Crossing the several scales of strain-accomplishing mechanisms in the hinterland of the central Andean fold–thrust belt, Bolivia

Abstract

Depictions of structures at outcrop, regional and tectonic scales enforce horizontal shortening and vertical thickening as the predominant style of deformation at all scales within the hinterland of the central Andean fold–thrust belt. Outcrop-scale structures document a progression of strain that created: (1) flexural-slip folds, (2) fold flattening via axial-planar cleavage, (3) vertical stretching via boudinage and late-stage faulting and, finally, (4) kink folding. These examples of intraformational deformation are generally concentrated just beyond the tip lines of thrust faults, where fault-propagation folds and related structures are well developed. Fault-propagation folding accommodated the accrual of strain indicated by outcrop-scale structures while the structures themselves indicate how deformation developed within each individual fold. Fault-propagation fold geometries at a regional scale emerge from the construction of regional balanced cross-sections. The sections were drawn with careful attention to: (1) known map relationships, (2) field inspection of key contacts, (3) bedding–orientation data, (4) local seismic control, and (5) principles of balance. The pervasive ESE–WNW shortening and vertical elongation seen at the outcrop and regional scales developed during the formation of the central Andean backthrust belt in the hinterland of the Andes. 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. Strain accrual within this west-verging zone of deformation is proposed to be a taper-building mechanism that allowed the fold–thrust belt to continue propagating eastward.