The central Andean backthrust belt is a large-scale west vergent thrust system along the western side of the Eastern Cordillera in the generally east vergent Andean fold-thrust belt of Bolivia. Although west vergent structures in the central Andes have been recognized previously, we describe the backthrust belt at a regional scale, emphasizing its implications for the kinematic development of the Andes and the subsequent influence of these kinematics on amounts of tectonic shortening. We use techniques such as line length balancing, restorability, and the viability of the progressive development of the structures to construct balanced cross sections across the backthrust belt and Altiplano. The cross sections are taken to a regional depth of detachment (basement) to examine the relationship between mapped surface structures and inferred subsurface structures. The relationship of the backthrust belt to the Altiplano suggests that the Altiplano basin is a crustal-scale piggyback basin created as a basement megathrust propagated up and over a half-crustal scale ramp located just west of the physiographic boundary of the Eastern Cordillera. This basement megathrust was the means by which a narrow Paleocene fold-thrust belt located to the west of the Altiplano propagated eastward and emerged in the present Eastern Cordillera. The relationship between the basement thrusts and the physiographic boundaries of the Central Andean plateau (as defined by Isacks ) suggests that extensive megathrust sheets (involving crystalline basement or quartzite) may play an important role in the formation of orogenic plateaus. The kinematic development of the Andean fold-thrust belt indicates that the backthrust belt developed as a taper-building mechanism after the basement megathrust overextended the system eastward. The mechanism proposed in this study for the development of the central Andean backthrust belt requires a minimum of 200 km of shortening within the Altiplano/Eastern Cordillera alone. This increases minimum shortening estimates across the fold-thrust belt in Bolivia to as much as 300-340 km.
ASJC Scopus subject areas
- Geochemistry and Petrology