Structure, petrology and calcite marble textural fabrics in the Nieves Unit (western Betic Cordillera, Spain): A record of deformation in the footwall to the Ronda mantle extrusion wedge

Strongly heterogeneous deformation and extreme metamorphic gradients characterize the dominantly carbonate Nieves Unit in the footwall to the Ronda peridotites (Betic Cordillera), the largest outcropping body of subcontinental lithospheric mantle on Earth. High strain is recorded within a several hundreds of meters-thick metamorphic aureole. Here, a well-developed foliation and mineral lineation occur in calcite/dolomite marbles, together with isoclinal intrafolial folds. Field structural analysis, as well as petrological, microstructural and EBSD textural data, all emphasize the role of both deformation and metamorphic processes post-dating final emplacement of the Ronda mantle extrusion wedge. On the other hand, our results point to a limited preservation of the non-coaxial finite strain component associated with NW-ward thrusting of the peridotites. Dominant coaxial strain is interpreted to result from the tectonic burial provided by the mantle wedge and overlying crustal rocks on thermally softened footwall carbonates, essentially following peridotite emplacement. Continued heat diffusion from the high-temperature peridotites led to static silicate growth and final zoning of the metamorphic aureole, as well as to local annealing of calcite marble textures. Following substantial cooling, renewed crustal shortening affected the whole Nieves Unit, resulting in widespread NE-trending meso-scale folds.