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The Mid-Atlantic Ridge at 13°N is regarded as a type locality for oceanic core complexes (OCCs), as it contains, within ∼70 km along the spreading axis, four that are at different stages of their life cycle. The wealth of existing seabed observations and sampling makes this an ideal target to resolve contradictions between the existing models of OCC development. Here we describe the results of P -wave seismic tomographic modelling within a 60 × 60 km footprint, containing several OCCs, the ridge axis and both flanks, which determines OCC crustal structure, detachment geometry and OCC interconnectivity along axis. A grid of wide-angle seismic refraction data was acquired along a series of 17 transects within which a network of 46 ocean-bottom seismographs was deployed. Approximately 130 000 first arrival traveltimes, together with sparse Moho reflections, have been modelled, constraining the crust and uppermost mantle to a depth of ∼10 km below sea level. Depth slices through this 3-D model reveal several independent structures each with a higher P -wave velocity (Vp) than its surrounds. At the seafloor, these features correspond to the OCCs adjacent to the axial valley walls at 13°20′N and 13°30′N, and off axis at 13°25′N. These high- Vp features display dipping trends into the deeper crust, consistent with the surface expression of each OCC's detachment, implying that rocks of the mid-to-lower crust and uppermost mantle within the footwall are juxtaposed against lower Vp material in the hangingwall. The neovolcanic zone of the ridge axis has systematically lower Vp than the surrounding crust at all depths, and is wider between OCCs. On average, throughout the 13°N region, the crust is ∼6 km-thick. However, beneath a deep lava-floored basin between axial OCCs the crust is thinner and is more characteristically oceanic in layering and velocity–depth structure. Thicker crust at the ridge axis suggests a more magmatic phase of current crustal formation, while modelling of the sparse Moho reflections suggests the crust–mantle boundary is a transition zone throughout most of the 13°N segment. Our results support a model in which OCCs are bounded by independent detachment faults whose dip increases with depth and is variable with azimuth around each OCC, suggesting a geometry and mechanism of faulting that is more complicated than previously thought. The steepness of the northern flank of the 13°20′N detachment suggests that it represents a transfer zone between different faulting regimes to the south and north. We propose that individual detachments may not be linked along-axis, and that OCCs act as transfer zones linking areas of normal spreading and detachment faulting. Along ridge variation in magma supply influences the nature of this detachment faulting. Consequently, not only does magma supply control how detachments rotate and migrate off axis before finally becoming inactive, but also how, when and where new OCCs are created. [ABSTRACT FROM AUTHOR]

The 13°N segment of the Mid-Atlantic Ridge is an example of a morphologically well-studied slow spreading ridge segment populated with oceanic core complexes (OCCs). In this paper we present the results of an ∼200-km-long 2-D seismic and gravity transect through this segment, the bounding fracture zones to the south and the ridge discontinuity to the north. We use this transect to consider the two end-member models of OCC evolution in which one, referred to as the Segment-scale model, implies they are interconnected with their detachments being part of a single segment-long feature, and the other, the Localized model, that each OCC is structurally isolated. We show, using the 7.5 km s−1 velocity contour as the base of crust marker, that the crust is consistently relatively thin ridge-parallel, at ∼5 km thick on average, and that, beneath the OCCs, the Moho marks the top of a velocity gradient transition into the mantle, rather than a distinct velocity discontinuity. Although each OCC is not traversed in an identical structural location, they show a different crustal velocity–density structure with depth, with along axis variations in this structure mirrored by the bathymetric deeps between them. Older OCCs have a contrasting velocity–depth signature to the currently active 13°20′N OCC. The 13°20′N OCC is distinct in that it does not show higher relative velocity at shallower crustal depth like its neighbours, while the 13°30′N OCC has an apparently thinner crust. Our combined P -wave seismic traveltime tomography and gravity forward modelling suggests that the OCCs of the 13°N segment are not interconnected at depth. To the north of the 13°30′N OCC, our modelling also suggests that the crust is being magmatically refreshed, or that the ridge axis is currently undergoing magmatic accretion with an associated ridge tip propagation occurring across the ridge discontinuity that marks its northern edge. The profile also crosses the Marathon and Mercurius fracture zones that mark the southern limit of the 13°N segment and the southern ridge-transform intersection outside corner. Along profile, Marathon fracture zone offsets younger (∼1 Myr versus ∼8 Myr) oceanic crust than Mercurius fracture zone (∼8 Myr versus ∼11 Myr). When considered in combination, both seismic and gravity modelling suggest crustal thinning in the direct vicinity of the bathymetric valley of Marathon fracture zone, coupled with a region of low density that, most likely, reflects serpentinization of the uppermost mantle. In addition, the crust captured between fracture zones appears relatively rotated about an E–W axis and uplifted to the north, with the upwards motion accommodated on the northern lateral edge of the bathymetric depression rather than in its centre. Both the outside corner and the crust bounded by fracture zones have velocity–depth characteristics similar to that of the 13°N segment OCCs rather than normally accreted oceanic crust, particularly in the upper-to-middle crust. Overall, our results support the Localized model of OCC evolution and suggest that fracture zones do not become locked immediately on transform-to-fracture transition as current models dictate. [ABSTRACT FROM AUTHOR]