Anisotropic uppermost mantle in young subducted slab underplating Central Mexico

Knowledge of the rate of plate-spreading at mid-ocean ridges is critical for estimating plate motions and the outward flux of heat from Earth’s interior. Magnetic lineations and anisotropy—crystals that have aligned themselves with mantle flow—preserved in oceanic lithosphere provide a means for estimating plate-spreading rates up to 180 million years ago. However, reconstructions beyond this time are difficult because most older oceanic lithosphere has been subducted into the mantle. Here we use converted seismic waveforms to show that anisotropy is preserved in the subducted part of the Cocos Plate beneath Central Mexico. We observe strong P- and S-wave anisotropy in the topmost 2–6 km of the subducted oceanic mantle. The strength of the anisotropy is comparable to that measured in the surface portion of the Cocos Plate. We also show that P-wave azimuthal anisotropy and plate-spreading rate at present-day mid-ocean ridges exhibit a linear relationship. On the basis of this relationship, we suggest that the subducted portion of the Cocos Plate formed at a half-spreading rate of about 8 cm yr^(−1) at the East Pacific Rise, about 15 million years ago. Our results imply that subducted oceanic lithosphere could preserve an archive of ancient plate-spreading rates on Earth