EVIDENCE FOR EXTENSION IN THE WESTERN ALPINE OROGEN - THE CONTACT BETWEEN THE OCEANIC PIEMONTE AND OVERLYING CONTINENTAL SESIA UNITS

In the Alps and other orogens, major tectonic contacts must have been established during convergence and overthrusting. What is less clear i s how strongly these contacts were modified as the orogen evolved. Dur ing Alpine collision, continental crust of the overriding plate, the S esia unit, is inferred to have been thrust northwest over oceanic mate rial of the Piemonte unit. Both units preserve, in parts, eclogite fac ies metamorphism. However, all structures along this contact in OUT st udy area indicate that final movement was southeast directed (with no clear evidence of the kinematics of earlier movement). These structure s 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 bu ckling and shortening earlier lithological contacts, was dominantly ex tensional with respect to pre-existing tectonic layering. The lower li mit 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 a nd does not re-emerge. It is therefore a shear which was net extension al relative both to the modern surface and to the palaeosurface. It ma y have contributed to the unroofing of eclogite facies rocks in the lo wer part of the Piemonte unit, although additional timing data are req uired to clarify this. Nearby, SE-directed shears buckle and imbricate earlier layering within the Piemonte unit. These are normally identif ied as backthrusts, yet they do not appear to breach the base of the S esia unit and might merge with the extensional shear. Even though thes e structures shorten layering, they too could have been extensional re lative to the Earth's surface. The significance of Alpine 'backthrusts ' should be reappraised in this context. This study indicates that hin terland-directed extension could have been an important phenomenon dur ing Alpine evolution.