Superposition of shear zones during orogenic development: an example from the NW Variscan Belt (Viana do Castelo, NW Portugal)

The Viana do Castelo region of NW Portugal is the only domain in the West Variscan where the transition from the D1 strike-slip structures to the D1 east vergent flat-lying structures, formed during Devonian–Carboniferous continental collision, has been recognized. Although intrusion of voluminous granite bodies has resulted in extensive recrystallization of host rocks, kinematic analysis of these rocks, using mesoscopic criteria and quartz CPOs (crystallographic preferred orientation) reveals shear senses coherent with the field evidence and suggests that D1 deformation was partitioned into transpressional and compressional components. We also interpret the strain patterns to the effects of non-cylindrical superposed shear zones. The above aspects can be useful to understand the complex tectonic evolution of the most internal zone of the Variscan Belt and of other internal zones of collisional orogenic belts with similar characteristics.