A geophysical study of oceanic core complexes and surrounding terrain, Mid‐Atlantic Ridge 13°N–14°N

Abstract

We describe a geophysical study of oceanic core complexes (OCC) and surrounding seafloor on the Mid‐Atlantic Ridge at 13°N–14°N and off‐axis to ∼1.9 Myr. Data include a detailed, deep‐towed side scan sonar, magnetic field and bathymetry survey, supplemented by concurrent sea‐surface bathymetry, magnetic field and gravity measurements. Using side scan and bathymetry, we infer areas and relative ages of seafloor volcanism, revealing a complex pattern of melt accretion across the median valley including close to its walls. We estimate tectonic and magmatic extension throughout the area, and find that average tectonic extension since chron 2 on plates containing OCCs is up to three times that on their conjugates. Deep‐towed magnetic data reveal asymmetric spreading (faster on OCC‐containing plates) and crustal magnetization that is highly heterogeneous on a scale of ∼5 km, suggesting that exhumed domes of OCCs have highly variable lithologies, perhaps comprising both serpentinized peridotite and gabbro. Improved fits to magnetic data are provided by models incorporating ∼45°of OCC footwall rotation. An axial zone of normal magnetization, of presumed Brunhes epoch, has highly variable width and amplitude, with parts of the ridge axis displaying very low or apparently reversed magnetization. Gravity requires that OCCs have dense cores capped by lower density zones several kilometers thick. Gravity data indicate longer term patterns of crustal thickness and melt distribution that are broadly consistent with numerical models of OCC formation and show that waxing magmatism may terminate OCCs.