Fluid-assisted strain localization in the shallow subcontinental lithospheric mantle

We report microstructural evidence for fluid-assisted ductile strain localization in a 50 m-wide mylonitic-ultramylonitic shear zone in the Ronda Peridotite massif, Southern Spain. Strain localization occurred at relatively low pressure ( mm). In the mylonites, olivine shows a crystal preferred orientation (CPO) coherent with dominant (001)[100] glide, probably due to the presence of interstitial fluids during deformation. In the ultramylonites, olivine CPO is weak to very weak, consistently with a decreasing contribution of dislocation creep to deformation. In contrast, fine-grained orthopyroxene in both mylonites and ultramylonites displays a clear CPO characterized by a [001] maximum normal to the foliation, which is not consistent with dislocation glide in any known slip system for orthopyroxene. We interpret this CPO as formed by oriented crystallization during dissolution-precipitation. In the present study, dissolution-precipitation creep predominates only in small-scale ultramylonite bands due to limited fluid availability and localized dynamic permeability. However, this process may be important in intermediate temperature domains of subduction zones, where it may lead to a feedback between strain localization and fluid transport. (C) 2016 Elsevier B.V. All rights reserved.