Particular Oceanic Core Complex evolution in an extremely low melt supply environment

Saint Paul is a major transform system in the Equatorial Mid-Atlantic Ridge. It consists of four transform faults and three short intra-transform ridge segments. This study focuses on peridotitic and gabbroic ridges and unusual Oceanic Core Complex (OCC)-related tectonics found at the St. Paul southern intra-transform segment. These structures display the same characters as the OCCs worldwide (termination, rafted blocks, corrugations, breakaway); however unusual features suggest that they have evolved in a particular way with respect to other OCCs along the Mid-Atlantic Ridge. Small ridge segments display an asymmetrical accretion through successive nucleations of detachment faults over more than 10 m.y. marked by crustal mylonitisation (Adrião et al., this session). Structural mapping and gravity models covering about 100 km on each ridge flank confirm the existence of four consecutive detachments, the more recent being still active, and provide an interpretative model of their spatiotemporal evolution. The unusual aspect is that each detachment appears to have been split on the two sides of the ridge axis. As a consequence, the breakaways are located on the American plate, while the conjugate terminations are drafted away on the African plate. We suggest that this unusual feature results from the rupture of the detachment surfaces by relocation of the ridge axis through westward small ridge jumps. This mode of expansion is somehow intermediate between the “normal” OCCs spreading and the Smooth Seafloor-type model described off-axis along the Southwest Indian Ridge (Sauter et al., 2013). It partly compensates the long-term asymmetric expansion of this ridge segment and is likely related to the extremely low melt supply and thick lithosphere inferred from other studies. Adrião et al., 2017. Mechanical mixing and metamorphism of mafic and ultramafic lithologies .... This Session Sauter et al., 2013. Continuous exhumation of mantle-derived rocks… Nat Geo, 201