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Pervasive silicification and hanging wall overplating along the 13°20'N oceanic detachment fault (Mid-Atlantic Ridge)

Abstract : The corrugated detachment fault zone of the active 13820 0 N oceanic core complex (Mid-Atlantic Ridge) was investigated with a deep-sea vehicle to assess the links between deformation, alteration, and magmatism at detachment fault zones. We present a study of 18 in situ fault rock samples from striated fault outcrops on the flanks of microbathymetric corrugations. All the samples are mafic breccias that are mostly derived from a diabase protolith, with two of them also showing mixing with ultramafic clasts. Brec-cias are cataclastic and display variable deformation textures, recording numerous slip events, and showing pervasive silicification throughout the fault zone. Deformation-silicification relationships are also complex, showing both static and syntectonic quartz precipitation; undeformed quartz overprints the fault breccia textures, and reflective and striated fault surfaces cross-cut silicified rocks. In situ detachment fault rocks are mainly fault breccias with almost exclusively basaltic clasts, with rare ultramafic ones, a lithology and texture never observed previously at other oceanic detachment fault zones. We propose the lower dyke complex in the hanging wall crust at the volcanic rift valley floor is the most plausible diabase source. Mechanical mixing of predominantly mafic and rare ultramafic clasts suggests an underlying ultramafic footwall and that mafic accretion operates in the shallowest crust (1-2 km), at the base of the dyke complex at temperatures >4008C. Silicification is produced by silica-rich fluids syntectonically channeled along the fault zone, and likely derived from hydrothermal alteration of basaltic rocks, likely mixed with serpentinization-derived fluids. Plain Language Summary This paper presents a textural, mineralogical, and microstructural study of the fault rocks recovered in situ on the 13820 0 N detachment fault zone (Mid-Atlantic ridge) during the ODEMAR cruise in 2013. This detachment is unique for the presence of mafic material integrated within the fault zone as breccias and the pervasive silicification observed throughout all the detachment surface. Our paper discusses the origin of the mafic breccias and proposes that they were captured from the base of the dyke complex within the hanging wall during the fault exhumation. Our study reveals furthermore that quartz mineralization occurred in depth during the exhumation and is likely linked with the presence of mafic material within the fault zone. Our study indicates a complex relationship between silicification and deformation during which quartz (re)crystallized under quasi-static conditions between periods of deformation. This work also demonstrates that extreme strain localization can be achieved in the absence of weak alteration phases (talc, serpentine) but with instead high-friction material (quartz), suggesting that elevated pore fluid pressures play an important role.
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D. Bonnemains, J. Escartin, C. Mével, M. Andreani, Anne Verlaguet. Pervasive silicification and hanging wall overplating along the 13°20'N oceanic detachment fault (Mid-Atlantic Ridge). Geochemistry, Geophysics, Geosystems, AGU and the Geochemical Society, 2017, 18 (6), pp.2028-2053. ⟨10.1002/2017GC006846⟩. ⟨hal-02330315⟩

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