Ordovician – Silurian transition in the Central-Iberian Zone: Just a stratigraphic boundary or a major element in the evolution of the Iberian Variscides?

Abstract

The Ordovician-Silurian boundary in the Central-Iberian Zone is often considered a normal stratigraphic transition. Nevertheless, in studies carried out in Portugal (e.g. Espinhosela, São Mamede and Buçaco areas) and Spain (e.g. Alcañices Synform) this contact is marked by the development of shear zones subparallel to the lithostratigraphic units. Ongoing structural mapping in Celorico de Basto (Marão region) shows that near Campanhó, the Ordovician (Pardelhas Formation) and the Silurian (Campanhó Formation) units are bounded by a regional shear zone with a normal kinematics (the Covelo do Monte Fault, here presented). Although their origin is debatable, the intraorogenic normal faults are common mechanisms active during Variscan times in the Central-Iberian Zone. In the case of the Ordovician- Silurian boundary, these detachments could be facilitated by shale and organic materials prone to control the development of shear zones.