The Late-Variscan fault network in central–northern Portugal (NW Iberia): a re-evaluation

Abstract

The fault network in central–northern Portugal, especially the fault system with mean strike N25°, has been used to help deduce the late Palaeozoic dynamics of Western Europe. On the other hand, the N80° strike–slip fault system was recognized in previous works as Late-Variscan and left lateral, but was scarcely mapped and its importance neglected. This study shows that the N25° faults were dextral in the late stages of the Variscan Orogeny and sinistral during the Alpine Cycle due to pervasive reactivation. Fault rocks and intrusions are clearly different, according to age: mostly high temperature quartz infillings, but locally muscovite, tourmaline, and aplite dykes in Late-Variscan times, and low temperature cataclasites, fault gouges and Mesozoic mafic dykes in younger Alpine times. We dated dextral N45° segments of the N25° fault system because they are less prone to reactivation by the NNW–SSE Alpine compression, and can thus preserve the Variscan record. We used K–Ar in muscovite concentrates and obtained a minimum age of the analysed faults of ca. 312 Ma, which sets a lower limit to the so-called Late-Variscan wrench-faulting period. The present study shows that the N80° fault set is pervasive and sinistral in central–northern Portugal, and therefore, that it only admits one brittle dextral conjugate, the N25° fault system. Both were generated by a maximum compressive stress bearing between N50° and N55° in azimuth. We did not find evidence of a Variscan sinistral strike–slip movement in the N25° fault system, and therefore, this kinematics is believed to represent only the displacements accommodated during the Alpine Cycle.