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 faul ting can be divided into those in which uplift is accomplished by flex ural failure and those in which uplift occurs via subvertical simple s hear. Each class of model predicts a different incremental strain hist ory that should be recorded in the footwall. In the Tauern Window (eas tern Alps), postmylonitic structures in the footwall of the Brenner Li ne normal shear zone predominantly consist of closely spaced, steep, w est down and east down microfaults. Formation of the west down faults before and at greater depths than the east down faults would be consis tent with unroofing via subvertical simple shear. Tn contrast, formati on of the two fault types as a conjugate set would be more indicative of unroofing via elastic processes. The field data alone do not provid e a sufficient test of the two hypotheses because crosscutting relatio ns are only rarely observed and there is no control on the depth at wh ich 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 macros copic west down and east down faults, obtaining density data for the i nclusions, and correlating these data with previously obtained geochro nologic data. The results indicate that the west down structures forme d at depths of 10-20 km and temperatures >450 degrees 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 degrees C in the mid-Miocene. The se data support the hypothesis that a ''rolling hinge'' was present in the footwall of the Brenner Line and that isostatically driven footwa ll deformation was accomplished predominantly by subvertical simple sh ear. The depths at which west down and east down faulting occurred, co upled with the angle of dip of the Brenner Line, yield a minimum later al 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 degrees C may reflect local variations in strain rate as the footwall rocks entered the zone of rolling hinge deformation.