Three-Dimensional Geophysical Structure of Oceanic Core Complexes Formed at the Mid-Atlantic Ridge, 30°N

Abstract

Gravity and seismic modeling of oceanic core complexes formed at the Mid-Atlantic Ridge near 30°N suggests that their shallow, domal 'core' may be dominated by mafic intrusive rocks. The youngest of the three core complexes in this area, Atlantis Massif, has been sampled to 1.4 km depth during Integrated Ocean Drilling Expeditions 304 and 305. The recovered sequence was gabbroic, in contrast to prior models of this domal core as an ultramafic mantle section, uplifted via long-lived detachment faulting. Rather than use a mantle Bouguer anomaly, as is typical for marine gravity studies, this study emphasizes anomalies after a seafloor Bouguer and a plate cooling contribution are removed from the Free Air Anomaly. Reanalysis of seismic refraction data at Atlantis Massif indicate that while a mantle velocity layer (~7.5 km/s) at subseafloor depths of less than 1 km provides a good fit to observed travel-times, models with greater along-strike variability in the structure of lower velocity units (<6.5 km/s) can fit the data equally well. These new gravity and seismic results support several structural and compositional interpretations. A 3-D gabbroic core with density 2900 kg/m3 and juxtaposed 3-D hanging wall of fractured basalt, density ~2600 kg/m3, can explain most of the Bouguer anomaly at Atlantis Massif. The detachment fault probably terminates along-strike or plunges northward beneath the surface where the corrugations end at the northern limit of the central dome. The southwest shoulder of the massif probably contains an upper crustal section of thickness ~1 km, whereas the peak and southeastern shoulder of Atlantis Massif have overall density more similar to the central dome. The older core complexes along the fracture zone in this study area are similar in size, depth, and distance of their peak from the transform fault. However, weathering probably has reduced their density somewhat compared to current values at Atlantis Massif.