Styles of footwall uplift along the Simplon and Brenner normal fault systems, central and Eastern Alps

The Simplon and Brenner extensional shear zones of the central and Eastern Alps, respectively, are low-angle detachments that accommodated orogen-para llel extension and unroofed midcrustal rocks in their footwalls. An integra ted field and fluid inclusion study of the Simplon low-angle shear zone rev eals several postmylonitic structure types that are consistent with passage of a complex rolling hinge through the Simplon footwall, including synthet ic (west down) and antithetic (cast down) semibrittle and brittle structure s, and strike-slip, extensional, and contractional structures, all of which are consistent with southwest directed extension along the main detachment zone. Fluid inclusion analysis of kinematically referenced inclusion array s indicates that west down, semibrittle, and brittle shears were active at conditions above 300 degrees -450 degreesC and 14-26 kill. Antithetic defor mation along brittle faults occurred at much shallower conditions of < 300 degreesC and 3-7 km. Deformation along these structures accommodated footwa ll uplift by a subvertical simple shear mechanism in the southern part of t he Simplon region. These results are strikingly similar to those from the B renner shear zone. In contrast with the Brenner region, however, the northe rn Simplon footwall shows strike-slip dominated postmylonitic deformation r elated to slip along the Rhone Line. The footwalls of these detachment syst ems define a unique class of core complex characterized by the development of a midcrustal, synthetic lower hinge that acts to steepen the detachment system and accommodate footwall uplift by a subvertical simple shear mechan ism. Both systems have poorly developed upper hinges that flatten the middl e to upper crustal ramp by only a few degrees of dip, which is in contrast to detachment fault systems of the western United States, where evidence of a deep hinge is rarely observed, and the upper hinge acts to deactivate th e flattened, updip part of the detachment. Our results emphasize the role o f tectonic setting buoyancy forces, and synkinematic fluids in controlling the observed style and mechanisms of large-magnitude extension during colli sion between continental masses.