Geology of the Ollo de Sapo antiform unit to the south of the Cabo Ortegal Complex (NW Spain)

Abstract

The Olio de Sapo antiform is located in the hinterland of the Hercynian Iberian belt in NW Spain. It consists mainly of Lower Paleozoic siliciclastic rocks with important lateral facies changes in the Lower Ordovician. These rocks underwent three main deformation phases during the Hercynian orogeny. D1 gave rise to close east-facing folds with an associated foliation (S1). D2 is responsible for the thrust emplacement of the Cabo Ortegal Complex on the rocks of the Olio de Sapo antiform. D3 resulted in the development of a major synform-antiform pair. The competent and massive character of the rocks of the Cabo Ortega! Complex conditioned the geometry of the D3 structures and resulted in an increase in the tightening of D3 folds towards the south and the development of a fan of folds in the southern part of the zone. D3 microstructures include crenulation, crenulation cleavage, differentiated cleavage and schistosity. A peraluminous leucogranite with K-feldspar megacrysts (Forgoselo granite) intruded in the final stages of D3 in a hinge dilation space of the major D3 synform and constitutes an example of folding related intrusion in a crustal shortening regime. This type of structural control for granite emplacement is tentatively proposed for some other granites outcropping in the adjoining West-Asturian Leonese zone, which belong to the same compositional group as the Forgoselo granite.