Abstract

Attempts to differentiate geological units of the Andean Principal Cordillera, between 31 degrees 30'S and 33 degrees S, faced several problems until very recently. The lack of geochronological data or the equivocal interpretation of some existing radiometric ages from thick volcanic and volcaniclastic sequences, along with their structural complexity and lithologic monotony, obscured the recognition and relationships between units. This made a tectonic interpretation difficult as the units cropping-out on the eastern side of the Pocuro Fault zone were initially assigned to Cretaceous formations (Los Pelambres Formation in Chile and Juncal Formation in Argentina). Structural-geological work carried out between 32 degrees and 32 degrees 30'S, together with 8 new U-Pb zircon datings, allowed us to recognize the presence of Upper Cretaceous levels (Salamanca Formation), underlying volcano-sedimentary younger units of Abanico and Farellones formations. The latter were interpreted as different facies associated with upper Oligocene-Miocene volcanism. Most of the Principal Cordillera at this latitude is composed by these units, at least until the Chile-Argentina border. In the western part of the region, the volcanic Abanico Formation presents the most proximal facies with mild deformation, while more distal units, near and across the border are intensely deformed in two events, one between 21 and 18 Ma, and other post 18 Ma. On the other hand, in the vicinity of the Pocuro Fault zone, Upper Cretaceous outcrops, underlying oligo-miocene deposits slightly deformed, suggest an oligo-miocene age for the last event of this fault zone.