HIGH TEMPERATURE, HIGH STRAIN-RATE FABRICS DEFINE A MAJOR DUCTILE SHEAR ZONE IN THE EASTERN SIERRAS PAMPEANAS, ARGENTINA

We recognize a crustal-scale high strain zone extending nearly continuously for at least 250 km NNW-SSE along the western margin of the Sierras de Córdoba, central Argentina. At its thickest point, the 'Tres Arboles' segment of the zone contains a 10-15 km thickness of ultramylonite and mylonite in the southern section, and is correlated along strike with a 2 km thick zone of mylonite, shear bands and minor pseudotachylyte in the northwest. The fault zone thrusts Cambrian I-type arc rocks and migmatized accretionary prism sediments to the west over Ordovician schists and psammites of the eastern Sierra de San Luis.

Geothermometry of hangingwall rocks in the Tres Arboles zone indicates amphibolite-grade deformation at temperatures >520°C. The biotite-rich ultramylonite matrix has a grain size up to 10 µm, and contains new sillimanite needles that define the foliation and wrap garnet and feldspar porphyroclasts. Along strike to the north and structurally higher in the zone, the greenschist-grade ultramylonite matrix is mostly biotite + chlorite, but feldspar porphyroclasts remain internally pristine. Shear sense is almost exclusively east-over-west, where apparent. Throughout the fault zone, the ultramylonite contains sigma, delta, and theta porphyroclasts, with complexly folded and disaggregated quartz ribbons, which suggests that grain boundary sliding and reaction-enhanced grain size reduction were the predominant deformation mechanisms and that strain rates were very high.

Deformation temperatures determined from geothermometry and microstructural analyses show a progressive shallowing of the Tres Arboles fault zone from a crustal depth of 15-20 km in the south, to 10-15 km in the north, coincident with a decrease in the thickness of the fault zone. Minimum horizontal shortening across the zone is 3-7 km, significantly less than is expected from a 10-15 km thickness of ultramylonite, or from the 'bow-and-arrow' rule of Elliot (1976). We suggest that these discrepancies may result from substantial early strike-slip motion along the Tres Arboles fault zone prior to the predominantly east-over-west thrust movement preserved in the rock fabric.