Contrasting evolution of upper crustal velocities in young oceanic crust: Insights from the Southern Mid-Atlantic Ridge

Abstract

Geophysical studies document rapid changes in upper crustal seismic velocities in young ocean crust that are attributed to infilling of cracks as a result of hydrothermal circulation. This study analyzes three seismic reflection profiles located on 0–6 Ma ocean crust on the slow-spreading southern Mid-Atlantic Ridge. Average velocities are ~2.5 km/s at 0 Ma, 3.2 km/s at 0.5 Ma, and 4.0 km/s at 3.5 Ma. Velocities are relatively constant from 3.5 to 6 Myr. Velocity trends are similar along all profiles and on both ridge flanks; this similarity suggests that the underlying processes associated with the velocity changes are widespread. The trends are similar to those observed at the slow-spreading equatorial Mid-Atlantic Ridge. In contrast, velocities at the intermediate-spreading Juan de Fuca Ridge increase more slowly with age than observed at the Mid-Atlantic Ridge. We argue that processes related to spreading rate are a significant factor in the age-related physical property changes. In particular, we highlight that compression associated with stresses related to the axial valley morphology may result in more rapid crack closure on the slow-spreading Mid-Atlantic Ridge than in similar age crust at the intermediate-spreading Juan de Fuca Ridge where axial valley morphology is less pronounced. A secondary control of spreading rate might simply be the increased amount of time slow-spreading crust resides near the ridge axis where the most vigorous hydrothermal circulation occurs. The rate of change of seismic velocity in young crust may be further moderated by the impact of sediment cover on upper crustal alteration.