STRUCTURAL AND METAMORPHIC EVIDENCE OF LOCAL EXTENSION ALONG THE VIVERO FAULT COEVAL WITH BULK CRUSTAL SHORTENING IN THE VARISCAN CHAIN (NWSPAIN)

The Vivero fault is a W-dipping, NS-striking ductile shear zone separa ting the Ollo de Sapo antiform in its western hangingwall and the Lugo dome in its eastern footwall. Two stages of deformation (F-1 and F-2) produced nearly coaxial folds with sub-horizontal axes. A crenulation cleavage S-2 transposes an older S-1. Three sets of shear bands in th e hangingwall define a pervasive fabric consistent with an E-W bulk sh ortening perpendicular to a composite S-1-2 foliation and NNE-stretchi ng parallel to L(2). The Vivero fault zone is marked by a mylonitic fo liation with a steeply NW-plunging stretching lineation and extensiona l crenulation cleavage (ECC) indicating normal slip. In the vicinity o f the fault, sub-horizontal NNE-trending F-3 folds, with a crenulation cleavage S-3, deform earlier-formed fabrics, including a mylonitic fo liation. Pressure-temperature conditions obtained from mineral assembl ages on both sides of the Vivero fault yield a minimum throw of 5.5 km . Andalusite-bearing pelite in the hangingwall was infolded by an F-2 synform into the kyanite field at 450-500 degrees C. The eastern edge of these rocks was later accreted to the footwall and heated to andalu site-staurolite conditions at similar to 600 degrees C. Slip on the Vi vero and Valdovino faults is kinematically related. East-west shorteni ng during F-2 involved folding and sinistral strike-slip on the Valdov ino fault which induced local extension along the newly generated Vive ro fault. Synkinematic emplacement of granitoids along the Vivero faul t is favoured by extension. Coeval slip on both faults took place duri ng the later stages of F-2 folding. Geometrical constraints caused nor thwards escape of the crustal block bounded by the Valdovino and Viver o faults, recorded by NNE-stretching defined by L(2).