Kinematics of the Alpine plate-margin: structural styles, strain and motion along the Penninic–Austroalpine boundary in the Swiss–Austrian Alps

Abstract

A structural and kinematic analysis of parts of the Alpine suture zone, i.e. the Arosa Zone and the edges of the overlying Austroalpine and underlying Penninic units, is presented. The Arosa Zone is interpreted as part of an accretionary wedge formed by the overriding of the Austroalpine units, the northern part of the Adriatic plate, over the Penninic units. Structures, strain and stretching trajectories indicate that two main deformation events followed each other discontinuously. The earlier event was a WSW- to NW-directed thrusting under noncoaxial deformation and led to melange-type structures. The second event, less penetrative in the Arosa Zone and with a downward increase in strain and noncoaxial flow, formed N- to NE-verging folds and imbrication zones. West-directed displacement commenced in the Early Cretaceous and the motion changed to a prevailing northerly direction during the Eocene, as revealed by sediments involved in thrusting and by radiometric dating of the mylonites. The kinematic history deduced from structural analysis agrees with the history obtained by recent work on plate margin sediments. We correlate the anticlockwise rotation of the Adriatic plate relative to Europe between about 130 and 60 Ma with the west-directed motion in the Arosa Zone. A change to relative N-S motion during the Lower Tertiary led to the N to NE directed displacements. A direct relationship thus exists between the motion of the Adriatic plate and the resulting deformation along its northern margin during the early history of the Eastern Alps. Partitioning in brittle and ductile deformation and structural slicing under noncoaxial deformation is responsible for the melange-type structures.