Abstract

The internally drained Altiplano plateau in the high-elevation hinterland of the central Andes contains an extremely thick (up to 12 km) succession of Tertiary nonmarine strata that recorded the topographic evolution of eastern and western plateau margins. In this study, temporal and spatial variations in detrital composition (based on 113 sandstone thin sections and 31 conglomerate clast counts), combined with sediment dispersal patterns and regional stratigraphic correlations, help delineate the Tertiary history of Altiplano basin development in relationship to thrust deformation and arc magmatism along plateau margins.

Stratigraphic and provenance data suggest limited subsidence and west-directed sediment dispersal during the early Tertiary history of the Altiplano region. By late Eocene time, development of a rapidly subsiding basin (probable foredeep) was underway, as recorded by extensive east-directed fluvial systems of the 3-6.5-km-thick Potoco Formation. Potoco compositional data are compatible with sediment sources in the incipient Western Cordillera and regions farther west, including (1) a probable thrust-belt source terrane composed of Paleozoic-Mesozoic strata and Precambrian granitic/gneissic basement and (2) a subordinate magmatic-arc source terrane composed of Tertiary volcanic rocks. Increased abundances of plagioclase grains and volcanic lithic fragments upsection indicate a progressively greater arc influence, due to either enhanced volcanism or eastward arc migration.

By late Oligocene time, west-directed fluvial and alluvial-fan systems of the upper Potoco Formation and the ∼3-km-thick Coniri Formation reached the Altiplano basin, providing a minimum uplift age for the backthrust belt along the western flank of the Eastern Cordillera. Paleozoic-clast conglomerate and relatively quartz-rich sandstone containing sedimentary and metasedimentary lithic fragments show unroofing of predominantly Paleozoic strata in the backthrust belt. Continued late Oligocene to Quaternary accumulation in the Altiplano reveals that the basin persisted as a depositional region during a well-documented period of Neogene shortening and uplift in the Andean thrust belt farther east. This intra-orogenic (hinterland) deposition between eastern and western highs during ongoing shortening suggests that the Altiplano basin existed as a crustal-scale piggyback basin, possibly in an internally drained plateau-like geomorphic setting, since ∼25 Ma.

Provenance data and independent structural and thermochronological evidence for spatially and temporally varying tectonic histories along the eastern and western plateau margins imply that several modes of basin development were active during growth of the Altiplano. Although flexural subsidence due to thrust loading along both plateau margins, initially in the west and subsequently in the east, may explain most Tertiary subsidence, possible closed-basin conditions due to structural damming could have promoted additional sediment accumulation from late Oligocene to Quaternary time.

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