Under the simplest models, the formation of the oceanic lithosphere is by cooling, and a seismically sharp interface is not expected. However, the physical processes that give rise to a seismically detectable interface are controversial, as is the attribution of the G discontinuity to the LAB. This interpretation implies that the transition from lithosphere to asthenosphere, or from conductive to advective regimes, is sharp on the scale of a seismic wavelength. Seismic studies have documented a seismic discontinuity in the upper mantle beneath ocean basins, historically known as the Gutenberg or G discontinuity, which has often been interpreted as the “lithosphere-asthenosphere boundary” or LAB. The lithosphere and asthenosphere are further distinguished thermally by conductive versus advective heat transfer, respectively, because the lithosphere does not convect internally, whereas the asthenosphere does. Plate tectonics is broadly defined as the steady movement of colder, more rigid lithospheric plates over hotter, more ductile asthenosphere. We conclude that the G discontinuity beneath the archipelago does not mark the boundary between rigid lithosphere and convecting asthenosphere. Results from seismic imaging, the compositions of Galápagos 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. 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. 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. We attribute areas of shallow discontinuity depth to the formation of the dehydrated layer near the Galápagos Spreading Center and areas of greater discontinuity depth to its modification over a mantle plume with an excess temperature of 115 ± 30☌. 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. The discontinuity appears deeper beneath the portion of the Nazca plate that we infer passed over the Galápagos mantle plume than elsewhere in the region. The mean depth of the discontinuity is 91 ± 8 km beneath the southeastern archipelago and 72 ± 5 km beneath surrounding regions. This work has significance in the current controversy as to the depth of penetration of the subducting slab into the lower mantle.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 Galápagos Archipelago. Moment tensor solutions, where available, will be used to account for amplitude effects. After successful mapping beneath the Tonga subduction zone, new areas such as the Izu-Bonin, Marianas, Java, Japan, and Kurile arcs will be investigated. This research is to use the study of the S-P converted phase S670P generated at the 670 km discontinuity beneath subduction zones to constrain the nature of the discontinuity and its dynamical significance. Primary Place of Performance Congressional District: Mark Richards (Principal Investigator) Sponsored Research Office:.Whitcomb EAR Division Of Earth Sciences GEO Directorate For GeosciencesįY 1990 = $45,953.00 FY 1991 = $42,273.00 REGENTS OF THE UNIVERSITY OF CALIFORNIA, THE Characteristics of the 670 km Seismic Discontinuity Beneath Subduction Zones NSF Org:
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