ABSTRACT. In the Argentinean side of the Andes at 34°40'S, the Cenozoic Andean orogeny produced the thick-skinned Malargüe fold-and-thrust belt and the easternmost basement uplift of the Cordillera Frontal. Integrating balanced structural cross-sections with previous studies of Cenozoic synorogenic rocks and 40 Ar/ 39 Ar ages of coeval volcanic and subvolcanic rocks, we propose a Miocene to Quaternary sequential structural evolution of this sector of the Andes. Andean deformation 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-skinned structures. The development of these genetically linked thick and thin-skinned structures finished with the insertion of a third basement wedge and its associated deformation in cover rocks along the Sosneado thrust, before the extrusion of 10.8 Ma volcanic rocks. These structures imply ~10 km of shortening, representing an important Middle Miocene compressive period (2.7 mm/a). Systems of fractures measured in basement, cover and subvolcanic rocks indicate an E-W direction of compression. A supercritical Coulomb wedge attained after the Middle Miocene deformation, was responsible of new basement-involved faulting into the foreland, the west-dipping Carrizalito thrust. The absence of efficient detachment levels in the cover rocks of the eastern region difficulted the development of large basement wedges and instead important backthrusting occurred in the hangingwall of the Carrizalito thrust. Widespread subvolcanic rocks (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 Mesozoic sequences over Cenozoic rocks although in the southern region it remains as a blind thrust. This different configuration along the strike of the Carrizalito thrust is due to an ENE trending oblique fault. Thick-skinned structures in the eastern sector, that involve ~4 km of shortening, were covered discordantly by horizontal basalts of ~0. RESUMEN. Evolución estructural Cenozoica de los AndesArgentinos a los 34°40'S: una estrecha relación entre deformación de piel fina y piel gruesa. En los Andes Argentinos a los 34°40'S, la orogenia Andina produjo durante el Cenozoico la faja corrida y plegada de Malargüe, de piel gruesa, y hacia el este el levantamiento del basamento en la Cordillera Frontal. Integrando secciones estructurales balanceadas con estudios previos de rocas sinorogénicas terciarias y edades 40 Ar/ 39 Ar de rocas volcánicas y subvolcánicas contemporáneas, proponemos una evolución estructural secuencial desde el Mioceno al Cuaternario para este sector de los Andes. La deformación andina en el área de estudio comenzó alrededor de los 14,5 Ma con el desarrollo de estructuras de piel gruesa en la región occidental, formadas por grandes cuñas de basamento que se pro...

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In this paper new geochemical, geochronological and isotopic Lu-Hf data from the Siempre Amigos Hills (SA), western Tandilia, Argentina, is presented. Outcrops in this small area include representative lithologies of the basement Río de la Plata craton (RPC) known as Tandilia Terrane. The new geochemical data characterize a heterogeneous Paleoproterozoic crust with a similar U-Pb SIMS magmatic age (~2.1 Ga) represented by a suite of mafic (amphibolites, charnockitic granulites), intermediate and felsic compositions (diorites, granodiorites and granitic gneisses), as well as intrusive granitic rocks (with Neoproterozoic age of ~652 Ma in zircon). Geochemical features suggest processes of magmatic differentiation from a mafic igneous protolith associated with the 'Camboriu orogeny' of the Trans-Amazonian-Eburnean cycle. Some amphibolites may represent sanukitoid magmas similar to those formed at the Archean-Proterozoic transition. This paper examines the hypothesis of a sanukitoid precursor for RPC Rhyacian magmatism. The potassic and calc-alkaline character of magmatic series, together with negative Nb-Ta-Ti anomalies and REE patterns, are compatible with classical modern subduction-related magmas. Precise Lu-Hf analyses of zircons collected from a granodiorite and a gneiss indicate a predominantly juvenile source (+εHf), with minor crustal recycling (-εHf). An inheritance age is recorded in a single Neoarchaean zircon core from a granodiorite (2771 ± 15 Ma), similar to calculated maximum Hf T DM (2.8 Ga). Considering the new available data from this region of the RPC, a sustainable petrogenetic model is proposed to explain the presence of geochemical and isotopic evidence of sanukitoid magmatism which may have occurred after 2.7 Ga, which seems to constitute a magmatic series which is a precursor to the development of the Rhyacian magmatic arc in the Tandilia Terrane. Medium-to high-grade metamorphic conditions probably resulted from Paleoproterozoic subduction followed by the intrusion of huge volumes of Rhyacian calc-alkaline magmatism. This model is compatible with RPC configuration as a collage of terranes, and also consistent with Trans-Amazonian-Eburnean amalgamation of various microcontinents in the formation of Atlantica.

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