El análisis estructural de los gneises Ollo de Sapo y de los metasedimentos adyacentes, realizado en el sector costero del Barquero-Vivero, provincia de Lugo, permite establecer que su estructura consiste en un manto de 4 km de espesor, que desarrolla una intensa zona de cizalla dúctil en su base y que corta a los pliegues D1 desarrollados en las rocas metasedimentarias. La foliación milonítica que presentan los gneises Olio de Sapo se generó simultáneamente con las fases de deformación variscas D1+2. Posteriormente, esta lámina cabalgante constituida por ortogneises fue afectada por fallas de desgarre y normales. El ascenso progresivo a niveles superficiales de la corteza de esta lámina de rocas calientes fue acompañada de una retrogradación en el bloque superior (Olio de Sapo) por transferencia de calor al bloque inferior, en donde se desarrolla una aureola dinamo térmica, de unos 100 m de espesor, pegada al cabalgamiento basal. La deformación en el bloque superior tiende a concentrarse en los niveles basales, en los que la presencia de fluidos, combinada con bajas temperaturas de deformación (~ 350ºC), produce cambios microestructurales muy significativos, que consisten en una intensa fracturación del feldespato, que facilita su alteración sintectónica a moscovita y cuarzo, y que lleva a una disminución dramática del tamaño de grano y de la proporción de feldespatos en los niveles ultramiloníticos basales.
The structural analysis along the Cantabric coastal section allows to improve the geology of this important structure of the Variscan belt. The upper and lower limits of the Ollo de Sapo augengneises consist of shear zones that cut D1 folds, and develop a highly evolved crenulation cleavage (S2). The textural types of the Ollo de Sapo augengneises are related to a mylonitization that gradually increased to the base. The attitude of the stretching lineations and shear sense criteria allows to characterise the first stages of deformation as thrust related, that can be coeval with both D1+2 structures of the foot wall. Thus the structure of the Ollo de Sapo gneisses can be interpreted as a thrust nappe, which evolved to a right reverse component during the last stages of D2, followed by strike slip and normal faulting affecting their upper part. The proposed thrust nappe structure accounts for the metamorphic evolution of the zone. The deformation in the hanging wall of this crystalline thrust nappe occurs under retrograde conditions. lt can be deduced that the Ollo de Sapo augengneises began to deform under temperatures in excess of 650ºC, and during their emplacement to upper crustal levels, temperature decreased to T ≤ 350ºC near the thrust surface, depicting an inverted thermal profile across the hangingwall, that can be due to heat transfer to a cold footwall at earlier stages of thrusting. The metamorphic evolution of the footwall record that prior to D2 the footwall rocks were stable in the andalusite field at shallow depth, followed by sin-D2 pressure increase, with kyanite overgrowing earlier quiastolite, and a post-D2 decompression. The presence of pre-S2 andalusite over distances of 100 m below the thrust surface would be interpreted as a «dynamothermal aureole» attached to the base of a hot (~ 650ºC) crystalline sheet. Subsequent pressure increase was attained at the end of thrusting event. The later decompression can be also explained by the action of the Barquero right normal fault.
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