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Detrital Record of Rapid Neogene Exhumation of the Cordilleran Real, Bolivia

Jessse Mosolf; Brian Horton; Lucas Wilson; Ramiro Matos

Department of Earth and Space Sciences, University of California

Los Angeles, CA

[email protected]

The precise kinematic history of the central Andean fold-thrust belt of Bolivia remains unresolved and has been strongly debated. This preliminary study addresses timing and exhumation issues through analysis of Neogene synorogenic strata bordering the eastern flank of the Cordillera Real. The Tipuani-Mapiri basin, located northeast of the Cordillera Real, may have collected sediment during Neogene exhumation, creating the Cangalli Formation. Previous field studies have mapped and interpreted the Cangalli Formation as a piggyback basin preserving a steep paleotopography in the wedge-top depozone of the active Andean fold-thrust belt. Our preliminary fieldwork shows the Cangalli deposit to be primarily conglomerates with interbedded sandstone and mudstone lithologies of fluvial and alluvial fan origin. The formation unconformably overlies highly deformed Ordovician metasedimentary rocks and exhibits minor folding, possibly suggesting deformation synchronous with sedimentation. Conglomerate clasts were predominantly derived from metasedimentary rocks with a small proportion derived from granitic rocks. Granitic detritus unequivocally links the Cangalli Formation to Neogene exhumation of the Cordillera Real and, therefore, provides a minimum age for unroofing of the range. Exhumation may be linked to crustal shortening and/or enhanced erosion due to climate change. Ongoing geochronological analyses of interbedded tuffs from the Cangalli Formation will constrain the timing of sedimentation and related exhumation. Understanding the links between the kinematics of shortening and the history of exhumation and sedimentation in the Bolivian fold-thrust belt could greatly aid hydrocarbon exploration and generate a predictive framework for other actively uplifting thrust belts and adjacent foreland and piggyback basins.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid