Tectonic and Climatic Controls on Rapid Exhumation Along the Altiplano-Eastern Cordillera Boundary, Bolivia

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Researchers from the University of California Los Angeles (UCLA) are investigating the role of tectonics and climate in driving rapid erosional exhumation in the central Andes Mountains of South America. In northern Bolivia, the central Andes are host to some of the highest topography, most rapid rates of exhumation, and greatest concentration of isotopic age data (thermochronological results) in the entire Andean mountain belt. The region also displays one of the most dramatic climatic gradients on Earth, from the extremely humid conditions of the Amazon watershed in the north to the arid conditions of the Parana watershed in the south. The focus of this multidisciplinary study is the evolution of a 4.5-6-kilometer-high narrow mountain range that defines the main physiographic divide between the Altiplano plateau and Amazon drainage system. Cenozoic uplift of this 30 by 250 kilometer range in the Eastern Cordillera had far-reaching effects, arguably (1) triggering growth of the 4-kilometer-high central Andean plateau, (2) inducing closed drainage in the Altiplano, (3) modifying regional climate by orographic effects, and (4) setting up a dynamic interplay between erosion and tectonics in a possible steady-state orogen. Previous isotopic ages from thermochronological studies generate conflicting interpretations which alternately invoke heating or cooling during either brief pulses (less than several million years) or prolonged episodes (several tens of millions of years) of deformation, erosion, and magmatism. The UCLA researchers are analyzing geologic structures, sedimentation, and thermal histories along two short (~30-kilometer-long) transects in northern Bolivia in order to understand erosional exhumation of the narrow mountain range in relationship to deformation and sedimentary basin development. Ongoing thermochronological analyses of the range and sedimentological studies of an adjacent basin are determining the function of individual structures in the range's cooling history as well as the timing of deformation and basin genesis. The project includes collaboration with Bolivian scientists and involves undergraduate and graduate students from the United States and Bolivia in field and laboratory research.Support for the project is provide by the Tectonics Program and by the Americas Group of the Office of International Science and Engineering.