La fábrica del olivino y las condiciones de la deformación de las rocas ultramáficas del Macizo de Herbeira (Cabo Ortegal, NO del Macizo Ibérico).

Zusammenfassung

The Herbeira ultramafic massif is located in the structural upper portions of the Cabo Ortegal Variscan Complex, NW Galicia, which form part of a thrust sheet that shows a complex deformational evolution. An important strike-slip fault, the Trans Herbeira Fault, divides the massif into two different zones. The eastern one is composed of amphibole bearing harzburgites, whilst the western one has a more complete lithological succession: basal harzburgite, lower dunite, medium pyroxenite and upper dunite. Equigranular and porphyroclastic microfabrics are observed in these rocks under optical microscope and SEM, providing evidence of a succession of several tectonic stages. The analysis of olivine CPO was carried out through electron backscattering diffraction techniques (EBSD). As a result the operating slip systems may be inferred. During de high temperature deformation stage DA the active slip systems were (010)[100] and {0kl}[100]. The activation of [001] axis along the slip direction occurred at lower temperatures (D1 tectonic stage). The estimation of tectonic stress and strain rate was performed by palaeopiezometry of olivine. During DA deformation the temperature reached more than 1000ºC, the tectonic stress level was very low (3-6 MPa) and the strain rate oscillated between 10-9 s-1 and 10-15 s-1, in accordance to the expected values at the mantle. Finally, the D1 deformation was developed at lower temperature (less than 1000ºC), moderately high tectonic stress level (8-35 MPa), and low strain rates (10-12 s-1 to 10-19 s-1), more coherent with the lower crust or uppermost mantle.