The structure of the OlIo de Sapo antiform in the Cantabrian coast (NW Spain) F. BASTIDA, J. ALLER and G. FERNANDEZ VIEJO Dpto. de Geologfa, Universidad de Oviedo, 33005 OVIEDO Abstract: The geological structure of the 0110 de Sapo antiform unit in the Cantabrian coast section (Central-Iberian zone) constitutes an anticlinorium developed during the first deformation phase (D.) of the Hercynian orogeny. This anticlinorium was strongly modified by subsequent deformation episodes. A thrust with a phyllonite zone was formed in the western part of the section during a second deforma- tion phase (D2). A post-D2 extensional episode produced the Viveiro fault and a ductile shear zone in the eastern part of the section. Later, an important refolding event (D3) gave rise to major folds and a large number of minor structures, developed in hectometric bands and characterized by geometrical variabi- lity. Finally, some late structures, mainly faults, were formed. These deformation events took place mainly under greenschist facies conditions. Quartz c-axis fabrics in quartzite and sandstones, with sym- metric small circle girdles, were developed during D,. These fabrics were passively folded by D3 folds. Key words: Hercynian belt, Central-Iberian Zone, structure, folds, foliations, shear zones, quartz c-axis fabrics. Resumen: La estructura del Antiforme del 0110 de Sapo (Zona Centroiberica) en el sector de la costa cantabrica consiste en un anticlinorio originado durante una primera fase de deformaci6n (D.), que ha si- do fuertemente deformado durante etapas posteriores. En una segunda fase (D2) se origin6 un cabalga- miento con una zona de filonitizaci6n en la parte occidental del corte. Posteriormente, se desarro1l6 un episodio distensivo durante el cual se produjo la Falla de Viveiro, 'asf como una zona de deformaci6n ductil en la parte oriental del corte. A continuaci6n, un importante replegamiento (D3) origin6 pliegues mayores y numerosas estructuras menores, que aparecen en bandas y se caracterizan por una gran varia- bilidad en su geometrfa. Por ultimo, se desarrollaron algunas estructuras tardfas, principalmente fallas. El desarrollo de la deformaci6n tuvo lugar principalmente bajo condiciones de la facies de los esquistos verdes. Las fabricas de ejes c del cuarzo se originaron principalmente durante la fase 1, y son en general guirnaldas de cfrculo menor simetricas y con fuertes maximos perifericos. En los casos estudiados, estas fabricas fueron rotadas pasivamente durante la formaci6n de los pliegues D3. Palabras clave: Macizo herciniano, Zona Centroiberica, estructura, pliegues, foliaciones, zonas de ciza- lla, fabric as de ejes c del cuarzo. Bastida, F.; Aller, J. and Fernandez Viejo, G. (1993): The structure of the 0110 de Sapo antiform in the Cantabrian coast (NW Spain). Rev. Soc. Geol. Espana, 6: 93 - 103. The 0110 de Sapo antiform unit (OSA) forms the northeastern part of the Central-Iberian zone, and fo- llows the trend of the Hercynian structures in the area (Fig. 1). The eastern boundary of the OSA in its nort- hern sector is the Viveiro fault, an extensional fault with a 10-13 km dip slip, that separates the OSA from the Westasturian-leonese zone (WALZ) on the east (Parga Pondal, 1967; Bastida et aI., 1984; Martfnez Catalan, 1985). The western boundary of the OSA is the Pontedeume-Valdovifio fault zone (Parga Pondal, 1963; Iglesias & Choukroune, 1980; Dfaz Garda, 1982), that separates the OSA from the Ordenes com- plex. The Cabo Ortegal allochthonous complex is th- rust onto the OSA unit along its northwestern margin (Ries & Shackleton, 1971). The basal thrust of the Ca- bo Ortegal Complex is the western boundary of the study section. The structure of the OSA is a good example of su- perimposed folding on a macroscopic scale. A contro- versy exists on the deformation history of the OSA. Matte (1968) first interpreted it to be the result of two superimposed folding phases depicting a type 3 inter- ference pattern (Ramsay, 1967). However, Martfnez Catalan et aI. (1977) re-interpreted the structure in this zone to be the result of a main folding event (D.) slightly modified by subsequent deformation. More re- cently, Dfaz Garda (1983) describes a set of structures of an intermediate age between the two folding phases of Matte (op. cit.) which forms a ductile shear zone (Dz) in the northwestern corner of the antiform. In mo- re recent reports on the area, the D3 folding phase is again stated as an important event (Gonzalez-Lodeiro et aI., 1982; Bastida et aI., 1984; Dfez Balda et aI., 1990; Azor et aI., 1992). The understanding of the Rev. Soc. Geol. Espal1a, 6 (3-4), (1993)