Three-dimensional geometry of large-scale fault-propagation folds in the Cantabrian Zone, NW Iberian Peninsula

In the western part of the Cantabrian Zone, a transition occurs from a southern zone mainly dominated by thrusts to a northern zone where folds predominate. Detailed structural analysis within the ‘transitional’ zone reveals that large-scale fault-propagation folds constitute the structural framework of this area. The topography, together with variations in plunge of the fold axes, allows us to visualize their three-dimensional geometry. They exhibit a complex internal structure due to thrusting before, during, and after major folding. Thrusts are the structures that accommodate most of the shortening in deep levels, whereas the shortening is accommodated stratigraphically upward by major fault-propagation folds, imbricate thrust systems and minor structures. In general, during the initial stages of fold amplification, the backlimb of the antiforms thickened, whereas during the latter stages their forelimbs thinned. After folding, the major folds were translated and rotated due to emplacement of younger thrusts located in the foreland. Although these natural large-scale fault-propagation folds are more complex than many theoretical models proposed up to date, certain specific structural features observed in the field examples can be explained by comparing them with theoretical models.