Fluid inclusion constraints on the kinematics of footwall uplift beneath the Brenner Line normal fault, eastern Alps

Dynamic models of isostatic footwall uplift in response to normal faulting can be divided into those in which uplift is accomplished by flexural failure and those in which uplift occurs via subvertical simple shear. Each class of model predicts a different incremental strain history that should be recorded in the footwall. In the Tauern Window (eastern Alps), postmylonitic structures in the footwall of the Brenner Line normal shear zone predominantly consist of closely spaced, steep, west down and east down microfaults. Formation of the west down faults before and at greater depths than the east down faults would be consistent with unroofing via subvertical simple shear. In contrast, formation of the two fault types as a conjugate set would be more indicative of unroofing via elastic processes. The field data alone do not provide a sufficient test of the two hypotheses because crosscutting relations are only rarely observed and there is no control on the depth at which the structures formed. However, both depth and timing constraints on the formation of the late structures can be obtained by correlating the orientations of fluid inclusion-lined microfaults with the macroscopic west down and east down faults, obtaining density data for the inclusions, and correlating these data with previously obtained geochronologic data. The results indicate that the west down structures formed at depths of 10–20 km and temperatures >450°C in the mid to late Oligocene and that the east down structures formed at 2- to 10-km depth and temperatures of 300 ± 50°C in the mid-Miocene. These data support the hypothesis that a "rolling hinge" was present in the footwall of the Brenner Line and that isostatically driven footwall deformation was accomplished predominantly by subvertical simple shear. The depths at which west down and east down faulting occurred, coupled with the angle of dip of the Brenner Line, yield a minimum lateral displacement on the fault of 15–26 km. Approximately coeval ductile shearing and brittle faulting at depths of 15–20 km and temperatures in excess of 400°C may reflect local variations in strain rate as the footwall rocks entered the zone of rolling hinge deformation.