ABSTRACT The sub-Andean zone of southern Bolivia is a typical thin-skinned fold-and-thrust belt with remarkable regularity in the geometry and spacing of the structures. This is a typical feature of fold-and-thrust belts where the basement is not involved in the deformation. However, when the structural geometry and evolution are analyzed in detail, many deviations from such regularity are evidenced. This paper has studied the processes that might have affected the development of some of the structures along the southern sub-Andean zone. Special attention is given to the La Vertiente structure, an elongated anticline with low relief developed at the latitude of the Pilcomayo River. According to the interpretation of growth strata based on 2-D seismic lines, the beginning of the deformation for this structure has been dated by several authors at around 6 Ma. Restoration of structural cross-sections suggests that the La Vertiente structure was originated with a spacing of 65 km (40 mi), which represents two to three times the maximum spacing that characterizes the rest of the structures of the southern sub-Andean zone. With the aim of explaining this “anomaly,” different factors that could lead to an increase in the maximum spacing of this order have been analyzed. We present a model where increase erosion capacity of the Pilcomayo River, as a result of the capture of a large drainage network area that was previously part of the Parapetí River, would have generated a dramatic decrease in the effective basal friction coefficient of the Silurian shales of the Kirusillas Formation. This change, in turn, would be responsible for the transient increase in spacing between structures in the La Vertiente structure.

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.