Opposite vergence of Nappes and crustal extension in the French‐Italian western Alps

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Abstract
In the Internal Pennine Zone of the French‐Italian Western Alps (Dora Maira, Monviso, Rocciavre massifs and Queyras region), the structural trends define an apparent “dome” geometry that is truncated to the east by the Pô Basin. This geometry is classically interpreted as a backthrusting system. The kinematic history of the area has been investigated using structural, metamorphic and geochronologic data, on the basis of which two main deformation episodes have been distinguished that occurred under differing metamorphic conditions: (1) The first deformation occurred under greenschist facies metamorphism. It caused a regional scale foliation and an EW trending lineation that lie parallel to ductile deformation zones and thrust contacts. Sense‐of‐shear indicators associated with the lineation indicate contradictory top‐to‐the‐east and top‐to‐the‐west senses of shear. From these, it is deduced that coeval east and west vergent thrust faulting accommodated major displacements between nappes. Phengite Ar‐Ar data yield late Eocene‐early Oligocene ages for the greenschist facies metamorphism. (2) The second deformation caused the widespread development of high‐angle shear bands and normal faults on opposite sides of the Dora Maira massif resulting in the apparent “dome” geometry of the massif. A steep (50–80°) network of west‐dipping normal faults is particularly well developed on the west side of the Dora Maira massif. Differences of 2–3 kbar and 100–150°C between juxtaposed Monviso + Rocciavre and Queyras nappes implies ≥8 km of vertical displacement along the Monviso upper tectonic contact. These normal faults formed in a more brittle regime. Only a post‐early Oligocene age can be estimated from microstructural studies for these final movements. These deformations postdate HP‐LT metamorphism and resulted in the juxtaposition of nappes of distinctly different origin, age, grade, and P‐T‐t history. A kinematic model is presented in which opposite vergence of nappes followed by large‐scale crustal extension occurred in response to uplift during the convergence between the European and South Alpine plates. This model may account for rapid uplift, rapid unroofing and cooling of high pressure‐low temperature rocks (thereby explaining the absence of amphibolite facies metamorphism overprint in this part of the Alps) that may have led to the preservation of coesite‐bearing rocks in the southern part of the Dora Maira massif.