The Schneeberg Normal Fault Zone: Normal faulting associated with Cretaceous SE-directed extrusion in the Eastern Alps (Italy/Austria)

The Cretaceous eo-Alpine collisional event in the European Eastern Alps is generally accepted to induce W-NW-directed thrusting both in basement and in sedimentary cover units. This study presents the first evidence of eo-Alpine W-NW directed normal kinematics along the Schneeberg Normal Fault Zone, which separates eo-Alpine high-pressure rocks in a footwall position from pre-Alpine basement rocks in a hanging wall position. New Garnet Sm-Nd data indicate that exhumation of the high-pressure rocks along the normal fault zone started around 95 Ma ago and continued up to low greenschist/brittle conditions at 76 Ma, as indicated by a Rb-Sr age from a low temperature mylonite. The occurrence of pre-Alpine basement rocks both in the hanging wall and the footwall of eo-Alpine high-pressure rocks suggests exhumation by extrusion processes. Despite the displacement or removal of parts of the lower portion of the high-pressure unit by Tertiary strike-slip faults, eo-Alpine top-to-ESE thrusting, as expected for the structurally lower part of an extruding wedge, was found at and below the base of the eo-Alpine high-pressure rocks. A Rb-Sr age of 77 Ma from a greenschist facies mylonite in this thrust shear zone shows the contemporaneity of deformation at the base and the top of the wedge. The tectonic transport direction within the extruding wedge was E-SE, opposite to the W-NW direction so far reported for the eo-Alpine event in the Eastern Alps. The contemporaneity of opposite tectonic transport directions during continental subduction may be explained by a double-vergent wedge model with a narrow zone of ductile flow, where the high-pressure rocks were exhumed. Π2005 Elsevier B.V. All rights reserved.