STRAIN LOCALIZATION MECHANISMS IN DEEP-SEATED LAYERED ROCKS

More than 160 shear zones of the Norwegian Caledonides and Italian Sou thern Alps, formed under high-temperature or high-pressure conditions, are analysed with respect to mechanisms of strain localization. In me tabasic rocks with a pre-existing (magmatic) layering strain localizes preferentially in mafic layers, although experiments in monomineralic rocks would predict a location in the weaker feldspar-rich layers. In addition, in some of these zones amphibole recrystallizes dynamically , whereas feldspars only show undulatory extinction. These are evidenc es for strength inversion between feldspars and mafic minerals. This i nverse strength behaviour is caused by a reduced grain size of feldspa rs and sometimes amphiboles in the pre-existing mafic layers, i.e. pri or to shearing. The localization of the shear zones is therefore a gra in-size-sensitive process (grain-size-dependent softening), nearly ind ependent of the composition of the deformed material. The numerous amp hibole-amphibole contacts favour dynamical recrystallization by grain- boundary migration and the rare feldspar-feldspar contacts prevent or suppress the same process in feldspars. Therefore, the feigned inverse strength is mainly caused by the interaction of reduced grain size an d grain-boundary effects in the original rock. This behaviour can lead to the preferential localization of shear zones in mafic layers, alth ough they are not favourably oriented with respect to the stress syste m.