Fine-Scale Crustal Velocity Structure at the Lucky Strike Segment of Mid-Atlantic Ridge From Full Waveform Inversion of Wide-Angle Seismic Data

Abstract

The Lucky Strike segment at the Mid-Atlantic ridge, characterized by a well-defined median valley with a central volcano, is an archetypical slow-spreading ridge segment and hence an ideal site for studying magmatic and tectonic processes at slow-spreading ridges. Here we present fine-scale velocity models of this segment, by applying full waveform inversion to wide-angle seismic data, that allows characterization of crustal accretion processes along the entire segment. Along ridge axis, the crust thins from ∼8.4 km at the center of the segment to ∼3.7–4.1 km at the segment ends. This large variation in crustal thickness is mainly accommodated by lower crustal thinning toward the segment ends. The ratio of the lower/upper crust thickness varies from 2.2 at the segment center to 0.1 at the segment ends, so upper crust at segment ends accounts for ∼90% of the crustal thickness, suggesting that the lateral dyking is the primary crustal accretion mechanism. The reduction of lower crustal velocity at the segment center indicates the presence there of melt within the lower crust, which is the source of melt delivery for dyke propagation. The upper crustal velocity gradually decreases from the segment center to segment ends, consistent with an increase in faulting and the presence of more evolved magma toward the segment ends. These observations demonstrate the presence of focused magma supply to the segment center. Off-axis, the upper crustal thickness shows little variation over ∼30 km on both flanks, suggesting the current magmatic accretion mode could have been active for 3 Myr.