(Ultra-) High-pressure metamorphism and orogenesis: An Alpine perspective

The Pressure–Temperature–time–deformation evolution of the high- to ultra-high-pressure units of the Western Alps has been progressively refined in the last 40 years, leading to several paradigm shifts in the understanding of orogenesis. This set of information, combined with Cretaceous–Tertiary plate kinematic reconstructions and Mesozoic palaeogeography, indicates that the Western Alps represent the amalgamation of a Cretaceous and an Eocene orogen, which developed at the expense of the Adriatic and European rifted margins, respectively. In the Cretaceous, NNE-directed drift of the Adriatic plate, parallel to the inherited Jurassic structural trends, led to the development of a highly oblique subduction boundary. In this setting, the Canavese Zone and parts of the Sesia Zone, derived from the hyper-extended Adriatic margin, underwent deformation and metamorphism at conditions ranging from sub-greenschist to eclogite facies. A Lower Eocene switch to NNW-directed motion, perpendicular to the rift-related Jurassic structural trends, culminated in the collision between the proto-Alps and the Briançonnais block at ca. 44 Ma. As a result, the Briançonnais Domain and parts of its hyper-extended margin, preserved in the (U)HP Piemonte Units, were accreted to the orogen. Continued convergence was subsequently accommodated by subduction of the more external Valaisan basin and of the thinned European margin. The arrival of thick European continental crust at the subduction zone at ca. 35–30 Ma marked the onset of the final continental collision.To a first order, the orogen grew through the progressive episodic accretion of units located towards north-westerly positions. Accretion of subducting units at the front of the orogen was coeval with kinematic reworking of tectonic contacts in the hangingwall units, locally resulting in renewed deformation/metamorphism.