Heterogeneity in texture and crystal fabric of intensely hydrated ultramylonitic peridotites along a transform fault, Southwest Indian Ridge

Fluid-assisted grain size reduction leads to strain localization in oceanic transform faults

“…The petrographic microstructural characters described here, i.e. the extensive grain size reduction, the interlayered olivine-rich and polymineralic domains, the occurrence of syn-deformational high-temperature hydrous phases 5 , 6 , 9 – 12 , 51 , and more rarely the orthopyroxene CPO mimicking that of the olivine 51 , have been reported in other transform domains along the MOR system. However, all these observations have not previously been interpreted as pertaining to fluid-assisted dissolution-precipitation creep, which could represent the leading mechanism during localization of deformation at OTFs.…”

“…However, all these observations have not previously been interpreted as pertaining to fluid-assisted dissolution-precipitation creep, which could represent the leading mechanism during localization of deformation at OTFs. Yet, recent petrological studies of various OTFs mylonites, such as those from the Shaka and Garret Fracture Zones 5 – 7 , 12 , support this assumption by showing positive feedback loop in the brittle-ductile transition zone between seismic-driven fluid infiltration, weakening, grain size reduction, shear and fluids.…”

“…Pervasive shearing at high pressure (>0.6 GPa) and high temperature (>750 °C) generates mylonites and ultramylonites, i.e. strongly foliated, very finely grained metamorphic rocks 5 , 6 , 9 – 12 , representing a rare occurrence on the seafloor. The deformation in the ductile region involves olivine-rich mantle-derived lithologies, containing variable amounts of pyroxene, spinel and occasional amphibole, here grouped under the general term peridotite.…”