The Role of Subslab Low-Velocity Anomalies Beneath the Nazca Ridge and Iquique Ridge on the Nazca Plate and Their Possible Contribution to the Subduction Angle

Subducting the buoyant crustal material of an aseismic oceanic ridge has been regarded as a dominant contributor to flat slab subduction. However, normal-dip subduction is also observed in some cases where ridges are subducting. In this study, we compare the subduction of two ridges on the Nazca Plate: Nazca Ridge (flat slab) and Iquique Ridge (normal-dip slab). Anisotropy determined by shear wave splitting observation suggests that the low-velocity anomalies found beneath the ridges are mapping anisotropic structure into isotropic velocities. After a tomographic inversion incorporating anisotropy models for both ridges, we find that the low-velocity anomalies found beneath the Nazca Ridge are not anisotropic and therefore likely represent warm mantle, and those beneath the Iquique Ridge are caused by anisotropy. We conclude that subslab mantle buoyancy has a larger impact on the subduction angle than the crustal material of the ridge.