Résumé
Detachment-mode spreading is recognised as playing an essential role in accommodating plate separation at a substantial portion of the slower spreading mid-ocean ridge system. The basic geometry of oceanic detachments is now well established, from study of oceanic core complexes (OCCs) on the Mid-Atlantic Ridge (MAR) supported by numerical models. With a concave upwards geometry, faults form and slip at a steep angle sub-surface, flattening through a 'rolling hinge' as the footwall is exhumed and flexurally rotated, exposing flat or gently domed fault planes on the seafloor, often with marked spreading-direction-parallel corrugations. In well-studied examples (e.g. 15 degrees 45N) strain localisation is achieved by formation and deformation of weak, relatively low-temperature phyllosilicates on the fault zone associated with penetration of water to deep levels on the detachment. Questions however remain about the style of deformation, controls and feedbacks between the locus of melt emplacement and locus of the deformation. In some cases gabbro bodies, emplaced within the detachment footwalls, are little affected by high-T crystal-plastic deformation (e.g. MAR at 15 degrees 45N), in which case deformation is strongly localised in a narrow, low-T (greenschist facies) fault damage zone. In other cases, best typified by Atlantis Bank on the SW Indian Ridge, gabbros are instead characterised by extensive high-T crystal-plastic deformation. New drilling results from Atlantis Bank (IODP Expedition 360) emphasise the fundamentally different deformation mechanisms, likely geometry of detachment faulting, and rheology of newly-forming lithosphere in this magma-dominated, ultraslow-spreading system. We suggest these contrasting regions represent end-members of a broader spectrum of deformation styles at OCCs, controlled mostly by the locus and amount of magmatic heat input to the system and consequent rate of cooling of the active zone of deformation.