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 Mondonedo Fold nappe. Both uplifted areas are hangingwall antiforms. The thrusts below these structures ramp do wnwards towards the hinterland and flatten towards the foreland. The f irst deformation events were controlled by low-angle shears? which at the cover level produced recumbent and asymmetric folds associated wit h a slaty cleavage in the a est, and the detachment of the Cantabrian zone in the frontal part of the orogen eastwards of the Narcea antifor m. These structures were modified as deformation progressed, and two g enerations of steep-dipping crenulation cleavages were locally formed. These crenulations occur in belts separated by others in which the sl aty cleavage remains undisturbed. The youngest structures are Hat-lyin g 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 vertic al crenulations, although in some cases they cross the boundary. The a lternation of belts with vertical and horizontal crenulations is found in all scales, from the kilometre to the microscopic scale. The hat-l ying structures indicate near vertical shortening, and together with a few W-dipping normal faults can be regarded as the result of orogenic collapse. (C) 1998 Elsevier Science Ltd. All rights reserved.