Small-scale structures formed during progressive shortening and subsequent collapse in the Navia-Alto Sil slate belt (Hercynian fold belt, NW Spain)

A prominent unit of the Hercynian orogen in north-western Spain is the Navia-Alto Sil slate belt, which is formed predominantly by Llanvirn-Caradocian slates (lower-upper Ordovician). From a structural point of view, the belt forms an infolded syncline between two uplifted areas, the Narcea Antiform and the Mondoٌedo fold nappe. Both uplifted areas are hangingwall antiforms. The thrusts below these structures ramp downwards towards the hinterland and flatten towards the foreland. The first deformation events were controlled by low-angle shears, which at the cover level produced recumbent and asymmetric folds associated with a slaty cleavage in the west, and the detachment of the Cantabrian zone in the frontal part of the orogen eastwards of the Narcea antiform. These structures were modified as deformation progressed, and two generations of steep-dipping crenulation cleavages were locally formed. These crenulations occur in belts separated by others in which the slaty cleavage remains undisturbed. The youngest structures are flat-lying crenulations, small chevron folds and kink bands which develop where the slaty cleavage has not been previously crenulated. The flat-lying small-scale structures end, in general, against the bands with vertical crenulations. although in some cases they cross the boundary. The alternation of belts with vertical and horizontal crenulations is found in all scales, from the kilometre to the microscopic scale. The flat-lying structures indicate near vertical shortening, and together with a few W-dipping normal faults can be regarded as the result of orogenic collapse.