Structural evolution of a subducted continental sliver: the northern Dora Maira massif, Italian Alps

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Abstract
Continental crust of the Dora Maira massif was subducted, in part, to pressures of 30 kbar during early stages of the Alpine orogeny. A new investigation in a transect across the north of the massif has shown that much of the dominant foliation in the upper Dora Maira (Pellice) unit was formed at high pressures of the order of 11 kbar, but at lower pressure than the locally preserved relicts of eclogite-facies metamorphism. This dominant deformation phase produced large recumbent isoclinal folds with intensely flattened limbs in the Pellice unit basement and the overlying Permian–Mesozoic cover (now white augen gneisses and calcschists). The rocks were thus part of the upper crust prior to their subduction, unless an unrealistically large thickness of Mesozoic rocks (now removed) is hypothesized. The Pellice unit was later emplaced over the Chisone unit in greenschist facies conditions. All shear sense indicators are top-to-west, implying a simple but prolonged history in which high-pressure units were emplaced westwards over lower pressure ones whilst being exhumed. The intense horizontal stretching recorded in the basement gneisses may have helped bring them closer to the surface by thinning. Indirect arguments suggest that such tectonic thinning is not, on its own, sufficient to explain the present setting of the eclogite units. One or more sheets of eclogite material may have been extruded from the Eoalpine subduction zone in addition. Such slabs would be bounded below by west-directed, and above by east-directed, shears. As the Dora Maira massif forms the structurally lowest Eoalpine eclogite unit in the Alps, the observed history is not incompatible with this model. East- or southeast-directed structures in the Alps are commonly identified as backthrusts, but an origin for some as the extensional shears predicted by this model should not be excluded.