THE GRENVILLIAN PIE DE PALO ULTRAMAFIC BELT, ARGENTINA:

CHERNICOFF, C.J., VUJOVICH, G.I., VAN STAAL. C.R.

San Luis

Simposio; XIII Reunión de Tectónica; 2006

Comisión de Tectónica, Asociación Geológica Argentina

he recent acquisition of a high-resolution aeromagnetic survey over the Pie de Palo uplift, western Sierras Pampeanas, has revealed an area of high magnetic anomalies associated with the Pie de Palo Complex. The Las Pirquitas thrust, which has translated and uplifted the Pie de Palo Complex, is recognized for at least 30 km in a roughly NE direction along the western boundary of the Pie de Palo Complex, beyond its limited outcrop. The sediments of the Caucete Group in the footwall of the Las Piquitas fault, which are regarded as the leading edge of the Precordillera terrane, show a much less pronounced magnetic signature. In addition, a conspicuous, NNE trending, broad magnetic high stands out in the survey, about 7 km to the east of the main outcrops of the Pie de Palo Complex; this broad magnetic anomaly bisects the Pie de Palo basement block, and continues further south up to at least 32º S, the present extend of the aeromagnetic data. This structure is interpreted to correspond to the Grenvillian Precordillera-Pie de Palo tectonic boundary zone, and would comprise the largest (though unexposed) part of the mafic-ultramafic belt. The geophysical modeling of the magnetic data indicates that the boundary zone dips to the east, also pointing to the existence of a set of synthetic east-dipping, west-verging thrusts, of which one major structure (Las Pirquitas fault) is exposed; the possible existence of other slivers of upthrust boundary zone material is not excluded. It is considered that the Pie de Palo Complex represents a small sliver upthrust from the unexposed boundary zone material (i.e. highly magnetic mafic-ultramafic rocks) . The east-dipping, west verging structures associated with the Pie de Palo Complex therefore represent an Ordovician reactivation of a Grenvillian suture zone developed when the Precordillera basement and Pie de Palo terrane docked; this reactivation probably resulted from the collision of the Cuyania terrane to the western margin of Gondwana.