A strain study of a granite–granite gneiss transition and accompanying schistosity formation in the Betic orogenic zone, SE. Spain

Publisher Website
Google Scholar
Abstract
An allochthonous granite-bearing tectonic unit in the Alpine mountain belt of SE. Spain shows all transitions from meta-granites with original igneous textures to granite-gneisses with a single, regionally subhorizontal schistosity. Locally, the transition may be recognized in ‘shear zone’ type structures and this is suspected on a regional scale. In some out-crops of deformed meta-granite, large feld-spar phenocrysts show preferred orientation with maximum pole densities of (010) faces perpendicular to schistosity. The symmetrical arrangement of the poles with respect to the tectonic fabric elements, and increasing preferred orientation with schistosity development favour an absence of preferred orientation prior to deformation. The minimum strain responsible for the preferred orientation may be determined by progressively "undeforming" the orientations of the phenocrysts until the assumed, original random distribution is achieved. In this way strains in excess of +150% within the schistosity, and more than −77% normal to the schistosity have been found. The large strains associated with a sub-horizontal schistosity could account for a considerable part of the displacements between flat-lying tectonic units of different competence. Furthermore, the large strain differences between different lithologies might account for the initiation of tectonic contacts and individualisation of nappes.