Geometric analysis of fold development in overthrust terranes

Abstract

Fault-bend folding, fault-propagation folding, and detachment (or décollement) folding are three distinct scenarios for fold-thrust interaction in overthrust terranes. Simple kink-hinge models are used to determine the geometric associations implicit in each scenario. Bedding maintains constant thickness in the models except in the forelimb of the fold. The forelimb is allowed to thicken or thin without limit. The models address individual folds, and the calculated fold geometries are balanced structures. Each mode of fold-thrust interaction has a distinct set of geometric relationships. Final fold geometry is adequate in itself to discern many fault-bend folds. This is not the case for fault-propagation and detachment folds. These two fold forms have very similar geometric relationships. Some knowledge of the nature of the underlying thrust or décollement zone is usually needed to distinguish between them. The geometry of a fold is altered, in a predictable fashion, by transport through an upper ramp hinge and by fault-parallel shearing of the structure. The shearing results in a tighter fold, whereas transport through the ramp hinge produces a broader fold. The viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the Canadian Cordillera. The geometric analysis is also used to evaluate cross-sections through subsurface structures. In an example from the Turner Valley oil field, the analysis indicates how the interpretation should be altered so as to balance the cross-section. The analysis reveals hidden assumptions and specific inconsistencies in structural interpretations.