ONE KILOMETRE-THICK ULTRAMYLONITE, SIERRA DE QUILMES, SIERRAS PAMPEANAS, NW ARGENTINA

We describe a 1km-thick ultramylonite forming the high strain base of the >3.5 km thick El Pichao shear zone in the Sierra de Quilmes. This shear zone thrusted granulite facies migmatites onto amphibolite facies schists during the 470 Ma Famatinian Orogen. Strain grades upwards from ultramylonites to weakly sheared migmatites across the 3.5 km zone and the mylonitic rocks define a geochemical field narrower than the protolith suggesting they underwent mixing and homogenization through shearing. Ultramylonites this thick are uncommon. The width of a shear zone, in the absence of significant compositional rheological contrasts controlling strain localization, is controlled by the balance between shear heat generation and diffusion. A strain rate of 10-12s-1 is required to form a 1 km- thick ultramylonite, and this is achieved when large movement velocities are imposed across the shear zone. We postulate that the El Pichao shear zone and its thick ultramylonite accommodated a significant fraction of convergence velocities driving the orogeny, and that the wide mylonitic shear zones characteristic of the Cambrian-Ordovician deformation of the Sierras Pampeanas result from the convergence movement being taken up by only a few active major shear zones.