Imaging Oceanic Lithosphere-Asthenosphere System beneath 155 Ma Pacific Seafloor using S-to-p Receiver Functions

The structure of the oceanic lithosphere-asthenosphere system is key to deciphering the evolution of tectonic plates and to resolving fundamental issues such as whether partial melt ponded at the lithosphere-asthenosphere boundary (LAB). In this study, we probe the lithosphere-asthenosphere system beneath 155 Ma western Pacific seafloor using teleseismic converted body waves recorded by the PLATE project ocean-bottom-seismometers. The observed S-to-p (Sp) receiver functions are limited in number, but present coherent signals implying distinct seismic structure beneath the stations.

In the binned Sp receiver functions, the LAB is detected at depths of 66-91 km with peak amplitudes at ~82 km. This result is broadly consistent with previous studies that sample old Pacific mantle and extends the flattening trend of LAB depth to seafloor ages greater than 70 Ma, coinciding with estimates from cooling plate models rather than the half-space cooling model. Preliminary waveform modeling suggests a ~10% shear velocity drop over 4 km. In simple cooling models, temperature changes alone are too gradual to produce a distinct LAB phase, demonstrating that an additional mechanism such as partial melt accumulated at the lithospheric base is required to sharpen the LAB velocity gradient. Partial melt in the asthenosphere appears to pond at the LAB and decrease in concentration downward, with the base of a low velocity melt-bearing layer detected by a deeper, weak positive phase at depth of 124-140 km.

Within the lithosphere, Sp receiver functions show a significant negative phase at 34-40 km, and this mid-lithosphere discontinuity (MLD) that is comparable in amplitude to the LAB phase. The depth is similar to the depth of wide-angle reflectors found in active source experiments conducted near the Shatsky Rise close to the PLATE deployment. Other receiver function studies in 129 Ma western Pacific and in 75-85 Ma eastern mid-Atlantic, as well as some SS precursor studies across the Pacific find discontinuities within a similar depth range, indicating that the oceanic MLD is probably widely distributed. The large amplitude of the MLD phase suggests a significant velocity decrease, which cannot be satisfied by a layer of azimuthal anisotropy found in prior work or a solidified basaltic melt layer. This result implies a volatile-rich component inside the trapped melt that produces a greater velocity drop, with the melt possibly originating from deeper asthenospheric melting beneath the flanks of the spreading centers.