GEOMORPHIC HISTORY OF RIVERS DRAINING THE EASTERN ANDEAN CORDILLERA (34–37°S) CONSTRAINED BY TEPHROCHRONOLOGY, U-SERIES DATING OF PEDOGENIC CARBONATE AND COSMOGENIC 3HE DATING OF BASALT FLOWS

Alluvial deposits and associated geomorphic features are dated by their relation with volcanic rocks. The age range and geologic setting requires a broad approach to constraining the history of rivers draining the Cordillera. Maximum age estimates are provided by identification of the ~ 450 ka Diamante Tuff in fill terraces. Along the Río Diamante this ash bed is observed >100 m above modern river level. In the Río Papagayos and Río Atuel drainages, the Diamante Tuff is associated with alluvial surfaces much closer to modern river level. Coarse Diamante pumice in the Río Atuel implies significant changes to the headwater drainage system since 450 ka. The maximum age constraint implied by occurrence of the Diamante Tuff in fill terraces has been successfully combined with minimum age estimates from cosmogenic ¹⁰Be approaching 350 ka (Baker et al., 2009). The relatively old age of alluvial surfaces in the region is additionally supported by U-series age estimates derived from pedogenic carbonate in volcanic soils. A minimum age in excess of 100 ka is conservative. The dated surface is underlain by a pumice/lapilli tephra deposit and basalt flows both of which have the potential to provide maximum age estimates for the surface. Conversely, the U-series data implies that the basaltic volcanism is older than 100 ka. Our age estimates of flows in several drainages are much younger. Cosmogenic ³He concentrations in hornblende from basaltic-andesites erupted along the Río Salado indicate exposure, and therefore eruption, ages younger than ~ 6 ka. These flows temporarily dammed the Río Salado in one location and bedrock incision below the level of the flows has occurred since. ³He concentrations in olivine from basaltic rocks at northeastern Volcán Payún Matru indicate a shield-building phase at ~ 40 ka. Recent basaltic aa flows from multiple vents are morphologically quite young and ³He exposure ages are forthcoming for one of them. The Río Grande has incised the older flows, and provides an average incision rate over a full glacial cycle. Combination of geochronological data from the region indicates provides accurate, if not tightly constrained, ages for alluvial surfaces and identifies spatially variable geomorphic rates influenced, in part, by contemporaneous volcanism.