Interplay between crystal-plasticity, fracturing and dissolution-precipitation creep in lower-crustal ultramylonite from hole U1473A, Atlantis Bank, Southwest Indian Ridge

Abstract

Microfabrics of a gabbroic ultramylonitic shear zone from the Atlantis Bank oceanic core complex have been studied to investigate strain localization processes during exhumation of the lower oceanic crust through detachment faults. The ∼0.5 cm wide ultramylonite band is hosted in a coarse-grained deformed gabbro. Microfabric analysis reveal undulatory extinction, kink bands, bent exsolution lamellae or twins and healed microfractures decorated by new grains within porphyroclasts of diopside and plagioclase in the ultramylonite, as well as the host gabbro close to the border of the ultramylonite. These microstructures suggest that strain was initially accommodated by dislocation glide associated with microfracturing, i.e. high-stress crystal plasticity. Heterogenous fluid influx along microfractures is suggested to have led to localized phase transformation, as amphibole is abundant in the fine-grained matrix of the ultramylonite but not in the host gabbro. Fluid-rock interaction resulted in the development of the ultramylonite via grain-size sensitive fluid-assisted granular flow. Strain localization is concomitant to increasing fluid-rock interaction and the replacement of the former deformed anhydrous mineral assemblage into a hydrated ultramylonite composed of a plagioclase-amphibole mixture. Our study highlights the importance of fluid-assisted metamorphic reaction in decreasing the viscosity of the metastable lower oceanic crust and creating zones of extreme rheological weakening that deform by diffusion creep on the long term. Fluid infiltration plays a key role in facilitating ductile deformation and tectonic spreading through large-scale detachment faults in the hot, lower oceanic crust.