Upper mantle structure beneath the Galápagos Archipelago from surface wave tomography

• Published in: Journal of Geophysical Research: Solid Earth 2007-07, Vol.112 (B7), p.n/a
• Main Authors: Villagómez, Darwin R., Toomey, Douglas R., Hooft, Emilie E. E., Solomon, Sean C.
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Galápagos hot spot; surface waves; upper mantle
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/2006JB004672

We present a Rayleigh wave tomographic study of the upper mantle beneath the Galápagos Archipelago. We analyze waves in 12 separate frequency bands (8–50 mHz) sensitive to shear wave velocity (VS) structure in the upper 150 km. Average phase velocities are up to 2 and 8% lower than for 0‐ to 4‐My‐old and 4‐ to 20‐My‐old Pacific seafloor, respectively. Laterally averaged VS is 0.05–0.2 km/s lower between 75‐ and 150‐km depth than for normal Pacific mantle of comparable age, corresponding to an excess temperature of 30 to 150°C and ∼0.5% melt. A continuous low‐velocity volume that tilts in a northerly direction as it shoals extends from the bottom of our model to the base of a high‐velocity lid, which is located at depths varying from 40 to 70 km. We interpret this low‐velocity volume as an upwelling thermal plume that flattens against the base of the high‐velocity lid. The high‐velocity lid is ∼30 km thicker than estimated lithospheric thickness beneath the southwestern archipelago, above the main region of plume upwelling. We attribute the thicker‐than‐normal high‐velocity lid to residuum from hot spot melting. The thickness of the lid appears to control the final depth of melting and the variability of basalt composition in the archipelago. At depths less than 100–120 km, plume material spreads in directions both toward and against eastward plate motion, indicating that plume buoyancy forces dominate over plate drag forces and suggesting a relatively high plume buoyancy flux (B ≥ 2000 kg/s).