Crustal structure of the Ontong Java Plateau

We present a new model for the crustal structure of the Ontong Java Plateau (OJP) constrained by joint reflection and refraction seismic tomography. A compressional velocity model with a Moho reflector along a 280 km-long transect is constructed by inverting 2717 Pg and 1775 PmP travel times. Uncertainty in our model parameters is estimated using nonlinear Monte Carlo analysis. Our analysis indicates that most of the velocity and depth nodes are well resolved with one standard deviation of 0.05 - 0.15 km/s and 1.8 km - 2.8 km, respectively. The bulk of the plateau is ∼28 km thick, and the P-wave velocity of its lower crust is found to be ∼5.8 - 7.1 km/s. Crustal velocity does not exceed 7.2 km/s over the entire transect. This velocity is significantly lower than earlier published values. On the basis of a mantle melting model that relates the effect of active upwelling, pre-existing lithospheric thickness, and mantle potential temperature to the observed crustal structure, it appears difficult to explain the plateau formation by the upwelling of a pyrolitic mantle source of anomalously high temperature, as is commonly assumed in the mantle plume head hypothesis. The melting of a more fertile mantle may be required to explain the observed combination of low velocity and considerable crustal thickness.