Mechanical stratigraphy influence on fault-related folds development: Insights from the Cantabrian Zone (NW Iberian Peninsula)

An excellently exposed outcrop of Carboniferous rocks in the Cantabrian Zone (Variscan foreland fold-thrust belt in NW Iberia) displays fault-bend, fault-propagation and detachment folds. To unravel the parameters that controlled their development, we constructed detailed cross-sections and analysed them. Detachment folds exhibit the greatest amounts of layer-parallel/bulk strain, forelimb dip and forelimb/hinge thickening and the lowest interlimb angle, whereas fault-bend folds have the lowest values except for the interlimb angle, with fault-propagation folds exhibiting intermediate values. The forelimbs of all these folds show some strain and thickening, and the detachment folds also show thickening and strain in the hinge area. Mechanical stratigraphy was determined to be the main controlling factor on the fold/thrust style; ramp folds developed in thick-bedded, isotropic, relatively strong and brittle rocks, whereas detachment folds developed in a thin-bedded, anisotropic, relatively weak and ductile unit. Competent rocks and smooth bedding surfaces induced fault-bend folding, whereas less competent and rough bedding surfaces favoured fault-propagation folding. The main detachments are located at the boundaries between mechanical units with substantial changes in their mechanical properties. The size of the structures depends on the occurrence of a basal detachment, variety of lithologies with different competences and smoothness of bedding surfaces.