Mechanisms and conditions of deformation in quartzites from the Cantabrian and west Asturian-Leonese zones, north Spain

Abstract

Deformation mechanisms and conditions have been studied, principally in Ordovician quartzites, from the Cantabrian and West Asturian-leonese zones, north Spain. The quartzites have been strongly deformed at shallow crustal levels during the Variscan orogeny. In the external Cantabrian zone, the dominant mechanism was cataclasis, which occurred both pervasively around large-scale folds by shear on a network of bedding-normal fractures, and as localised deformation along large faults. Solution transfer was confined to grain boundaries. Grain boundary sliding operated in sandstones with porous microstructures, which deformed by small-scale chevron folds. This contrast in deformation modes is shown to be a consequence of different initial microstructures. In the West-Asturian leonese zone ( towards the internal part of the orogen), solution transfer extended along stylolites, and progressively increasing amounts of crystal plasticity became the fold accommodating mechanism. This can be followed by an increase in the intracrystalline extinction angle as grain boundary migration and sub-grain rotation contributed to further dynamic recrystallisation. The late stages of deformation were cataclastic. In all examples of cataclasis, fractures clustered together to form deformation zones of high density fracturing: this is an inherent part of the deformation mechanism, which is interpretted as a localisation phenomenon in response to changed material properties. The changes in mechanisms from the external to internal parts of the orogen correlate with an increase in temperature from 0-2500 to 311 - 411°. Effective confining pressures were from 30 to 7S MPa. and differential stresses from 50 to 300 MPa. These conditions of deformation are linked with the deformation
modes to define nine deformation facies; the sequence of facies, the deformation path, is plotted for the four localities studied in detail, and it is suggested that the study area can be subdivided into four sub-areas, each characterised by one path.