests that this hotspot was not responsible for the formation of the OJP or, alternatively, that the hotspot has drifted significantly relative to the Earth's spin axis (as the Hawaiian hotspot appears to have done; e.g., Tarduno et al., 2003). Kroenke et al. (2004) also note the presence of linear gravity highs in the western OJP, which they speculate may indicate formation of the OJP close to a recently abandoned spreading center. Antretter et al. (2004) point out that the paleomagnetic paleolatitude of the OJP (~25°S) determined by Riisager et al. (2003a, 2004) further increases the discrepancy with the location of the Louisville hotspot. Zhao et al.'s (2004) investigation of the rock magnetic properties of basalt from the OJP shows that original and stable magnetic directions are preserved, allowing robust estimates of paleolatitude. The discrepancy between the paleolatitudes calculated from the paleomagnetic data and from the fixed-hotspot reference frame is interpreted by Riisager et al. (2003a, 2004) as evidence for movement between hotspots. Antretter et al. (2004) show that the Louisville hotspot may have moved southward over the past 120 m.y. and that taking into account both hotspot motion and true polar wander reduces the discrepancy and makes the formation of the OJP by the Louisville hotspot barely possible, if still unlikely.
The determination of high-quality paleolatitudes is a major achievement of Leg 192, but these are not the only results to come out of paleomagnetic studies on the basaltic basement. Riisager et al. (2003b) note that unaltered basaltic pillow-rim glasses preserve a record of geomagnetic paleointensity at the time of their formation. These authors carried out Thellier experiments on basaltic glass recovered during Leg 192 in order to determine the intensity of the Early Cretaceous magnetic field. Hall et al. (this volume) found strong anisotropy of magnetic susceptibility in basalt sampled close to the boundaries between eruptive units. They interpret this to be a result of flow during emplacement and use the azimuths of the maximum anisotropy axis to infer lava flow directions. These azimuths are consistent within each site but differ from site to site.
The thickest exposures of the OJP basement rocks in the Solomon Islands are found on the remote island of Malaita (Fig. F1). Petterson (2004) presented the results of geological surveys that reveal a monotonous succession of Early Cretaceous tholeiitic pillow basalt, sheet flows, and sills (the Malaita Volcanic Group) 3–4 km thick. Rare and very thin interbeds composed of laminated pelagic chert or limestone suggest high eruption frequency and emplacement into deep water. The Malaita Volcanic Group is conformably overlain by a 1- to 2-km-thick Cretaceous–Pliocene pelagic sedimentary cover sequence, punctuated by alkaline basalt volcanism during the Eocene and by intrusion of alnöite during the Oligocene.