CRUSTAL STRUCTURE OF THE ONTONG JAVA']JAVA AND MANIHIKI OCEANIC PLATEAUS

Air gun-sonobuoy precision echo recorder data required on the Ontong Java Plateau and the Manihiki High Plateau demonstrate that these features have upper crustal seismic velocity structures strikingly similar to those observed in deep ocean basins. Each of the crustal layers of the oceanic plateaus, however tends to be proportionally thicker than its ocean basin counterparts. Our measurements show that the crustal layers of the Ontong Java Plateau and the Manihiki Plateau are 5.0 and 3.1 times thicker than ocean basin crust, respectively. The propertionality also holds for other oceanic plateaus, such as the Shatsky Rise, where the structure reported by Den et al. (1969) is 2.7 times thicker than ocean basin crustal layers. Layer 2A on the three plateaus is poorly predicted by this model. The oceanic plateaus considered in this paper have essentially the same mean crustal density as the much thinner ocean basin crust. These observations, in addition to the ocenaic affinities and apparent age progression of the upper crustal rocks of the Ontong Java Plateau, lead us to conclude that this region, as well as most other oceanic plateaus, probably formed as an integral part of the lithosphere at an accreting plate margin.-Authors