Accumulation of partial melt beneath the Cascadia forearc constrained by teleseismic S reflections and P receiver functions

Discontinuities in the mantle lithosphere hold important clues about plate tectonics and thus have received persistent attention from the seismology community. However, despite decades of effort in seismically imaging lithospheric discontinuities, many key questions remain unanswered due to limitations of traditional methods, such as P and S receiver functions, in imaging lithospheric discontinuities, as well as the lack of agreement between different methods. Here, we find a sub-horizontal negative-polarity interface at a depth of ~80 km beneath the Cascadia forearc on both teleseismic-S-reflection and P-receiver-function images. Our preliminary waveform-modeling results show that this interface represents a ~12% isotropic Vs drop, which is highly unlikely without the presence of partial melt. We thus interpret this negative interface as a thin layer with a significant content of partial melt. The depth of this thin layer coincides with the top of a low-velocity body suggested by previous tomography studies, which was interpreted as the accumulation of warm oceanic-asthenosphere material from the subducting plate. Therefore, we speculate that our negative interface may represent the accumulation of partial melt derived from the warm region below. Our results highlight the importance of combining different seismic methods in imaging lithospheric discontinuities.