PURE SHEAR AND DEFORMATION TEMPERATURE ESTIMATES FROM ORDOVICIAN MYLONITES AND ULTRAMYLONITES, SIERRAS PAMPEANAS, CENTRAL ARGENTINA

Crystalline basement rocks in the Eastern Sierra Pampeanas (ESP), Argentina, now uplifted on Mesozoic and Tertiary faults, record several fault reactivation events that are preserved in high-strain mylonite zones. The mylonites contain preserved microstructures that allow estimation of the temperature at which the rocks were deformed and show the kinematic history by their orientations, symmetry, and shapes.

This study focuses on a steep, east-dipping mylonite zone on the eastern edge of the Sierras de Cordoba, ESP. A petrological study of the mylonites shows evidence for two separate deformation events. Preserved polygonal recrystallized feldspar grains indicate that the first event was at high temperature (550-650°C); this event is thought to be associated with the regional Pampean Orogeny (525-509 Ma). Preserved microstructures in feldspar and epidote indicate that a second, mylonite- and ultramylonite-forming event occurred in the temperature range of 400-500°C. However, quartz recrystallization textures in ultramylonites indicate apparent deformation temperatures as low as 300-400°C, most likely the result of extremely high strain rates. We therefore used epidote, amphibole and feldspar microstructures for mylonite formation temperature estimates.

Mineral lineations in the mylonites and ultramylonites are down-dip, and shear sense indicators display a reverse, east over west movement, consistent with regional trends. Forward- and back-rotated grains from eight ultramylonites were analyzed for general shear conditions using the porphyroclast hyperbolic distribution method. The results suggest that at least in the final stages of ultramylonite formation, general shear was dominated by pure shear. The mylonites are probably post-470 Ma, on the basis of correlation with high strain zones in adjacent regions. 428 Ma pseudotachylyte veins cross cut the mylonites, but appear genetically related. This Ordovician deformation event correlates with the emplacement of several major tectonic terranes to the west of the Sierras de Cordoba. We propose that initial thrusting during terrane collision was followed by a steepening of the shear zone dip, which would have caused pure shear to dominate in the shear zone during its final movement.