Dating fault fabrics using modern techniques of <superscript>40</superscript>Ar/<superscript>39</superscript>Ar thermochronology: evidence for Paleozoic deformation in the Eastern Sierras Pampeanas, Argentina

Steven J. Whitmeyer

New 40Ar/39Ar age data of ductile and brittle fault fabrics within the Sierras de Cordoba of the Eastern Sierras Pampeanas, Argentina, reveal episodes of Silurian and Early Carboniferous deformation. Laser ablation 40Ar/39Ar analyses of pseudotachylyte from the La Calera fault zone in the eastern Sierras de Córdoba yielded a weighted mean age of 429.3±3.5 Ma that is equivalent, within error, to existing age data. Pseudotachylyte from the Los Tuneles fault zone, along the northwestern margin of the Sierras de Córdoba, yielded weighted mean ages of 345±5 Ma and 348±2 Ma. These results are broadly coeval with ages of 341±5 Ma and 343±5 Ma obtained from ultramylonite biotite within the Tres Arboles fault zone along the southwestern margin of the Sierras de Córdoba.

These 40Ar/39Ar dates and kinematic data document Silurian brittle fault rocks that cross-cut regional metamorphic fabrics, and early Carboniferous ductile and brittle fault rocks that are interpreted as minimum ages for deformation. The close agreement between the ages for the Los Tuneles and Tres Arboles fabrics is consistent with existing models that equate the shallow, brittle fault fabrics of the Los Tuneles fault zone with deep, ductile fabrics of the Tres Arboles fault zone as a single extensive deformation zone along the western margin of the Sierras de Córdoba. The early Carboniferous deformation may represent the final stage of convergent tectonism along the western margin of Gondwana prior to the assembly of Pangaea.