Superposed deformations in the Eastern Alps: strain analysis and microfabrics

Abstract

Superposition, mainly of two Eoalpine (Cretaceous) deformation events, can be observed in vertical sections and horizontal traverses throughout a decollement zone in the Eastern Alps consisting of crystalline basement and autochthonous and allochthonous cover stacked in several nappes. The first event is an episode with a non-coaxial strain history caused by W-directed thrusting, the second a N-directed shortening by folding and fold imbrication. Strain analysis based both on deformed pebbles and on the preferred orientation of phyllosilicate grains (March theory), microfabric analysis, and observations of the relative timing of deformation and metamorphism, together indicate that the superposition of structures caused by the two events amounts to one continuous, progressive act of deformation. We attribute gradual and consistent variations in intensity and axial ratio of the tectonic strain to a history influenced by: (a) the position of samples in the pile; (b) the relative importance of the strain increments caused by the two events; and (c) rock ductility. We interpret variations of the c-axis fabrics of quartz in the same way, and draw tentative kinematic conclusions for the Eoalpine orogeny in the Eastern Alps.