Controls on thrusts spacing: An example from the sub-Andean fold-and-thrust belt of Southern Bolivia

ROCHA EMILIO; CRISTALLINI ERNESTO OSVALDO

Petroleum basins and hydrocarbon potential of the Andes of Peru and Bolivia

American Association of Petroleum Geologists

Lugar: Boulder ; Año: 2017; p. 1 - 16

The sub-Andean zone of southern Bolivia is a typical thin-skinned fold-and-thrust belt withremarkable regularity in the geometry and spacing of the structures. This is a typical featureof 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 fromsuch regularity are evidenced. This paper has studied the processes that might have affectedthe 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 lowrelief developed at the latitude of the Pilcomayo River. According to the interpretation ofgrowth strata based on 2-D seismic lines, the beginning of the deformation for this structurehas been dated by several authors at around 6 Ma. Restoration of structural cross-sectionssuggests that the La Vertiente structure was originated with a spacing of 65 km (40 mi), whichrepresents two to three times the maximum spacing that characterizes the rest of the structuresof the southern sub-Andean zone. With the aim of explaining this ?anomaly,? differentfactors that could lead to an increase in the maximum spacing of this order have beenanalyzed. We present a model where increase erosion capacity of the Pilcomayo River, as aresult of the capture of a large drainage network area that was previously part of the ParapetíRiver, would have generated a dramatic decrease in the effective basal friction coefficient ofthe Silurianshales of the Kirusillas Formation. This change, in turn, would be responsible forthe transient increase in spacing between structures in the La Vertiente structure.