A Mid‐Lithospheric Discontinuity Detected Beneath 155 Ma Western Pacific Seafloor Using Sp Receiver Functions

This study probes the lithosphere‐asthenosphere system beneath 155 Ma Pacific seafloor using teleseismic S‐to‐p receiver functions at the Pacific Lithosphere Anisotropy and Thickness Experiment project ocean‐bottom‐seismometers. Within the lithosphere, a significant velocity decrease at 33–50 km depth is observed. This mid‐lithospheric discontinuity is consistent with the velocity contrast between the background mantle and thin, trapped layers of crystallized partial melt, in the form of either dolomite or garnet granulite. These melts possibly originated from deeper asthenospheric melting beneath the flanks of spreading centers, and were transported within the cooling lithosphere. A positive velocity increase of 3%–6% is observed at 130–155 km depth and is consistent with the base of a layer with partial melt in the asthenosphere. A shear velocity decrease associated with the lithosphere‐asthenosphere boundary at 95–115 km depth is permitted by the data, but is not required. Plain Language Summary Using seismic waves from distant earthquakes recorded by seismometers deployed on the seafloor, we investigated the seismic velocity structure beneath old Pacific seafloor. Within the lithosphere, we see a decrease in velocity at depths of roughly 40 km, which can be modeled as crystallized layers of partial melt that were emplaced in the lithosphere when it was young. The data indicate the presence of a velocity increase at depths of 130–155 km, and are consistent with (but do not require) a velocity decrease at depths of 95–115 km. These boundaries could mark the lower and upper boundaries of a layer in the asthenosphere that contains higher fractions of partially molten rock. Key Points Lithosphere and asthenosphere beneath 155 Ma western Pacific crust are measured by S‐to‐p receiver functions using the Pacific Lithosphere Anisotropy and Thickness Experiment ocean‐bottom seismometers data A mid‐lithospheric discontinuity at ∼40 km depth is observed and can be explained by layers of crystallized partial melt A partial melt‐bearing layer from 95 to 155 km depth is consistent with the receiver function observations