Middle Jurassic shear zones at Cap de Creus (eastern Pyrenees, Spain): a record of pre-drift extension of the Piemonte–Ligurian Ocean?

Abstract

The Cap de Creus peninsula in NE Spain consists of greenschist- to amphibolite-facies metasediments and granitoid bodies of the Variscan Axial Zone of the Pyrenees, overprinted in the north by anastomosed greenschist-facies shear zones. Current tectonic interpretations ascribe these shear zones to the waning stages of the Variscan orogeny. We present muscovite 40Ar/39Ar data from the shear zones, yielding Middle Jurassic ages between 159.33 ± 0.43 and 175.18 ± 1.10 Ma and one Tertiary age of 58.57 ± 0.55 Ma. We suggest that the present-day structure at Cap de Creus resulted from Variscan deformation and HT–LP metamorphism, followed during the Jurassic by crustal stretching and development of ductile normal faults reflecting pre-drift continental extension related to opening of the Piemonte–Ligurian basin east of Iberia. Tilting during Alpine convergence caused steepening in the northern part of the penisula, with the ductile normal faults rotated to their present orientations appearing as dextral reverse shear zones. The shear zone yielding a Tertiary age could reflect either an Alpine structure or reactivation of an earlier, presumably Jurassic shear zone. The Cap de Creus structure may thus represent a continental margin that has undergone ductile stretching equivalent to the now-buried west Iberian or Newfoundland margin.