The kinematic history of the central Andean fold-thrust belt, Bolivia: Implications for building a high plateau

Abstract

This paper presents a model for the kinematic evolution of the central Andean plateau based on balanced cross sections across the Bolivian Andes. The proposed model links the formation of the Andean plateau to the development of the Andean fold-thrust belt through the creation and propagation of two large basement mega thrusts. Support for large, basement- involved thrust sheets is found in significant steps in both the topography and the exposed structural elevation of the Andean fold-thrust belt. The structurally highest basement thrust raised folds and faults in predominantly lower Paleozoic rocks of the Eastern Cordillera with respect to Tertiary rocks in the broad, internally drained basin of the Altiplano to the west, and east- verging folds and faults in upper Paleozoic rocks of the Interandean zone to the east. The Interandean zone was in turn raised (both structurally and topographically) with respect to the frontal folds and faults of the fold-thrust belt (the Subandean zone) by a second, structurally lower basement thrust sheet. Thus, these two megathrusts divide the Andean fold-thrust belt into four areas of markedly different structural elevations. The Eastern Cordillera can be further subdivided into two zones of west- and east-vergent folds and thrusts. Shortening accommodated by the fold-thrust belt can be divided among these tectono-structural zones and linked to shortening accommodated by the inferred basement megathrusts. The proposed kinematic model suggests that the eastward propagation of the structurally highest basement thrust fed ∼105 km of slip into the Eastern Cordillera along east-vergent and west-vergent faults. This structure also fed ∼90 km of eastward slip into the Interandean zone. The initiation and eastward propagation of a lower basement thrust structurally elevated the Interandean zone with respect to the foreland while feeding ∼65 km of slip into the Subandean zone. Out-of-sequence basement thrusting to the west is proposed to have elevated the western edge of the plateau and accommodated ∼40 km of shortening within the Altiplano. Total cumulative shortening within the cover rocks of the Andean fold-thrust belt (300–330 km) can be balanced by an equivalent amount of shortening along two basement megathrusts. To the first order, the eastern margin of the central Andean plateau (defined by the 3 km topographic contour) is contiguous with the leading edge of the upper basement megathrust. This relationship between the basement highs and the physiographic boundaries of the Andean plateau suggests that extensive megathrust sheets (involving strong rocks such as crystalline basement or quartzite) play an important role in the formation of the central Andean plateau, and a similar link between megathrust sheets and plateaus may be found in other orogens.