Links between extensional tectonics and nature of fluid-rock interactions in the oceanic lithosphere

Slow-spreading ridges with reduced magma supply are associated with the formation of detachment faults that exhume rocks from the lower crust and mantle at the seafloor, and with their alteration by seawater. To better understand the mechanisms of deformation occurring in these systems, we conducted a detailed study of fault rocks collected in situ with a submersible from the 13°20’N oceanic detachment fault surface (Mid-Atlantic Ridge). We conducted microstructural and mineralogical analyses that we coupled with a fluid inclusions study. This work reveals the unprecedented observation of a pervasively silicified fault zone consisting mostly of mafic breccias. We propose that mafic material was incorporated into the fault zone from the base of the upper magmatic crust, and that the 13°20’N fault faces a well-developed magmatic crust with a lower dyke complex hydrothermally altered under amphibolite facies conditions. Silicification takes place during the fault exhumation, but quartz crystallized under quasi-static conditions at the sample scale. This alteration is linked with fluid flow driven by the presence of a high-temperature hydrothermal circulation at the base of the dyke complex that acts as a reaction zone. Fluids are then channelized by the fault that acts as a discharge zone. We also propose a new approach to recognize ophiolite complexes for which the oceanic lithosphere was formed in this context of extension that exhumed rocks from the mantle. Mantle rocks exposed in ophiolites systematically show variable degrees of serpentinization. As the composition of serpentinization products is dependent on the temperature of the system, we propose a magnetic, mineralogical and geochemical approach that can bring a new argument to distinguish between ophiolites serpentinized at the ridge axis (high-temperature) from those serpentinized off-axis (lower temperature).