Footwall Structure of Oceanic Core Complexes: New Insights from Geophysical Data for Footwall Capture of Ascending Melt

Oceanic core complexes (OCCs) are the footwalls of long-lived detachment faults which form in response to magma-poor crustal accretion processes along the mid-ocean ridge. Although OCC formation is expected to occur at intermediate levels of melt supply to the ridge axis (e.g. Buck et al., 2005), sidescan sonar data have shown that surficial volcanism is absent during part of the OCC life cycle (MacLeod et al., 2009). This implies that footwall capture of ascending melt is an active process during OCC formation. Here, we present the results of a shipboard gravity and deep-towed magnetic survey across actively forming OCCs on the Mid-Atlantic Ridge between 12-14°N. Forward modelling of magnetic data show that the oldest parts of OCCs generally have zero magnetisation, and thus we interpret these areas as comprising non- or low-magnetic upper crustal material such as sheeted dikes and gabbros. In contrast, the younger, domal sections of OCCs often record a very heterogeneous magnetisation pattern, indicative of significant local variations in footwall magnetisation and/or composition over distances of < 6 km (our track spacing). Furthermore, 2.5D and 3D modelling of gravity data reveal a zone of relatively low density material within the ultramafic footwalls of near-axis OCCs. For a density of 2900 kg/m^3, this low density zone (LDZ) must extend for ~3-4 km beneath the seafloor. We interpret this LDZ as comprising a mixture (based on magnetic results) of serpentinised peridotite and gabbroic material that has accreted within the OCC footwall as the detachment fault has captured ascending melt beneath the ridge axis. Older near-axis OCCs in the region are generally associated with a thicker LDZ, which most likely represents more pervasive serpentinisation and melt accumulation with age. References: Buck, W. R., L. L. Lavier & A. N. B. Poliakov, 2005. Modes of faulting at mid-ocean ridges, Nature, 434, 719-723. MacLeod, C. J., R. C. Searle, B. J. Murton, J. F. Casey, C. Mallows, S. C. Unsworth, K. L. Achenbach & M. Harris, 2009. Life cycle of oceanic core complexes, Earth and Planetary Science Letters, 287, 333-344.