Evidence for extension in the western Alpine orogen: the contact between the oceanic Piemonte and overlying continental Sesia units

Abstract

In the Alps and other orogens, major tectonic contacts must have been established during convergence and overthrusting. What is less clear is how strongly these contacts were modified as the orogen evolved. During Alpine collision, continental crust of the overriding plate, the Sesia unit, is inferred to have been thrust northwest over oceanic material of the Piemonte unit. Both units preserve, in parts, eclogite facies metamorphism. However, all structures along this contact in our study area indicate that final movement was southeast directed (with no clear evidence of the kinematics of earlier movement). These structures include SE-verging folds, SE-directed shear bands, and larger normal faults downthrowing to the southeast. It is inferred that there was a SE-directed shear regime below the contact which, although locally buckling and shortening earlier lithological contacts, was dominantly extensional with respect to pre-existing tectonic layering. The lower limit of this shear regime has not been identified, although SE-directed shear is common throughout the upper, greenschist facies part of the Piemonte unit. We show that this shear passes beneath the Sesia zone and does not re-emerge. It is therefore a shear which was net extensional relative both to the modern surface and to the palaeosurface. It may have contributed to the unroofing of eclogite facies rocks in the lower part of the Piemonte unit, although additional timing data are required to clarify this. Nearby, SE-directed shears buckle and imbricate earlier layering within the Piemonte unit. These are normally identified as backthrusts, yet they do not appear to breach the base of the Sesia unit and might merge with the extensional shear. Even though these structures shorten layering, they too could have been extensional relative to the Earth's surface. The significance of Alpine ‘backthrusts’ should be reappraised in this context. This study indicates that hinterland-directed extension could have been an important phenomenon during Alpine evolution.