Quartz fabric and strain partitioning in sheath folds: an example from the Voltri Group (Western Alps, Italy)

Abstract

A mesoscopic sheath fold, defined by an almost pure quartzitic level in quartz-mica-schists of the Voltri Group (southeastern border of the Western Alps), was sampled and studied in detail. Microstructural analysis was carried out on thin sections from different structural domains in the quartzitic layer, with the emphasis to analyse quartz-shape fabrics and the crystallographic preferred orientation of the c-axes of this mineral. Quartz was dynamically recrystallized but its degree of preferred orientation is not as high as it would be expected in a high shear-strain regime required for the development of sheath folds. This may suggest strain partitioning between the quartzitic level and the micaceous matrix. Crystallographic preferred orientation of c-axes and grain-shape fabric asymmetry indicate the same sense of shear on both fold limbs in the XZ-plane; this is in agreement with a model of sheath-fold development by the passive rotation of material lines. Clustering of the c-axis poles shows a configuration indicative of a different strain state in different domains of the sheath fold and suggests an evolution of the fold characterized by combined simple shear and coaxial progressive deformation.