Compensation of the Ontong Java Plateau by surface and subsurface loading

Gravity data over Ontong Java Plateau reveal deep crustal structure support ing the plateau's topography. At long wavelengths (>500 km), new shipboard I:ravity and bathymetry profiles indicate local isostatic compensation. At intermediate wavelengths (250-500 km), however, Bouguer anomalies have larg e amplitudes, too high to be explained by models of Airy isostasy or volcan ic loading on the surface of an elastic lithospheric plate. This finding is also evident in admittance functions, generated from maps of bathymetry an d satellite-derived Bouguer anomalies, which show high values at intermedia te wavelengths. In addition, coherence between two-dimensional Bouguer grav ity and bathymetry spectrums decreases to zero, beginning at intermediate w avelengths. Two possible models can explain the high admittance and low coh erence at intermediate wavelengths. The first model explains the high admit tance by large scale lithospheric folding and explains the low coherence by sedimentation and erosion that is uncorrelated with the igneous crustal st ructure. Lithosheric might result from tectonic Lithospheric folding might results from tectonic compression imposed on Ontong Java by the Solomon are and trench system. The second model involves a multiple-stage accretion hi story with surface magmatism on a thin elastic plate and magmatic underplat ing beneath a thick plate. The thick plate during underplating is required to explain the high admittance values at intermediate wavelengths, while de correlation between underplating and the surface loaded magmas can explain the low coherence. The thick plate also suggests that the lithosphere coole d appreciably since the first magmatic stage and therefore magmatic underpl ating occurred at a significantly later time than the initial eruption. Cal culations predict that the underplated material could comprise as much as 5 0% of the total crustal volume, suggesting that the later eruptive stage(s) may have been as, or more voluminous than, the first.