Seismic Evidence for Large-Scale Compositional Heterogeneity of Oceanic Core Complexes

Long-lived detachment faults at mid-ocean ridges exhume deep-seated rocks to form oceanic core complexes (OCCs). Using large-offset (6 km) multichannel seismic data we have derived two-dimensional seismic tomography models for three of the best-developed OCCs on the Mid-Atlantic Ridge. Our results show that large lateral variations in P-wave velocity occur within the upper ~0.5-1.7 km of the lithosphere. We observe good correlations between velocity structure and lithology as documented by in situ geological samples, and we use these correlations to show that gabbros are heterogeneously distributed as large (10s of km²) bodies within serpentinized peridotites. Neither the gabbros nor the serpentinites show any systematic distribution with respect to along-isochron position within the enclosing spreading segment, indicating that melt extraction from the mantle is not necessarily focused at segment centers, as has been commonly inferred. In the spreading direction, gabbros are consistently present toward the terminations of the detachment faults, suggesting that they may have been emplaced by late-stage decompression melting during footwall exhumation. Heat introduced into the rift valley by flow and crystallization of this melt could result in formation of new faults and thus may explain abandonment of the detachments that formed the OCCs. Detailed seismic studies of the kind described here have tremendous potential to elucidate the internal structure of the crust and upper mantle and thus to understand the tectonic and magmatic processes by which they were emplaced.