Petrology and structure of mylonites and cataclasites derived from marbles in the southern end of the Sierra de Comechingones (Córdoba): Implications in the tectonic evolution of the Las Lajas shear zone.

Abstract

Las Lajas shear zone is one of the local strain zones developed in Sierras de Córdoba. Located in the southern part of Las Lajas shear zone, the Cerro El Potrero Sur displays a marble mylonites and cataclasites sequence interbedded with minor undeformed marbles. The intrinsic properties of marble tectonites provided new petro-structural insights allowing this research to reveal a part of the evolution of Las Lajas shear zone and to complement the previous knowledge based on silicic mylonites. During a first shearing, marbles were transformed into mylonite sequences. Mineral assemblages, microtextures, and calcite twins of marble mylonites constrained their formation conditions from amphibolite to greenschist facies, eventually microscale ductile processes defined an equilibrium under sub-greenschist conditions. Subsequently, the shearing developed local marble cataclasites under sub-greenschist conditions that were indirectly determined by petro-structural analysis as well as related deformation mechanisms (cataclasis, pressure solution, and cataclastic flow). Regarding the shear kinematic in the Cerro El Potrero Sur, the structures related to marble mylonites indicated a reverse movement with hanging wall W-uplift and variable strike-slip component. The cataclastic structures suggested a similar spatial arrangement in the movement paths, characterized by CO₂-rich fluid circulation and at least two cataclastic pulses. Overall, marble tectonites in Las Lajas shear zone are the result of localized deformation that evolved progressively under shallower metamorphic conditions (amphibolite to sub-greenschist facies) through ductile to ductile-brittle domains along an extensive time span (Famatinian to post-Achalian?), implying a continuous deformation coeval to the basement exhumation under retrograde conditions.