The making of a high elevation plateau: Insights from the central Andean Plateau, Bolivia

摘要

The structural link that ties the formation of the central Andean plateau to the evolution of the Andean fold-thrust belt is formation and propagation of basement megathrusts. Balanced cross-sections through a regional west-vergent thrust system in the hinterland of the eastward-verging Andean fold-thrust belt suggest that the Altiplano basin is a crustal-scale, piggy-back basin created as a basement megathrust propagated up and over a half-crustal scale ramp located on the eastern edge of the Altiplano. The backthrust belt, detached above the basement thrust sheet, acted as a crustal-scale, passive-roof duplex building taper after the megathrust overextended the system eastward. Pervasive shortening and vertical elongation seen at the outcrop and regional scales supports the development of the backthrust belt as a taper-building mechanism. The interrelationship of the backthrust belt, basement megathrust, and Andean plateau is shown in a sequential, kinematic model based on balanced cross sections from the volcanic arc to the foreland. The model links the formation of the plateau to east- and west-verging portions of the Andean fold-thrust belt through the eastward propagation of two large basement megathrusts. The megathrusts can explain topographic and structural steps in the Andean fold-thrust belt and accommodate a minimum of 300-330 km of shortening which matches shortening estimates in the tightly folded and faulted cover rocks. To a first approximation, 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. The 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. Combining the history of foreland basin migration with palinspastically restored regional cross sections across the Bolivian Andes between 18°-20°S argues for an eastward migrating fold-thrust belt/foreland basin system since the late Cretaceous. The longer time span for the Andean orogeny implies greater shortening amounts, a decrease in shortening rates with time and a high (3-4 km) Andean plateau by 20 Ma.