The geometry and timing of orogenic extension: an example from the WesternItalian Alps

Contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements. Low-P upon high-P contacts are commonly interpreted as extensional (i.e. material points on either side o f the contact moved apart relative to the palaeo-horizontal), but dating of deformation and metamorphism is essential in testing such models. In the W estern Alps, the Piemonte Ophiolite consists of eclogites (T approximate to 550-600 degrees C and P approximate to 18-20 kbar) structurally beneath gr eenschist facies rocks (T approximate to 400 degrees C and P approximate to 9 kbar). Mapping shows that the latter form a kilometre-wide shear zone (t he Gressoney Shear Zone, GSZ) dominated by top-SE movement related to crust al extension. Rb-Sr data from micas within different GSZ fabrics, which dyn amically recrystallized below their blocking temperature, are interpreted a s deformation ages. Ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from m apping. They show that the GSZ had an extensional deformation history over a period of c. 9 Myr between c. 45-36 Ma. This overlaps in time with the ec logite facies metamorphism. The GSZ operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was ther efore responsible for eclogite exhumation. The discrete contact zone betwee n eclogite and greenschist facies rocks is the last active part of the GSZ and truncates greenschist facies folds in the footwall. These final movemen ts were therefore not a major component of eclogite exhumation. Pressure es timates associated with old and young fabrics within the GSZ are comparable , indicating that during extensional deformation there was no significant u nroofing of the hangingwall. Since there are no known extensional structure s younger than 36 Ma at higher levels in this part of the Alps, exhumation since the final juxtaposition of the two units (at 36 Ma) seems to have bee n dominated by erosion.