Oceanic Core Complex Structure from Seismic Evidence

Prominent topographical highs occurring at slow- and intermediate-spreading mid-ocean ridges have been interpreted as oceanic core complexes (OCC's) created by a detachment fault that is unroofing lower crustal and/or upper mantle rocks (gabbros, serpentinized peridotite). We performed a seismic refraction and wide- angle line across the Mid-Atlantic Ridge at 22°15 N, crossing the domed structure of an OCC in a mid- segment setting. The dome coincides with a gravity high and magnetic anomaly 2 was identified over the summit of this feature, indicating an age of 1.8 Ma. During the cruise M60/2 of the German research vessel METEOR 15 ocean-bottom hydrophones were deployed along a 150 km long seismic transect. Results from tomographic travel time inversion show a large variation in P-wave velocity structure in the uppermost 2 km of the igneous crust and a profound asymmetry between the western-flank core complex and the eastern-flank hanging wall. Joint modelling of seismic travel times and free-air gravity data has been used to constrain the crust/mantle transition zone (Moho), that display a distinct assymetric structure across the spreading center. We interpret the crustal and uppermost mantle structure as formed by a long-lived detachment fault. Our results are consistent with recent observations from other OCC's. The core complex has been developed at a magmatically starved portion of the Mid-Atlantic Ridge. However, the occurrence of wide-angle reflections from the seismic Moho (PmP) under the core complex suggests an abrupt transition and the presence of lower crustal gabbroic rocks formed by magmatic accretion. Thus, detachment faulting occurred during a magmatic active phase leading to an asymmetric accretion of oceanic crust. While the eastern flank is still generating 'normal' but thickened oceanic crust the western side exposes lower crust material by detachment faulting. Rafted blocks covering the core complex fault supports a 'rolling hinge' model for OCC formation.