Timing the Bolivian Subandean Foreland Evolution: Disciminating Between Tectonics and Climate Forcing

Uba, Cornelius Eji¹, Axel K. Schmitt², Manfred R. Strecker³ (1) University of Potsdam, Potsdam, Germany (2) University of Los Angeles, California, Los Angeles, CA (3) University of Potsdam, Potsdam, Germany

The tectonically active Subandes fold and thrust belt of Bolivia reflects ongoing deformation, uplift, exhumation and sedimentation in the southern central Andes. To characterize sedimentation and to better constrain the timing of the Subandean evolution, and to particularly discriminate between tectonics and climate forcing in the deposition of 7.5-km-thick terrestrial sedimentary rocks, we used field observations of facies changes, structural cross sections, seismic stratigraphy, sediment budget calculations, and radiometric dating of hitherto poorly constrained sections.

U-Pb dating of zircons from 7 volcanic ash samples intercalated in sedimentary rocks constrains the onset of thrusting in the Subandes at 12.4 Ma. Detailed sedimentologic observations including facies and paleocurrent studies as well as subsidence analysis show that this area was characterized by relatively high subsidence and very low sedimentation rates of craton-sourced ephemeral fluvial systems that coexisted with ephemeral lacustrine depositional environments between 27 to 6.9 Ma. Our observations suggest that the creation of accommodation space resulted from the eastward advance of thrusting and coeval tectonic loading. As of 7 Ma we observe a further eastward advance of the deformation front and depocenters, which was coupled with the development of fluvial megafans. The sedimentologic analysis and sediment budget estimates suggest that between about 7 and 5 Ma this region was characterized by a rapid sediment supply and sedimentation rates of 1.5 mm/yr. These environmental changes toward a more humid climate are synchronous with changes in paleo-vegetation cover as indicated by trends in stable C-isotopes derived from paleosols.