Cenozoic structural evolution of the Argentinean Andes at 34º40’S: A close relationship between thick and thin-skinned deformation

Abstract

In the Argentinean side of the Andes at 34°40’S, the Cenozoic Andean orogeny produced the thick-skinnedMalargüe fold-and-thrust belt and the easternmost basement uplift of the Cordillera Frontal. Integrating balanced structuralcross-sections with previous studies of Cenozoic synorogenic rocks and 40Ar/39Ar ages of coeval volcanic and subvolcanicrocks, we propose a Miocene to Quaternary sequential structural evolution of this sector of the Andes. Andeandeformation in the study area begun around 14.5 Ma with the growth of thick-skinned structures at the western region,formed by large basement wedges that propagated along detachment horizons within the cover generating thin-skinnedstructures. The development of these genetically linked thick and thin-skinned structures finished with the insertion ofa third basement wedge and its associated deformation in cover rocks along the Sosneado thrust, before the extrusionof 10.8 Ma volcanic rocks. These structures imply ~10 km of shortening, representing an important Middle Miocenecompressive period (2.7 mm/a). Systems of fractures measured in basement, cover and subvolcanic rocks indicate anE-W direction of compression. A supercritical Coulomb wedge attained after the Middle Miocene deformation, wasresponsible of new basement-involved faulting into the foreland, the west-dipping Carrizalito thrust. The absence of efficientdetachment levels in the cover rocks of the eastern region difficulted the development of large basement wedgesand instead important backthrusting occurred in the hangingwall of the Carrizalito thrust. Widespread subvolcanicrocks (10.5 to 5.5 Ma) exposed in this region suggest a close relationship between magmatism and antithetic faulting.With continuing compression, the Carrizalito thrust breaks through to the surface thrusting pre-Jurassic and Mesozoicsequences over Cenozoic rocks although in the southern region it remains as a blind thrust. This different configurationalong the strike of the Carrizalito thrust is due to an ENE trending oblique fault. Thick-skinned structures in the easternsector, that involve ~4 km of shortening, were covered discordantly by horizontal basalts of ~0.5 Ma. A shorteningrate of 0.42 mm/a from Late Miocene to Middle Pleistocene indicates that Andean contraction diminished, which is inagreement with most geodynamic models of plate tectonics.