Crustal thickness along the western Galapagos Spreading Center and the compensation of the Galapagos hotspot swell

Wide-angle refraction and multichannel reflection seismic data show that oceanic crust along the Galapagos Spreading Center (GSC) between 97degreesW and 91degrees25'W thickens by 2.3 km as the Galapagos plume is approached from the west. This crustal thickening can account for similar to52% of the 700 in amplitude of the Galapagos swell. After correcting for changes in crustal thickness, the residual mantle Bouguer gravity anomaly associated with the Galapagos swell shows a minimum of -25 mGal near 92degrees15'W, the area where the GSC is intersected by the Wolf-Darwin volcanic lineament (WDL). The remaining depth and gravity anomalies indicate an eastward reduction of mantle density, estimated to be most prominent above a compensation depth of 50-100 km. Melting calculations assuming adiabatic, passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of similar to30degreesC. The associated thermal expansion and increase in melt depletion reduce mantle densities, but to a degree that is insufficient to explain the geophysical observations. The largest density anomalies appear at the intersection of the GSC and the WDL. Our results therefore require the existence of compositionally buoyant mantle beneath the GSC near the Galapagos plume. Possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the Galapagos Islands that is later transported to the GSC. Our estimate for the buoyancy flux of the Galapagos plume (700 kg s(-1)) is lower than previous estimates, while the total crustal production rate of the Galapagos plume (5.5 m(3)s(-1)) is comparable to that of the Icelandic and Hawaiian plumes. (C) 2002 Elsevier Science B.V. All rights reserved.