| We image the lithosphere-asthenosphere boundary beneath eastern North America using P-to-S and S-to-P converted phases. Modeling the properties of the associated velocity gradient indicates that the boundary is strong, and sharp, a 5-10% velocity drop that occurs over 11 km or less at 85-105 km depth. Therefore, a mechanism besides thermal gradients, such as a boundary in depletion combined with a boundary in dehydration and/or a change in anisotropy, or melting in the asthenosphere is required.; We also present attenuation tomography results from the mantle wedge beneath Costa Rica and Nicaragua that illuminate strong along-arc variations between Costa Rica and Nicaragua. The mantle wedge beneath Nicaragua is characterized by larger zones of more intense attenuation than that beneath Costa Rica. These results indicate that the Nicaraguan wedge is more hydrated and/or hotter. Either of these mechanisms are consistent with greater degrees of melting over larger areas beneath Nicaragua. These results are consistent with geochemical indicators in the region, which suggest that melting in Nicaragua is more hydrated, with greater degrees of melting, and perhaps greater depths of melting. |
