Journal of Structural Geology. Vol. ~. Nos. 3/4, p p 415 to 4311, 1986 0191-8141/86 $1131111 + 0.110 Printed in Great Britain © 1986 Pergamon Press Ltd. Structural, metamorphic and magmatic history of the Mondofiedo nappe (Hercynian belt, NW Spain) F. BASTIDA*, J. R. MARTINEZ-CATALANt and J. A. PULGAR* * Departamento de Geotect6nica, Facultad de Geologia. Universidad de Oviedo. Spain + Departamento de Geotect6nica. Facultad de Ciencias, Universidad de Salamanca, Spain (Received 1 July 1984; accepted in revised form 26 September 1985) Abstract--The Mondofiedo 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 Dt E-verging recumbent structures, and thrusted several tens of km towards the east during a D~ 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 Di and before or during D 2, were deformed in this basal zone. The geometry of D~ 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 (Dr) followed by another in which the deformation tended to concentrate in a subhorizontal ductile shear zone (D2) 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 Mondofiedo 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. Resum(--La nappe de Mondofiedo est une des unites principales des zones internes de la chaine hercynienne de la Peninsule lberique. Elle est compos6e par des metasediments du Precambrien Superieur et du Pal6ozoique, affectes par des plis couches (DL). L'ensemble est chevauche vers l'Est sur plusieurs km lors d'une deuxi6me d6formation (D2). Dans la partie inferieure de la nappe, I'on trouve une zone de cisaillement de 3 km d'epaisseur. Des corps granitiques intrusifs dans cette zone ont et6 d6form6s pendant la deuxi~me phase. On d6crit la g6ometrie des structures et celle des isogrades, et on 6tablit des relations entre le m6tamorphisme et la d6formation. On peut d6duire une evolution structurale qui aurait commenc6 par un episode /~ comportement ductile gen6ralis6 (D I), suivi par un autre h d6formation concentr6e dans une zone de cisaillement ductile subhorizontale (D2) qui a fini par chevaucher au long d'une fracture importante. La zone de cisaillement basale a 6t6 developpee, d'apr6s les donn6es sur le m6tamorphisme, ~ une temperature de l'ordre de 550--600°C. Les caract6ristiques tectoniques, m6tamorphiques et magmatiques de la nappe de Mondofiedo suggerent qu'elle est en relation avec un cisaillement profond intracontinental de la plaque a la quelle appartient la nappe qui peut bien 6tre une cons6quence de sa collision avec une autre masse continentale situ6e a l'Ouest. INTRODUCTION THE HERCYNIAN belt of NW Spain comprises several superimposed allochthonous units, some of which have undergone displacements of tens or hundreds of kilo- meters. The town of Mondofiedo, to the north of the province of Lugo, gives its name (Marcos 1971) to one of the largest of these in the internal zone, a thrust sheet whose eastern limit is a thrust fault or imbricate stack extending over some 165 km along strike (Fig. 1). To the west, the unit is cut by the Vivero fault (Parga Pondal et al. 1967, Matte 1968), a N-S striking fault downthrown to the west. Its throw, comprising that of the fault itself and that of an associated ductile shear zone, is about 10 km, as may be inferred from stratigraphic, structural and petrological data (Martfnez-Catal~in 1981). The thrust sheet was folded by two systems of large open antiforms and synforms whose hinges and axial surfaces are more or less perpendicular, producing type 1 inter- ference patterns (Ramsay 1967) to the west of the unit. Three principal domes were formed in this area. In the two northern ones, the base of the nappe and the footwall rocks (Gistral and Monte Carballosa tectonic windows) may be seen, while in the southernmost one, known as the Puertomarfn dome, only the basal zone of the nappe crops out, the thrust fault not having been exposed by erosion. Because of its dimensions and outcrop conditions, the Mondofiedo nappe offers particular facilities for the study of the evolution of a large allochthonous structure in the internal zone of an orogenic belt, including (i) its geometry, (ii) the temporal relationship between the different structures related to the nappe and thrust tectonics, (iii) the deformation mechanisms of the rocks, (iv) the relationships between the development of the structures, metamorphism and plutonism and (v) the link between the history of a unit of this kind and the general evolution of the orogen. REGIONAL SETTING The Hercynian belt of the NW Iberian Peninsula has been divided into three zones with different geological 415