LEG 130

Ontong Java Plateau

The Ontong Java Plateau formed prior to 113 Ma along a west-northwest spreading ridge and, during its northward shift, has accumulated over 1000 m of pelagic sediments. In the late Oligocene, the southwest part of the plateau encountered the Outer Melanesian subduction zone, resulting in the intrusion of dikes and sills along the outer trench rise and ending subduction of the Pacific Plate beneath the arc. Subduction ceased at ~ 25 Ma, when the convergent plate boundary shifted, and resumed at ~ 10 Ma south of the Solomon Islands. Eastward subduction of the Indo- Australia Plate beneath the Pacific Plate caused collision between the Woodlark Spreading Ridge and Solomon Island Arc at ~ 4 Ma, elevating and folding the plateau.

During Leg 130, a depth transect of five sites (Site 803 to Site 807) was drilled to investigate the global ocean dynamics and climate during the past 25 m.y. and the origin and tectonic history of the world's largest oceanic plateau. Periods of low sedimentation, dominant in the late early to early middle Miocene, are characterized by hiatuses at some sites and condensed sections at others. The onset of the period of low accumulation coincides with a marked change in the carbon isotope composition of the ocean, the result of an increased difference in productivity between the deep sea and coastal waters. The period of high sedimentation centered in the late Miocene to earliest Pliocene displays some of the highest rates recorded in open-ocean pelagic sediments (~ 50 m/m.y.) The timing of this period suggests that these high rates may be associated with the closing of the Tethys seaway and with important phases of mountain-building, leading to an increased supply to the ocean of continent-derived materials. A large number of acoustic reflectors are synchronous and are associated in time with important paleoceanographic events. Many reflectors line up with sudden changes in carbonate accumulation while others mark the position of hiatuses, which in turn line up with condensed sections, emphasizing the importance of carbonate dissolution pluses. The recovery of the K/T boundary at two sites demonstrates the presence of a deep carbonate compensation depth across the transition; one sequence is calcareous, the other is not. There is also evidence of volcanic activity at that time.

Basement is composed of predominantly olivine-bearing basalts, erupted during the mid- Cretaceous, followed by the eruption of thin-pillowed flows, deposition of limestone-claystone facies, and a second sequence of pillow flows during waning volcanism. Magnetic paleolatitudes suggest that the plateau has moved coherently with the Pacific Plate since the Early Cretaceous.