Structures and mechanisms associated with development of a fold in the Cantabrian Zone thrust belt, NW Spain

A detailed history of progressive fold development based on mesostructural analysis is provided for a metre-scale fold in the Cantabrian fold and thrust belt of northwestern Spain. The well-exposed asymmetric syncline displays a wide variety of structures including extensional faults, thrusts, veins, tectonic stylolites, bedding plane faults, drag folds, and a main limb thrust that transects the entire fold. Important constraints on fold development are provided by the spatial distribution and style of deformation, angular relations, and cross-cutting relations among the structures. Fold mechanisms varied as a function of structural position; while the northwest limb was subjected to pure shear shortening in the form of contractional faults, the southeast limb was bent under tangential longitudinal strain, resulting in a combination of outer-arc extensional faulting in dolostones and simple shear along interbedded shales. Sequential development of the Bلrzana fold consisted of; (i) initial flexural slip folding until the southeast limb attained a dip of 60°; (ii) a phase of pressure solution upon locking of bedding plane faults; (iii) shortening accommodated by bed-confined mesofaulting and the propagation of a limb thrust; (iv) unlocking of bedding plane faults and resumption of flexural slip; and (v) propagation of a new limb thrust due to offset of the main thrust by flexural slip. This study demonstrates that a number of very different mechanisms can operate during the course of fold development, and that folding processes and geometries are strongly dependent upon structural position, mechanical stratigraphy, and magnitude of tectonic shortening.