Mantle deformation during slow seafloor spreading constrained by observations of seismic anisotropy in the western Atlantic

During mantle flow at mid-ocean ridges, viscous shear deformation imparts a structural fabric that remains in the lithosphere as it cools, preserving a record of ridge processes over time. This fabric can be detected by seismic imaging, as it produces an azimuthal anisotropy in the velocity of seismic waves that propagate in the shallow mantle. Using data from a novel seismic refraction survey in the western Atlantic, we found that the maximum P-wave velocity in the upper 10 km of mantle lithosphere formed at the slow-spreading Mid-Atlantic Ridge is parallel to the paleo-spreading direction, consistent with viscous shear deformation dominated by corner flow. The magnitude of the P-wave anisotropy is 3.4±0.3%, approximately one-half that found in lithosphere formed at faster spreading rates in the Pacific. Weaker anisotropy in the Atlantic suggests that more pervasive conductive cooling at slow spreading ridges increases the proportion of localized (brittle) deformation in the mantle lithosphere, thereby limiting the degree of viscous deformation. By scaling our field observations to laboratory experiments, we estimate that total viscous strain during slow seafloor spreading is of order 0.5, as opposed to ∼1-2 for the fast-spreading case. Finally, our travel-time observations display an azimuthal asymmetry that can be interpreted in one of two ways: either the average elastic tensor in this region is not orthorhombic as is commonly assumed, or the underlying shear fabric is rotated ∼15° relative to fossil spreading.