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Cambrian initiation of the Las Pirquitas thrust of the western Sierras Pampeanas, Argentina: Implications for the tectonic evolution of the proto-Andean margin of South America

¹ Department of Geology, University of California, Davis, California 95616, USA
² Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA
³ Berkeley Geochronology Center, Berkeley, California 94709, and Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA

The ⁴⁰Ar/³⁹Ar hornblende crystallization ages from the Las Pirquitas thrust of the Sierra de Pie de Palo, Argentina, indicate that imbrication of the proto-Andean margin had initiated by ca. 515 Ma. The ages do not permit the Pie de Palo block to represent basement of the adjacent Argentine Precordillera terrane, but rather require it to be a separate fragment not juxtaposed with the Precordillera prior to the Ordovician. The Pie de Palo block was likely autochthonous to the Famatina arc of the proto-Andean margin in the Early Cambrian. When combined with existing regional ages of magmatism, metamorphism, and deformation, the ages suggest that east-dipping subduction beneath the Famatina arc initiated by the Early to Middle Cambrian. The newly established convergent margin resulted from a westward shift of the plate boundary at the end of the Pampean orogeny in the eastern Sierras Pampeanas. The Famatina arc persisted as an Andeantype active margin until the collision and accretion of the Precordillera terrane in the Middle Ordovician.

Key Words: ⁴⁰Ar/³⁹Ar • Argentine Precordillera • Sierras Pampeanas • Las Pirquitas thrust • Cambrian • Ordovician • proto-Andean margin • convergent margin • terrane accretion