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Tertiary Exhumation of the Cordillera Real: Linkages with Sedimentation and Shortening in the Advancing Andean Fold-Thrust Belt, Bolivia

Mosolf, Jesse 1; Horton, Brian K.2; Matos, Ramiro 3
1 Earth Science, UCSB, Santa Barbara, CA.
2 Department of Geological Sciences and Institute for Geophysics, University of Texas, Austin, TX.
3 Instituto de Investigaciones Geologicas y Medio Ambiente, Universidad Mayor de San Andres, La Paz, Bolivia.

The Cenozoic kinematic history of the central Andean fold-thrust belt of Bolivia remains unresolved. This study addresses the issues regarding timing and exhumation processes of the central Andes through the analysis of Neogene strata (Cangalli Formation) in the Tipuani-Mapiri basin. The Tipuani-Mapiri basin lies below the eastern flank of the Cordillera Real, a ~4.5-6 km-high mountain range along the Altiplano-Eastern Cordillera boundary, and served as a major sediment catchment during the Neogene.

Seven individual lithofacies and three lithofacies associations for the Cangalli Formation are interpreted to be braided-river, overbank, and shallow-lacustrine deposits. Provenance data link sedimentation in the Tipuani-Mapiri basin to exhumation of the Cordillera Real. (U-Th)/He cooling ages for granitic detritus sampled from the Cangalli Formation suggest the Cordillera Real to have undergone an estimated ~5.5 km of exhumation between ~42 and 18 Ma, with average cooling rates of ~5 to 18 °C/Myr. Cooling ages likely correlate with rapid exhumation of the Cordillera Real, as determined by previous thermochronometric studies.

The nearly flat-lying Cangalli Formation rests unconformably on folded and faulted Paleozoic rocks, suggesting that the local style of deformation changed prior to sedimentation in the Tipuani-Mapiri basin. 40Ar/39Ar isotopic dating of sanidine from an interbedded volcanic tuff yielded an age of 9.159 ± 0.032 Ma, providing a minimum age constraint for the onset of sedimentation in the Tipuani-Mapiri basin and the cessation of local shortening in the Bolivian fold-thrust belt.

The driving force behind sediment accumulation in the Tipuani-Mapiri basin remains unclear. Climate change and intensified erosion; deformation of the Subandes and resultant sediment ponding; emplacement of large thrust sheets in the lower crust and associated flexure; or a combination of the above processes are considered potential factors driving the onset of Neogene sedimentation in the Bolivian fold-thrust belt.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009