QUARTZ FABRIC AND STRAIN PARTITIONING IN SHEATH FOLDS - AN EXAMPLE FROM THE VOLTRI GROUP (WESTERN ALPS, ITALY)

A mesoscopic sheath fold, defined by an almost pure quartzitic level i n quartz-mica-schists of the Voltri Group (southeastern border of the Western Alps), was sampled and studied in detail. Microstructural anal ysis was carried out on thin sections from different structural domain s in the quartzitic layer, with the emphasis to analyse quartz-shape f abrics 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 hig h shear-strain regime required for the development of sheath folds. Th is may suggest strain partitioning between the quartzitic level and th e micaceous matrix. Crystallographic preferred orientation of c-axes a nd grain-shape fabric asymmetry indicate the same sense of shear on bo th fold limbs in the XZ-plane; this is in agreement with a :model of s heath-fold development by the passive rotation of material lines. Clus tering of the c-axis poles shows a configuration indicative of a diffe rent strain state in different domains of the sheath fold and suggests an evolution of the fold characterized by combined simple shear and c oaxial progressive deformation. (C) 1997 Elsevier Science Ltd.