Rayleigh-wave tomography of the Ontong-Java Plateau

The deep structure of the Ontong-Java Plateau (OJP) in the westcentral Paci fic is investigated through a 2-year deployment of four PASSCAL seismic sta tions used in a passive tomographic experiment. Single-path inversions of 2 30 Rayleigh waveforms from 140 earthquakes mainly located in the Solomon Tr ench confirm the presence of an extremely thick crust, with an average dept h to the Mohorovicic discontinuity of 33 km, The thickest crusts (38 km) ar e found in the southcentral part of the plateau, around 2 degrees S, 157 de grees E. Lesser values remaining much thicker than average oceanic crust (1 5-26 km) are found on either side of the main structure, suggesting that th e OJP spills over into the Lyra Basin to the west. Such thick crustal struc tures are consistent with formation of the plateau at the Pacific-Phoenix r idge at 121 Ma, while its easternmost part may have formed later (90 Ma) on more mature lithosphere. Single-path inversions also reveal a strongly dev eloped low-velocity zone at asthenospheric depths in the mantle. A three-di mensional tomographic inversion resolves a low-velocity root of the OJP ext ending 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.