Journal of Structural Geology, Vol. 15, No. lZ, pp. 1405 to 1419, 1993 0191--s141193 $06.00+0.00 Printed in Great Britain © 1993 Pergamon Press Ltd Anatomy of the Mondofiedo Nappe basal shear zone (NW Spain) J. ALLER and F. BASTIDA Departamento de Geologia, Universidad de Oviedo, Spain (First received 17 March 1992; accepted in revised form 14 January 1993) Abstract-This paper analyses a major shear zone from the Iberian Hercynian belt which forms the basal thrust of the Mondoiiedo Nappe. The shear zone developed by ductile deformation under amphibolite facies metamor- phic conditions and later by brittle-ductile deformation ·in greenschists facies. Folds in the shear zone are asymmetric, very tight, Ie or similar class and frequently developing sheath geometries. The sheath folds originated by non-coaxial flow superimposed on earlier irregularities. The fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non-mylonitic tectonites. c-axis fabrics vary across the shear zone, but show a dominant monoclinic symmetry. The blastomylonitic rocks include the fabrics representing the highest temperatures. The main foliation ofthe schists results from flattening of an earlier foliation, recording occasional microfolds. The use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen. INTRODUCTION THE Mondoiiedo Nappe basal shear zone is well suited for the study of problems arising in this type of structural association, and has already been the subject of some previous papers (Bastida & Pulgar 1978, Pulgar 1980, Martinez-Catalan 1985, Bastida et at. 1986). The shear zone, 3.4 km wide, is located in the northern part of the Iberian Hercynian belt (Fig. 1), presenting a continuous outcrop with strong ductile deformation all along the Cantabrian coast. It is bound at its base by a major thrust developed in brittle-ductile conditions during the last stage of the shear zone deformation history. The shear zone outcrops again, further to the east, in an area closer to the front of the Mondoiiedo Nappe (Fig. 1). Here the shear zone is characterized by a brittle-ductile defor- mation zone not wider than 100 m, above the basal thrust. GEOLOGICAL SETTING The shear zone is the lower boundary of a large thrust sheet, the Mondoiiedo Nappe, situated in the western part of the Westasturian-Leonese zone, a major tectonic zone in the hinterland of the Iberian Hercynian belt. The Mondoiiedo Nappe unit is made up of a stack of recum- bent, almost isoclinal, large amplitude E-facing D 1 folds (Fig. 1) with an associated tectonic foliation (Sl)' These folds are affected by the nappe basal thrust and the associated shear zone. This shear zone is important in the western part of the nappe, where a large number of shear related E-facing minor folds (D z ) are developed. In this part, Sl has been flattened (Sl+z), crenulated or replaced by a new mylonitic foliation (Sz). Finally, the whole nappe has been folded into a gentle synform in the east and an antiform in the west (D 3 ), both with a subvertical axial plane and a large wavelength. D 3 minor folds and S., crenulation cleaval!e occur locallv. D., folds homoaxially overprint D 1 folds producing a Type 3 interference pattern (Ramsay 1967) that can be seen both on the map and in cross-section (Fig. 1). Uplift due to the D 3 major antiform has resulted in exposure of the thrust and the basal shear zone around the Xistral semi-window (Fig. 1) where amphibolite facies metamorphism was reached (Martinez-Catalan 1979, 1980). It is in this western outcrop of the shear zone, in the section between the coastal localities of Burela and Foz (Fig. 1), that most of this work was focused. GEOLOGY OF THE CROSS-SECTION The rocks affected by the shear zone as well as those of the Xistral semi-window, belong mainly to the Cindana Group (Lower Cambrian) (Walter 1966, 1968). Precam- brian rocks, consisting of micaschist with common sand- stone intercalations and porphyroids outcrop just at the base of the nappe. The Candana Group consists of four units, only two of which are affected by the shear zone: the lower Candana formed by coarse-grained felds- pathic sandstones and quartzite with intercalated schists, and the middle Candana formed by micaschist, commonly with some sandstone, and minor black slate, carbonatic and amphibolitic layers. The macroscopic structure shown in Fig. 2 consists of several very tight D 1 folds, downward-facing due to their location in the western limb of a large gentle D 3 syn- form. The basal thrust, also in a tilted position, can be seen in the western part of the limb. The ductile defor- mation of the shear zone is shown by an association of minor structures that developed in the nappe footwall (shaded area in profiles of Fig. 1). The general features of metamorphism and its chronological relationship with the deformation in the Mondoiiedo Nappe have been pointed by out Bastida & Pullmr (1978). Martinez-Catalan (19R5) and Ba!'\tida et