The origin of the lithosphere-asthenosphere boundary (LAB) remains enigmatic. Tomographic studies argue in favor of a thermal LAB, but detections of the Gutenberg (G) seismic discontinuity, often associated with the LAB, cannot be explained by thermal effects alone. In this study, we modeled 3-D seismic anisotropy and velocity in the Pacific upper mantle and compared our models with the location of SS precursor detections of the G. Our results are consistent with a purely thermal LAB, but we found that the G is associated with vertical changes in anisotropy within the lithosphere. This implies that the two are not equivalent interfaces even though they may overlap in some locations. We propose that the G results from dehydration under mid-ocean ridges, generating a depleted, viscous layer that becomes overprinted by lowered temperatures as the plate cools down. Partial melt may also be present at the LAB where the two interfaces coincide and help explain enhanced detections of the G near upwellings.
|Last updated||Key Points|