Seismic structure and crustal magmatism at the Mid‐Atlantic Ridge, 35°N

Abstract

We have determined the three‐dimensional P wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the Mid‐Atlantic Ridge (MAR) near 35°N. The best fitting one‐dimensional model is ∼0.8 km/s lower in velocity at a given depth than for fast spreading ridges, while the isotropic three‐dimensional velocity structure shows a strong lateral heterogeneity (up to 1.4 km/s peak‐to‐peak variation at 1 km depth) in both the along‐ and cross‐axis directions. An anomalous region, with velocities as much as 0.8 km/s lower than average and 3–5 km in lateral extent, is located beneath a near‐axis seamount. We interpret this feature as a region of near‐solidus temperatures with possibly a small degree of partial melt. From the variation of travel time residuals with azimuth we determine a best fitting P wave anisotropy model that has 4% anisotropy from 500 m to 1 km depth, 2% anisotropy in the depth range 1–1.5 km, and no anisotropy at greater depth. This finding is consistent with vertical cracks aligned parallel to the axial valley. We find no evidence for the shallow, high‐velocity, ridge‐parallel feature commonly observed at the axis of fast spreading ridges and attributed to high‐velocity dikes beneath a thin extrusive layer. The absence of this anomaly at the MAR implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25,000 years. An unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes, which may account for the generally lower velocities beneath the MAR compared with the East Pacific Rise, and a lateral variation in upper crustal lithology.