Strike-slip shear zones of the Iberian Massif: Are they coeval?

Strike-slip shear zones of the Variscan orogen are used to derive the evolution of paleostrain and discuss the kinematics of the waning stages of the Gondwana-Laurussia collision during the amalgamation of Pangea. In the Iberian Massif, the recognition of three late Carboniferous deformation events related to strike-slip tectonics (D-3, D-4, D-5) in the Trancoso-Pinhel region (Portugal) reveals that late orogenic transcurrent deformation was episodic and occurred in a short period of time (<15 m.y.). Early stages of strike-slip deformation included dextral and sinistral shear zones and orogen-parallel upright folds (D-3; ca. 311 Ma). These structures followed the development of extensional shear zones (D-2) during the tectonothermal reequilibration of the orogen. D-3 structures were deflected and folded by the sinistral D-4 Juzbado-Penalva do Castelo shear zone, dated as ca. 309-305 Ma by SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon dating of synkinematic granitoids. D-3 and D-4 structures were folded under east-west compression (D-5) influenced by the strike-slip movement of the dextral Porto-Tomar shear zone. Variscan movement along the Porto-Tomar shear zone started ca. 304 Ma (onset of the Bucaco basin and syn-D-5 granites), but ceased before ca. 295 Ma (age of the final closure of the Ibero-Armorican arc and crosscutting granites). The contrasting geometry, kinematics, and timing of these strike-slip shear zones are explained by deformation partitioning upon a rheologically inhomogeneous crust with structural and tectonothermal anisotropies generated during previous deformation. The convergence vector between Gondwana and Laurussia during D-3-D-5 remained the same, and was equivalent to the vector that explains the previous tectonic record (D-2) in central and northwestern Iberia.