Revised deformation history of the central Andes: Inferences from Cenozoic foredeep and intermontane basins of the Eastern Cordillera, Bolivia

摘要

Investigation of Cenozoic deformation, sediment accumulation, and provenance in the Eastern Cordillera of Bolivia at 17-21°S indicates major shortening (60-140 km) and foredeep development followed by limited internal shortening and intermontane basin development. Contrasting histories of shortening, deposition, sediment dispersal, and detrital composition distinguish a formerly extensive Paleogene foredeep exposed along an eastern belt of synclines from a zone of principally Neogene intermontane basins in the central Eastern Cordillera. New ~(40)Ar/~(39)Ar ages of 25-17 Ma for interbedded tuffs reveal latest Oligocene-early Miocene accumulation in intermontane basins at rates < 80 m/Myr, several times lower than Andean foredeeps. Although poorly dated, foredeep evolution probably coincided with middle Eocene-Oligocene deformation and denudation in adjoining regions to the west. A mid-Cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement-involved tectonic wedge beneath the Eastern Cordillera. In this interpretation, wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 Ma, whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas. Regardless of the regional subsurface structural geometry, ~(40)Ar/~(39)Ar ages define a 25-21 Ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the Eastern Cordillera and possibly the age of initial shortening in the Interandean and Subandean zones to the east. In contrast to several popular models, the revised tectonic history presented here suggests significant pre-Neogene shortening in the Eastern Cordillera and underscores the large uncertainties in estimates of long-term shortening rates in the Andes.