STRUCTURAL EXPRESSION OF A ROLLING HINGE IN THE FOOTWALL OF THE BRENNER LINE NORMAL-FAULT, EASTERN ALPS

The kinematic and temporal sequence of structures observed to overprin t mylonites along the Brenner Line low-angle normal fault may record p assage of the footwall through two rolling hinges, at the top and bott om of a ramp in the shear zone. The structures comprise west down brit tle and brittle-ductile structures and east down brittle structures. P T conditions of formation (250 degrees to >400 degrees C and 2-23 km d epth), obtained from analysis of oriented fluid inclusion planes, indi cate that west down structures were formed at greater depths and tempe ratures, and therefore earlier, than the east down structures. These d ata suggest that the brittle structures formed under conditions that p ermit crystal-plastic deformation at long-term geologic strain rates a nd therefore probably reflect transient rapid strain rates and/or high fluid pressure. Structures inferred to have formed at a lower hinge a re consistent with viscous flow models of rolling-hinge deformation an d support the concept of a crustal asthenosphere. Such high temperatur es at shallow crustal depth also suggest significant upward advection of heat by extensional unroofing of warm rocks, which may have reduced the flexural rigidity of the footwall and thus affected mechanical be havior at the upper rolling hinge. Exposed mylonitic foliation within a few hundred meters of the Brenner line and on top of the east-west t rending anticlines in the footwall dips similar to 15 degrees west. Ou r data favor a ramp dip of similar to 25 degrees but permit a dip as g reat as 45 degrees. Fluid inclusion data suggest that structures relat ed to the hinge at the base of the ramp formed at depths of 12-25 km. If the average dip of the Brenner shear zone to those depths was 20 de grees, intermediate between the favored ramp dip and the dip of expose d foliation, then the horizontal component of slip could be as high as 33-63 km. The two discrete sets of structures with opposite shear sen ses, formed in the temporal sequence indicated by PT data, are consist ent with subvertical simple shear models of rolling-hinge strain. This kinematic pattern is not predicted by the flexural-failure model for rolling hinges. However, the predominance of normal slip at the upper hinge, which extends rather than shortens the mylonitic foliation, fai ls to match the subvertical simple shear model, which predicts shorten ing of the foliation there. One possible solution is that superpositio n of regional extension upon hinge-related stresses modified the rolli ng-hinge kinematics. Such a modified subvertical shear model can accou nt for the observed small foliation-parallel extensional strains if th e foliation was bent <5 degrees-10 degrees passing through the upper h inge. If more bending than that occurred, the data suggest rolling-hin ge kinematics in which deformation is achieved by uniform-sense simple shear across the shear zone as in the subvertical simple shear model but in which material lines parallel to the shear-zone foliation and t he detachment fault undergo very small length changes, presumably indi cating that footwall rocks retained significant resistance to shear an d underwent minimal permanent strain. The mechanics that would generat e such a rolling hinge are uncertain but may incorporate aspects of bo th subvertical simple shear and flexural failure. An important kinemat ic consequence of such a rolling hinge is that all of the net slip acr oss a normal fault, not only its horizontal component, is converted in to horizontal extension. This implies a significantly larger magnitude of crustal extension across dipping normal faults whose footwalls pas sed through a rolling hinge than for those that did not develop along with a hinge.