Controls on thrusts spacing; an example from the Sub-andean fold-and-thrust belt on southern Bolivia

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 Parapeti 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.