INITIAL SHORTENING IN THE NORTHERN ALTIPLANO: STRATIGRAPHIC, STRUCTURAL AND GEOCHRONOLOGICAL CONSTRAINTS FROM THE AYAVIRI BASIN, PERU

The onset of shortening in the central Andes is variably defined, with estimates spanning from Late Cretaceous to late Miocene time. The Ayaviri basin in the northernmost Altiplano is an elongate, NW-trending, fault-bounded basin at ~3900 m elevation in southern Peru. It is bounded by the NW-striking Ayaviri and Pasani thrust faults. Up to 4 km of Cenozoic clastic fill rests on Paleozoic strata to the north and Cretaceous carbonates to the south. Interbedded ignimbrites provide a geochronological framework for Cenozoic basin development. New measured sections, lithofacies analyses and conglomerate clast counts characterize Members A, B, C and D of the ~1.8 km thick Tinajani Formation (~27–14 Ma), and help reveal the sedimentological response to basin-margin shortening. Members A, B and C consist of fine- to coarse-grained, thin-bedded (<1 m), horizontally laminated to cross-bedded fluvial deposits with limited paleosol and lacustrine carbonate intervals. Member D consists of thick-bedded (<5 m), crudely stratified to massive, cobble-boulder conglomerate alluvial-fan successions with laterally discontinuous, massive or cross-bedded coarse-grained sandstones. Paired with existing geochronology of interbedded volcanic rocks, we employ new detrital zircon U-Pb geochronology to further define the accumulation history of the Tinajani Formation. The abrupt coarsening represented by Member D commenced at ~17 Ma, with a coeval increase in sedimentation rates from ~150 to ~300 m/Myr. Concurrent with this coarsening and increased accumulation is a provenance shift from approximately south- to north-directed sediment dispersal. Conglomerate clast counts corroborate this shift in transport direction. The uppermost Tinajani Formation is incorporated into a footwall growth syncline as a result of syndepositional thrust displacement along the Pasani fault. New structural analyses and geologic mapping identify hanging-wall fault relationships and define the footwall stratigraphic levels marking the onset of growth strata in Member D. These new data help constrain the Oligocene-Miocene sedimentation history during Ayaviri basin evolution, suggesting that the major shifts in depositional systems and onset of growth strata accumulation at ~17 Ma were genetically linked to the onset of Pasani fault motion.