Crustal and upper mantle structure beneath the Galapagos arechipelago from seismic tomography

To explain the origin of several distinct aspects of the Galapagos volcanic hotspot, such as the broad geographical extent of recent volcanism and the unusual pattern of geochemical anomalies, we conducted seismic tomography studies of the upper mantle and crust beneath the Galapagos Archipelago. The studies combine measurements of group and phase velocities of surface waves and delay times of body waves. We find that upper mantle seismic velocities are lower than those beneath other regions of comparable age in the Pacific and consistent with an excess temperature of 30 to 150°C and ∼0.5% melt. We attribute the excess temperature and presence of melt to an upwelling thermal mantle plume. Crustal seismic velocity is up to 25% lower than that of very young crust at the East Pacific Rise (EPR) and is comparable to that of Hawaii, which we attribute to heating by increased intrusive activity above the Galapagos plume and the construction of a highly porous volcanic platform. In addition, we find that the Galapagos hotspot is underlain by a high-velocity region whose thickness varies from 40 to 100 km. The tomographic images reveal that the upwelling mantle plume tilts northward (towards the nearby Galapagos Spreading Center) as it rises and then spreads laterally when it reaches the bottom the lid. The lid, which we attribute to residuum from melting, is thickest where it is farthest from the spreading center, suggesting that ridge processes may affect the generation and amount of thinning of the residuum layer. In addition, the thickness of the lid correlates well with the geographical pattern of geochemical anomalies of erupted lavas, suggesting that the lid may control the final depth of decompression melting. We conclude that many of the distinct characteristics of the Galapagos can be attributed to the interaction of the upwelling plume with the lid and the nearby ridge. We further suggest that the ridge affects the geometry of plume upwelling in the upper mantle and also the pattern of lateral spreading of the plume due to its effect on the thickness of the residuum layer.;This dissertation includes previously published co-authored material.