Deformation and metamorphism in the Trois Seigneurs massif, Pyrenees - evidence against a rift setting for its Variscan evolution

Abstract

Low grade metamorphic rocks in the Trois Seigneurs massif characteristically exhibit steep foliations axial planar to tight folds, indicating horizontal shortening. An older deformation phase is indicated by a considerable spread of intersection lineations with bedding on the steep foliations. The steep foliations have been folded into a kilometre-scale inclined fold, which has a crenulation cleavage subparallel to the axial plane. Rotation sense of porphyroblasts, in medium to high grade rocks, is different in each limb of this fold. The fold is argued to result from late Variscan vertical shortening and horizontal extention following crustal thickening. The massif is transected by the Cap de las Costes fault, inferred to be a strike-slip fault with 5-6 km sinistral offset. Variscan metamorphism in the Trois Seigneurs massif started after the shortening phase and reached peak temperatures after the extensional phase. It is not related to a rifting event, as proposed by Wickham & Oxburgh (1985), but to late orogenic extension after considerable crustal thickening.