CRUSTAL STRUCTURE OF THE ONTONG JAVA PLATEAU - MODELING OF NEW GRAVITY AND EXISTING SEISMIC DATA

Seismic refraction and gravity-based crustal thickness estimates of th e Ontong Java oceanic plateau, the Earth's largest igneous province, d iffer by as much as 18 km. In an attempt to reconcile this difference we have evaluated available seismic velocity data and developed a laye red crustal model which includes (I) a linear increase in velocity wit h depth in the Cenozoic sediments and the uppermost extrusive basement and (2) a reinterpretation of deep crustal and Moho arrivals in some deep refraction profiles. Previously, Moho had commonly been interpret ed from later arrivals and in some cases constrained by precritical ar rivals. However, if first arrivals at distal offsets are interpreted a s Moho refractions, the maximum depth to Moho is reduced by about 10 k m. Two-dimensional gravity modeling along two transects from well-dete rmined oceanic crust in the Nauru Basin across the central Ontong Java Plateau to the Lyra Basin, based on the reinterpreted crustal model, is regionally consistent with satellite altimetry derived and shipboar d gravity fields yielding a 8.0 km/s Moho velocity at a depth of simil ar to 32 km under the central plateau. The crust features a thick ocea nic, three-layer igneous crust comprising an extrusive upper crust a 6 .1 km/s middle crust and a similar to 15 km thick 7.1 km/s lower crust . The total Ontong Java Plateau crustal volume is calculated at 44.4 x 10(6) km(3) and 56.7 x 10(6) km(3) for off-and on-ridge emplacement s ettings, respectively. On the basis of velocities and densities we int erpret the lower crust on the plateau to consist of ponded and fractio nated primary picritic melts, which due to deformation and/or fluid in vasion may have recrystallized to granulite facies mineral assemblages . The melts were emplaced during lithospheric breakthrough of a mantle plume in an oceanic, near-ridge plate tectonic setting.