Internal zone extension and external zone shortening in the Alps

The eclogites of the Piemonte zone in the Alps were unroofed by SE-directed extensional shear from 100 km to 30 km depth. This shear zone, the Gressoney shear, was a surface subparallel to the subduction zone but with opposite movement sense. This scenario is tightly constrained by our mapping and geochronological work: unroofing occurred in the period 45-36 Ma. In the external zones, a marine foreland basin was actively migrating away from the orogen, and receiving clastic detritus at this time. This migration is of the same order of magnitude as the displacement induced by internal zone extension. They must have been strongly dynamically coupled, and local buoyancy played a key role. Although the eclogites are dense, they form a relatively thin sliver. Attachment to, or influence by, upwardly flowing continental material is likely to have been important. Numerical modelling confirms profound coupling between internal zone and external zone processes and between different levels in the lithosphere. It leads us to reappraise the dynamics of foreland basin evolution in this mountain belt.