Discovery of flat seismic reflections in the mantle beneath the young Juan de Fuca Plate

Crustal properties of young oceanic lithosphere have been examined extensively, but the nature of the mantle lithosphere underneath remains elusive. Using a novel wide-angle seismic imaging technique, here we show the presence of two sub-horizontal reflections at similar to 11 and similar to 14.5km below the seafloor over the 0.51-2.67Ma old Juan de Fuca Plate. We find that the observed reflectors originate from 300-600-m-thick layers, with an similar to 7-8% drop in P-wave velocity. They could be explained either by the presence of partially molten sills or frozen gabbroic sills. If partially molten, the shallower sill would define the base of a thin lithosphere with the constant thickness (11km), requiring the presence of a mantle thermal anomaly extending up to 2.67Ma. In contrast, if these reflections were frozen melt sills, they would imply the presence of thick young oceanic lithosphere (20-25km), and extremely heterogeneous upper mantle. Applying seismic imaging methods on ocean bottom hydrophone data, the authors here describe a horizontal, flat lithosphere base plus lithosphere-asthenosphere boundary beneath the young (0.51 to 2.67Ma) Juan de Fuca plate.