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

Abstract

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 tothe D1 east vergent flat-lying structures, formed during Devonian Carboniferous continental collision, has been recognized. Although intrusion ofvoluminous granite bodies has resulted in extensive recrystallization of host rocks, kinematic analysis of these rocks, using mesoscopic criteriaand quartz CPOs (crystallographic preferred orientation) reveals shear senses coherent with the field evidence and suggests that D1 deformationwas partitioned into transpressional and compressional components. We also interpret the strain patterns to the effects of non-cylindricalsuperposed shear zones. The above aspects can be useful to understand the complex tectonic evolution of the most internal zone of the VariscanBelt and of other internal zones of collisional orogenic belts with similar characteristics.