Fluid-Assisted Ductile Strain Localization in the Shallow Subcontinental Lithospheric Mantle (Ronda Massif, Betic Cordillera, S-Spain)

The Ronda massif (S-Spain) is the largest (ca. 300km2) of several orogenic peridotite massifs exposed in the Betic and Rif (northern Morocco) mountain belts in the westernmost part of the Alpine orogen that was tectonically emplaced during early Miocene times. Here we report a microstructural study of strain localization in a mylonitic peridotite shear zone that have been formed during the latest ductile history of the massif. Strain localization is associated with a sudden decrease of grain size and the redistribution of orthopyroxene in the finest grained microstructural domains (ultramylonites). Olivine crystallographic preferred orientation (CPO) shows [100]-axes subparallel to the lineation and [001] at the pole of the foliation, whereas orthopyroxene mimics the CPO of that of olivine. Although low-angle misorientations in neighboring pixels are frequent in both phases, olivine and orthopyroxene CPO cannot be explained by coeval deformation via dislocation creep mechanisms. Based on microstructural and major element geochemical data, we propose strain localization at low temperature assisted by interstitial fluids that has led to the dissolution of orthopyroxene porphyroclasts and reprecipitation along the ultramylonite bands.