Resumen de La estructura de la región de Sanabria. Noroeste de España
- español
En la zona de Sanabria se han distinguido tres unidades cuyas relaciones nos permiten proponer una evolución geológica nueva para esta zona del autóctono de la cordillera Varisca. Las unidades gneísica (UG) y metasedimentaria (UM) se encuentran separadas por un despegue extensional que lleva asociada una intensa deformación dúctil, que ocasiona en la UG una deformación generalizada en condiciones retrógradas, mientras que en la UM la deformación se concentró en la base, acompañada de la formación de biotita, con un gradiente geotérmico muy alto. La tercera unidad está constituida por rocas graníticas porfiroides (UGIS), instruidas a lo largo de fallas en niveles superficiales de la corteza con posterioridad al desarrollo del despegue. Presentan una sola foliación vertical, desarrollada en condiciones de bajo grado (zona de la clorita), que se corresponde con la del plano axial de los pliegues variscos, erguidos, relacionados con la estructura mayor del antiforme de Sanabria. La formación del despegue extensional, posteriormente intruido por las rocas de la UGIS, se corresponde con los movimientos extensionales que tuvieron lugar el margen de Gondwana durante el Ordovícico y que permitieron la formación de profundas cuencas sedimentarias y la adición de importantes volúmenes de magmas graníticos a la corteza.
- English
The Sanabria area (autochthonous domain of the Variscan belt) is characterized by an extensive outcrop of ortogneisses and porphyroids that record U/Pb ages from 500 to 465 Ma and that appear below the Ordovician metasediments at the core of the Sanabria antiform. Three main units have been distinguished, two of them being the lower gneissic unit and the upper metasedimentary unit, which are brought together by a top-to-East extensional detachment that controls the metamorphic evolution of both units. The detachment related-deformation in the gneissic unit developed under retrograde conditions from HT-LP metamorphism and partial melting in the lower parts to strong chloritization of the mylonitic rocks that appear distributed throughout the footwall unit, but particularly concentrated at the top. The metasedimentary unit constitutes the hanging wall to the detachment showing a highly evolved suhhorizontal crenulation cleavage in the basal level. The metamorphism of this unit is prograde and reaches the biotite zone in the basal part, which suggests a high geothermal gradient. The third unit consists of porphyroid granitic rocks which intruded in two units after the ductile development of the detachment, along normal faults that affect it, which probably channelled the ascent of the granitic magma. In contrast to the rocks of the gneissic and metasedimentary units, which are affected with different intensity by the subhorizonal shearing, porphyroid rock show only a low temperature vertical foliation, that may be coeval with the vertical foliation developed in the other two units, where it usually appears as a crenulation cleavage related to the upright variscan folding event. Therefore, the Upper Ordovician tectonic evolution of the Sanabria area seems to be related to the extensional tectonics developed in the continental margin of Gondwana that allows the formation of deep basins and the addition of huge volumes of granitic magma to the crust.
