Multiple geometries and modes of fault-propagation folding in the Canadian thrust belt

Abstract

A multitude of fold models have been proposed to explain the variety of fold geometries which develop in front of thrust faults. Detailed field, fabric, and photogrammetric studies of 4 fault-cored asymmetrical folds in the thin-skinned Canadian thrust belt were used to test models of fault-propagation folding. Fold geometries include combinations of angular and rounded fold surfaces, highly contorted anticlinal hinge areas, and minimal penetrative deformation or changes in bedding thickness. Interlimb angles generally decrease with increasing shortening, indicating progressive fold tightening about fixed anticlinal hinges. Extensive flexural slip thrusting toward the anticlinal axes of angular folds suggests that kink folding in thin-skinned thrust belts is aided by material transfer from both fold limbs into hinge areas. Fold geometries change dramatically along the strike of individual structures, demonstrating the non-uniqueness of fault-propagation fold geometries. No single mode of fault-propagation folding can explain the diverse fold geometries seen in the Canadian thrust belt. This geometric variability can be ascribed to the complex interplay of multiple modes of folding. In strata near the causal thrusts, oblique shear and flexural slip in triangular shear zones distribute thrust displacements into both rounded and angular folds. Simultaneous angular folding of overlying strata commonly occurs by progressive kink folding where folds tighten by flexural slip on all fold limbs until the thrust breaks through the fold. Regional and local differences in the amount of pervasive, top-to-the-craton shear needed for progressive kink folding may be partially responsible for the variability of fault-propagation fold geometries in thin- and thick-skinned orogens world-wide.