Structural, metamorphic and magmatic history of the Mondon̄edo nappe (Hercynian belt, NW Spain)

Abstract

The Mondon̄edo nappe is one of the main units in the internal zone of the Hercynian belt in the Iberian Peninsula. It consists of low to medium grade metasediments, late Precambrian and Palaeozoic in age, folded by large D 1 E-verging recumbent structures, and thrusted several tens of km towards the east during a D 2 deformation episode. At the base of the thrust sheet, a shear zone of up to 3 km thick was developed. Granitoid bodies, which were intruded after D 1 and before or during D 2, were deformed in this basal zone. The geometry of D 1 and D 2 structures and that of the isograds are described, as well as the relationships between metamorphism and deformation. A structural evolution is inferred which shows an episode of generalized ductile deformation ( D 1) followed by another in which the deformation tended to concentrate in a subhorizontal ductile shear zone ( D 2) and culminated with the thrusting along an important fracture. The shear zone originated in a thermally softened level which, according to metamorphic data, was at temperatures in the range 550–600°C. The structural, metamorphic and magmatic features of the Mondon̄edo nappe suggest a relationship with a crustal scale overthrusting of the continental mass to which the nappe belongs. This could be a consequence of the collision against another continental mass to the west.