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

Abstract

The Simplon and Brenner extensional shear zones of the central and Eastern Alps, respectively, are low-angle detachments that accommodated orogen-parallel extension and unroofed midcrustal rocks in their footwalls. An integrated field and fluid inclusion study of the Simplon low-angle shear zone reveals several postmylonitic structure types that are consistent with passage of a complex rolling hinge through the Simplon footwall, including synthetic (west down) and antithetic (east down) semibrittle and brittle structures, 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 arrays indicates that west down, semibrittle, and brittle shears were active at conditions above 300°-450°C and 14-26 km. Antithetic deformation along brittle faults occurred at much shallower conditions of <300°C and 3-7 km. Deformation along these structures accommodated footwall uplift by a subvertical simple shear mechanism in the southern part of the Simplon region. These results are strikingly similar to those from the Brenner shear zone. In contrast with the Brenner region, however, the northern Simplon footwall shows strike-slip dominated postmylonitic deformation related to slip along the Rhône Line. The footwalls of these detachment systems 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 mechanism. Both systems have poorly developed upper hinges that flatten the middle 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 the flattened, updip part of the detachment. Our results emphasize the role of tectonic setting buoyancy forces, and synkinematic fluids in controlling the observed style and mechanisms of large-magnitude extension during collision between continental masses.