POLYPHASE MIOCENE DEFORMATION IN THE ARGENTINE ANDES: EVIDENCE FROM THE VELADERO NORTH AREA

The geometry and timing of Tertiary deformation is crucial to tectonic models for the Andes of Chile and Argentina. In the Veladero North area (29°S; Argentina) near the Chile-Argentina border, thrusts that dip steeply (70°-90°) to the west and back thrusts that dip steeply (70°-90°) to the east define the overall geometry of the Miocene fold and thrust belt from east to west. Permian (250-260 Ma; U-Pb zircons) rhyolitic volcanic and volcaniclastic rocks and late Oligocene to Miocene (24-23 Ma; U-Pb zircon) andesitic to rhyodacitic lavas, pyroclastic rocks, and sedimentary rocks of the Tilito Fm. form the thrust panels. Each structural sheet contains a unique stratigraphic sequence not repeated in other sheets. Volcaniclastic rocks ~16Ma old (U-Pb zircon), equivalent to the Cerro de las Tórtolas Fm., overlie a regional low-relief unconformity, the Frontera Deidad (FD) surface. These rocks and subjacent erosional surface at Veladero North dip shallowly (~20° E) or are sub-horizontal. The FD surface lies at 5200 m elevation west and east of Veladero North, whereas it lies at 4200 m elevation at Veladero North. A younger regional low-relief pediment, the Azufreras-Torta (AT) surface, is cut on tilted 16-Ma rocks and is locally covered by 11 Ma volcanic rocks. This surface is topographically higher than the FD surface near Veladero North, an elevation difference inconsistent with regional relations throughout the El Indio belt. The FD surface elsewhere is 200-300 m topographically higher than the AT surface. The regional relations thus indicate two Miocene deformation events near Veladero North. A major fold and thrust belt formed between 23 Ma and 21 Ma, if regional data is considered. The FD surface bevelled that terrane. Tilting of the FD surface after 16 Ma is best explained by large amplitude folds. Steeping of early Miocene thrusts to near vertical dips likely occurred at this time. The AT surface cut the tilted rocks before 11 Ma. Post-11 Ma normal slip on northerly-striking faults reactivated older thrusts faults, further disrupting the Miocene fold and thrust belt and regional erosional surfaces.