Structural and metamorphic evidence of local extension along the Vivero fault coeval with bulk crustal shortening in the Variscan chain (NW Spain)

Abstract

The Vivero fault is a W-dipping, N-S-striking ductile shear zone separating 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 the hangingwall define a pervasive fabric consistent with an E-W bulk shortening perpendicular to a composite S 1–2 foliation and NNE-stretching parallel to L 2. The Vivero fault zone is marked by a mylonitic foliation with a steeply NW-plunging stretching lineation and extensional crenulation cleavage (ECC) indicating normal slip. In the vicinity of the fault, sub-horizontal NNE-trending F 3 folds, with a crenulation cleavage S 3, deform earlier-formed fabrics, including a mylonitic foliation. Pressure-temperature conditions obtained from mineral assemblages 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°C. The eastern edge of these rocks was later accreted to the footwall and heated to andalusite-staurolite conditions at ∼600°C. Slip on the Vivero and Valdoviño faults is kinematically related. East-west shortening during F 2 involved folding and sinistral strike-slip on the Valdoviño fault which induced local extension along the newly generated Vivero fault. Synkinematic emplacement of granitoids along the Vivero fault is favoured by extension. Coeval slip on both faults took place during the later stages of F 2 folding. Geometrical constraints caused northwards escape of the crustal block bounded by the Valdoviño and Vivero faults, recorded by NNE-stretching defined by L 2.