Areal balancing and estimate of areal reduction in a thin-skinned fold-and-thrust belt (Cantabrian zone, NW Spain): constraints on its emplacement mechanism

The Cantabrian zone is a curved thrust-and-fold belt forming the frontal part of the Hercynian orogen in northwestern Spain. The present day structure is the result of interference between different kinds of structures describing the arc and a set of cross folds, showing a radial pattern. Some of the structures can be traced all around the arc, while some others are of limited lateral extent, so that the displacement they produce is transferred to some other kind of structure. Taking into account geometrical data and field evidence on the direction of thrust motion an area-balanced model has been obtained. This model permits the restoration of the Cantabrian zone to its pre-deformed stage, and shows that the areal reduction has been about 50%. Globally considered, the structure of the Cantabrian zone corresponds to a detached and deformed wedge of sediments which was being thrusted and folded as it was being subjected to translation. The wedge before deformation was flat-topped and had a basal backslope, but as it was being deformed a surface foreslope was generated. Slope and thickness evaluations fulfil the required theoretical conditions for the décollement to be possible under compressional conditions. Lack of tectonic denudation at the back, persistence of the basal backslope and the general structure of the belt indicate compression and crustal shortening. Only in the most frontal part of the belt, gravitational gliding of rock slabs took place and chaotic mixtures were formed, contributing to the infilling of the Carboniferous basin in the very core of the arc. © 1986.