Upper Mantle Anisotropic Shear Velocity Structure at the Equatorial Mid-Atlantic Ridge Constrained by Rayleigh Wave Group Velocity Analysis From the PI-LAB Experiment

The evolution of ocean lithosphere and asthenosphere are fundamental to plate tectonics, yet high resolution imaging is rare. We present shear wave velocity and azimuthal anisotropy models for the upper mantle from Rayleigh wave group velocities from local earthquake and ambient noise at 15–40-s period recorded by the Passive Imaging of the Lithosphere Asthenosphere Boundary experiment at the equatorial Mid-Atlantic Ridge covering 0–80-Myr-old seafloor. We find slow velocities along the ridge, with faster velocities beneath older seafloor. We image a fast lid (25–30-km thick) beneath the ridge that increases to 50–60 km beneath older seafloor. Punctuated, ∼100 km wide low velocity anomalies exist off-axis. There are multiple layers of azimuthal anisotropy, including (i) a lithosphere (20–40 km depth) characterized by strong anisotropy (4.0%–7.0 %) with fast-axes that rotate from ridge subparallel toward the absolute plate motion/spreading direction at distances >60 km from the ridge, and (ii) weak anisotropy (1.0%–2.0%) at >40 km depth. Our results are consistent with conductive cooling of lithosphere, although with some complexities. Thickened lithosphere beneath the ridge supports lateral conductive cooling beneath slow-spreading centers. Undulations in lithospheric thickness and slow asthenospheric velocities are consistent with small scale convection and/or partial melt. Lithospheric anisotropy can be explained by vertical flow and a contribution from either fluid or mineral filled cracks organized melt beneath the ridge and plate motion induced strain off axis. Deep azimuthal anisotropy is suggestive of upwelling beneath the ridge and three-dimensional flow possibly caused by small scale convection off-axis.