The active source seismic component of the Gravity Lineations, Intraplate Melting, Petrologic and Seismic Expedition ( GLIMPSE) experiment investigated the velocity structure and crustal thickness of the Pacific plate along a similar to 450 km line paralleling the southern East Pacific Rise where prominent intraplate ridges lie within lows of the lineated free-air gravity pattern. Compressional wave traveltime data collected by 10 ocean bottom seismometers indicate that the median crustal thickness is similar to 6.2 km and increases to similar to 8.2 km beneath the Sojourn Ridge. The conspicuous absence of crustal thinning beneath both the Sojourn and Hotu-Matua ridge systems is strong support against a boudinage-style model for the formation of these volcanic complexes. Our results suggest the ridge loads are partially supported by both plate flexure and underplating, with an additional mantle component inferred from the analysis of gravity data, Rayleigh wave tomography, and teleseismic body wave delays. Small-scale convection could produce the observed crustal thickness pattern but fails to explain the geochemical and formation age trends along the ridges. We prefer a model where the channelized flow of low-density material in the upper mantle, possibly undergoing small-scale convection, is advected toward the ridge crest. Lows in the gravity field reflect the density contrast within the asthenosphere, and decompression melting amid upwelling limbs of the convective cells sustains the development of the associated ridge systems.
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