An upper mantle seismic discontinuity beneath the Galapagos Archipelago and its implications for studies of the lithosphere-asthenosphere boundary

摘要

An upper mantle seismic discontinuity (the Gutenberg or G discontinuity), at which shear wave velocity decreases with depth, has been mapped from S-to-p conversions in radial receiver functions recorded across the Galapagos Archipelago. The mean depth of the discontinuity is 918 km beneath the southeastern archipelago and 725 km beneath surrounding regions. The discontinuity appears deeper beneath the portion of the Nazca plate that we infer passed over the Galapagos mantle plume than elsewhere in the region. We equate the depth of the G discontinuity to the maximum depth extent of anhydrous melting, which forms an overlying layer of dehydrated and depleted mantle. We attribute areas of shallow discontinuity depth to the formation of the dehydrated layer near the Galapagos Spreading Center and areas of greater discontinuity depth to its modification over a mantle plume with an excess temperature of 11530 degrees C. The G discontinuity lies within a high-seismic-velocity anomaly that we conclude forms by partial dehydration and a gradual but steady increase in seismic velocity with decreasing depth after upwelling mantle first encounters the solidus for volatile-bearing mantle material. At the depth of the solidus for anhydrous mantle material, removal of remaining water creates a sharp decrease in velocity with depth; this discontinuity may also mark a site of melt accumulation. Results from seismic imaging, the compositions of Galapagos lavas, and rare-earth-element concentrations across the archipelago require that mantle upwelling and partial melting occur over a broad region within the dehydrated and depleted layer. We conclude that the G discontinuity beneath the archipelago does not mark the boundary between rigid lithosphere and convecting asthenosphere.