Three‐dimensional structure of oceanic core complexes: Effects on gravity signature and ridge flank morphology, Mid‐Atlantic Ridge, 30°N

Abstract

Our gravity modeling of oceanic core complexes formed at the Mid‐Atlantic Ridge near 30°N suggests that their shallow, domal “cores” could be dominated by mafic intrusive rocks, consistent with recent drilling results at Atlantis Massif. The three‐dimensional gravity analysis incorporates additional underway geophysics data in a new compilation and uses a higher‐resolution bathymetry model to remove the gravity contribution of seafloor topography. The additional detail is required in order to confidently relate few‐kilometer‐scale gravity anomalies to specific morphologic/tectonic blocks. Different models of subseafloor core complex structure and density are tested to determine which minimizes the local gravity anomaly. A 3‐D core with density 2900 kg/m3, as measured in the gabbroic section drilled at the central dome, and juxtaposed 3‐D hanging wall of fractured basalt, density ∼2600 kg/m3, satisfactorily explains most of the Bouguer gravity anomaly at Atlantis Massif. The capping detachment fault terminates or plunges northward beneath the seafloor at the northern limit of the central dome. The southwest shoulder of the massif has lower density, consistent with an upper crustal section ∼1 km thick, whereas the summit and southeastern shoulder have overall density similar to the central dome. The older core complexes distributed along Atlantis fracture zone are similar in size, depth, and distance of their summit from the transform fault. However, weathering/alteration probably has reduced their density somewhat compared to Atlantis Massif. Bathymetric embayments occur adjacent to the fracture zone in several places on the ridge flanks and are consistently associated with core complexes.