EVIDENCE FOR EXTENSION IN THE LOMA DE LAS CHACRAS, SIERRA DE LA HUERTA, WESTERN SIERRAS PAMPEANAS, ARGENTINA

Our study of the western Sierra de la Huerta in the western Sierras Pampeanas, Argentina, finds a NNE trending, SE dipping, mylonite/ultramylonite zone with down to the SE kinematics. Kinematic indicators suggest both top to the NW and top to the SE motion of the mylonite zone. Previous studies in the area recognize a top to the NW compressional event, but SE extension in the region has not been documented. The 200 m wide shear zone is centered in the southern hills of the Loma de las Chacras; a geomorphologic dome comprised of high-P moderate-T metamorphic units, in the western Sierra de la Huerta.

Structurally below the shear zone, high-grade metamorphic units predominate. Migmatites localize within kyanite-sillimanite bearing metapelites. Amphibolite boudins display partial melting at their edges. Garnet bearing granitic sills and dikes are common, within which fabrics range from strongly deformed banded mylonites and augen gneisses proximal to the shear zone, to moderately deformed fabrics below the zone.

Structurally above the shear zone, metapelitic schists record lower grade metamorphic conditions. Upper plate units are predominately garnet-muscovite schist, with lenses of calc-silicate and quartzite, and rare boudins of both granitic and amphibolitic composition. Amphibole porphyroblasts in the upper plate boudins display a prograde core to rim zoning. Penetrative foliation in the metapelitic schist strikes NNE with minor S-C fabrics developed.

The juxtaposition of lower grade on top of higher-grade metamorphic assemblages across the high strain shear zone, combined with the kinematic indicators and prograde upper plate porphyroblasts, suggest an extensional event coeval with high-T conditions in the lower plate.

The mylonitic fabric dips steeply SE in the southern reaches of Loma de las Chacras, with the dip angle shallowing northward to horizontal, and appears to be contiguous with a NE striking, NW dipping shear zone approximately 5 km to the north. Earlier studies of units to the north indicate lower grade assemblages overlie the gneiss here as well. This arching of the shear zone, with migmatitc gneiss doming up in the center and lower grade units to the south and possibly to the north, suggests the Loma de las Chacras has the requisite morphology to be a gneiss dome associated with extension.