Active tectonics in the Subandean belt inferred from the morphology of the Rio Pilcomayo (Bolivia)
Résumé
Recent thrust tectonic mechanisms of southern Bolivia are inferred from the incision of the Rio Pilcomayo, the main river that crosses the thin-skinned Subandean thrust belt. The fluvial shear stress model, and more specifically its adaptation to calculate a nondimensional form of the excess critical shear stress, is used to relate channel river properties to the rate of fluvial incision into bedrock. The following parameters have been compiled to apply this model: (1) grain size of the bedload from field work; (2) drainage area from a digital elevation model; (3) slope of the river from 1:50,000 topographical maps and global positioning system measurements; (4) width of the river from field work and topographical maps; and (5) water discharge from published data. A comparison between the nondimensional shear stress (excess critical shear stress) of the Rio Pilcomayo and a structural cross section through the southern Subandean zone shows that the excess critical shear stress increases in the hanging wall of three thrust faults. These faults show field evidence of recent thrust activity, and we estimate the relative hanging-wall vertical motion for the thrusts based on the excess critical shear stress within the Tertiary sediments at their hanging wall. Our analysis indicates that the frontal structure and the third thrust at the boundary between the eastern and western zones of the Subandean belt are active, but the shortening on the second thrust (Aguarague) is more than half of the total thrust motion. We use and adaptation of the Suppe (1983) equations to estimate the variation of the uplift rate through a folded structure. The ratio between uplift rate and horizontal shortening varies from 1.1 in the western flank to 0.25 in the eastern flank of the Aguarague structure. A nondimensional index of erodibility (ratio between the excess critical shear stress and the horizontal shortening) is estimated from the relative uplift rate and the excess critical shear stress. This index of erodibility varies weakly, from 10 ± 2 for the Tertiary sandstones to 6 ± 2 for upper Devonian rocks. The excess critical shear stress reaches its maximum at the transition between the Subandean zone and Interandean zone. This high value cannot only be driven by tectonics, and a lithological effect (low erodibility) is inferred.