Cenozoic South American Land Mammal Ages (SALMAs) have historically been correlated to the geologic time scale using 40 Ar/ 39 Ar dating and magnetostratigraphy. At Gran Barranca (68.7°W, 45.7°S)-one of South America's key areas for constraining SALMAs-existing radioisotopic ages have uncertainties of up to 4 m.y. To better constrain the ages of mammalian assemblages, we employed high-precision (±<40 k.y.) U-Pb dating using single zircon crystals. We dated nine tuffs from the Sarmiento Formation containing middle Eocene-early Miocene faunas (Barrancan, Mustersan, Tinguirirican, Deseadan, Colhuehuapian, and "Pinturan"). The new dates span from 39.861 ± 0.037 Ma to 19.041 ± 0.027 Ma. The La Cancha Tuff, occurring within the Tinguirirican faunal level yielded an age of 33.581 ± 0.015 Ma, confi rming that the Vera Member contains the only fossiliferous geologic section encompassing the Eocene-Oligocene transition in the Southern Hemisphere. The pre-Deseadan fauna, La Cantera, is ≤30.77 Ma, the age of the Colhuehuapian is expanded to 21.1-20.1 Ma, and the Pinturan may be as old as ca. 19 Ma.The new U-Pb dates confi rm that atmospheric temperatures and vegetation remained constant across the Eocene-Oligocene transition in Patagonia and that hypsodonty occurred in South American ungulates much earlier than on any other conti-nent. Additionally, refi nement of the SALMA boundaries will eventually provide the context necessary to compare faunal transitions across continents, although currently too much data are missing to allow such comparisons. Finally, the new ages provide a highresolution age model from which hypotheses about rates of environmental and evolutionary change at Gran Barranca can be tested.

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Available and new palaeomagnetic data reveal transpressional deformation in the Argentine Precordillera fold and thrust belt contemporaneous with Juan Fernández ridge subduction. Localized changes in the orientation of palaeomagnetic directions indicate a vertical axis rotation pattern linked to local, oblique brittle-ductile shear zones that overprint the regional structure. The nearly homogeneous clockwise block rotation pattern from Western-Central Precordillera shows localized rotation nulls along NNW-trending left-lateral oblique belts, revealing that overprinting anticlockwise tectonic rotations could have balanced previous clockwise rotations. Conversely, clockwise rotations in Eastern Precordillera are only localized in the vicinity of these NNW-trending structures, and are controlled by rigid block rotations linked to basement-involved deformation.These results combined with available regional palaeomagnetic data in the forearc would indicate a regional bending of the upper plate margin between 27° S and 33° S that seems to be related to the subduction of the Juan Fernández aseismic ridge.

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-In the southern Central Andes, the Andean foreland was deformed due to Neogene shallowing of the Nazca slab beneath the South America plate. In this 27-33º S Pampean flat-slab segment, the N-trending Argentine Precordillera transpressional fold-and-thrust belt and the Sierras Pampeanas broken foreland developed as a consequence of inward migration of the orogenic front. At 28º S, a NNE-trending westward-dipping, thick Neogene synorogenic sequence is exposed in the Sierra de los Colorados, which shares deformation features of the Precordillera and the Sierras Pampeanas. Integration of new structural and kinematic data and available structural, kinematic, geophysical and palaeomagnetic information allows consideration of the Sierra de los Colorados area as part of the northern sector of the Precordillera during the middle Neogene. At c. 9 Ma, basement block exhumation started with the uplift of the Sierra de Umango-Espinal that was triggered by deformation along the NE-trending Tucumán oblique belt. This stage marked the beginning of compartmentalization of the incipiently deformed Vinchina foreland. Since c. 6.8-6.1 Ma, basement block uplift linked to the Miranda-Chepes and Valle Fértil NNW-trending sinistral transpressional belts, as well as kinking of the Neogene sequence by localized WNW-striking cross-strike structures, resulted in multiple segmentation that produced a complex mosaic of basement-block pieces. The overprint of these regional, basement-involved, oblique, brittle-ductile transpressional and cross-strike megazones could be related to high interplate coupling. Localized mechanical and rheological changes introduced by magmatism favoured this thick-skinned deformation overprint.