Rayleigh-wave tomography of the Ontong-Java']Java Plateau

The deep structure of the Ontong-Java Plateau (OJP) in the westcentral Pacific is investigated through a 2-year deployment of four PASSCAL seismic stations used in a passive tomographic experiment. Single-path inversions of 230 Rayleigh waveforms from 140 earthquakes mainly located in the Solomon Trench confirm the presence of an extremely thick crust, with an average depth to the Mohorovicic discontinuity of 33 km, The thickest crusts (38 km) are found in the southcentral part of the plateau, around 2 degrees S, 157 degrees E. Lesser values remaining much thicker than average oceanic crust (15-26 km) are found on either side of the main structure, suggesting that the OJP spills over into the Lyra Basin to the west. Such thick crustal structures are consistent with formation of the plateau at the Pacific-Phoenix ridge at 121 Ma, while its easternmost part may have formed later (90 Ma) on more mature lithosphere. Single-path inversions also reveal a strongly developed low-velocity zone at asthenospheric depths in the mantle. A three-dimensional tomographic inversion resolves a low-velocity root of the OJP extending as deep as 300 km, with shear velocity deficiencies of similar to 5%, suggesting the presence of a keel, dragged along with the plateau as the latter moves as part of the drift of the Pacific plate over the mantle. (C) 2000 Elsevier Science B.V. All rights reserved.