STRATIGRAPHY AND DETRITAL ZIRCON PROVENANCE OF THE MULASI ANTICLINE, SW BOLIVIA, AND INSIGHTS INTO SEDIMENT ROUTING IN THE PROTO-ANDEAN FORELAND BASIN

The central Andes are dominated by two rugged, high-altitude (peak heights >5000m) mountain chains, the Western and Eastern Cordillera, which run parallel to the coastline. These two mountain chains bound an internally drained, high-altitude (average elevation >4000m) hinterland basin in the Altiplano, and cratonward of the Eastern Cordillera a low relief foreland basin extends thousands of kilometers eastward into South America. The Western Cordillera has been the site of an active volcanic chain since the Late Cretaceous and remains so today. Andean exhumation began in the Western Cordillera in the Late Cretaceous and continued through the early Eocene before the deformation front underwent a rapid eastward migration, focusing exhumation on the Eastern Cordillera in the Eocene through early Miocene. The tectonically active foothills of the Eastern Cordillera, the Subandean zone, continue to expand today as the deformation front propagates further eastward. Presntly, the Eastern Cordillera constitutes an imposing topographic barrier that precludes major sediment routing from the Altiplano to the foreland. However, the timing of the formation of this barrier remains ambiguous, and possible paleodrainage linkages between these two basins have not been well constrained.

The Mulasi Anticline in southwestern Bolivia exposes a stratigraphic sequence mapped as Cretaceous Puca Group and the Paleocene - Eocene Potoco Formation. A suite of 14 detrital zircon samples from sandstones of this sequence provide new chronological constraints on the timing of deposition as well as insights into paleodrainage patterns in the Altiplano. Comparison to zircon data from age-equivalent strata exposed in the Camargo Syncline reveal differing sediment sources, suggesting the presence of drainage divide at about the location of the modern Eastern Cordillera as early as ~52 Ma. This drainage pattern could be the result of either a pre-existing topographic barrier that predates estimates of Eastern Cordilleran uplift by tens of millions of years, or by a depocenter in a well-defined foreland basin system which later experienced basin inversion as the deformation front migrated from the Altiplano to the Eastern Cordillera. To further distinguish between these two possibilities, detrital thermochronology may be necessary as a complement to traditional provenance reconstruction.