A reappraisal of the orogenic cycle : thermal mechanical evolution of orogens along convergent plate boundaries

Sixty years ago, the advent of plate tectonics (Wilson, 1965; Le Pichon, 1968; Morgan, 1968) provided a framework to account for the relationship between lithospheric plate convergence and orogenic evolution. Metamorphic belts with gradients ranging from high-pressure/low-temperature (HP/LT) to low-pressure/high--temperature (LP/HT) nourished the concept of alpinotype and hercynotype orogens (Zwart, 1967) and of hot vs cold orogens (Chardon et al., 2009) attributed to secular cooling of the Earth (Brown, 2007). It also led to the distinction between subduction-type orogens, currently represented by the Cordilleras along the Pacific Ocean, and collision-type orogens exemplified by the Alpine-Himalayan belt (Dewey and Bird, 1970). In this view, plate convergence is first accommodated by subduction and is followed by continental collision, which marks the end of the Wilson orogenic cycle (Wilson, 1966) owing to the low density of the continental crust that impedes subduction (McKenzie, 1969). The concept of subduction-type orogen has been extended in the one of accretionary orogens marked by prolonged subduction of an oceanic plate and successive opening and closure of back-arc basins and associated tectonic accretion of terranes (Collins, 2001; Cawood et al., 2009). In turn, the concept of collision-type orogen has fed the model of indentation based on the India-Asia collision (Molnar & Tapponnier, 1975). This description of the orogenic cycle has been challenged by the documentation of UHP metamorphism attributed to continental subduction (Chopin, 1984) and of extension of previously thickened crust in zones of active plate convergence (Coney & Harms, 1984) ascribed to gravitational collapse (Dewey, 1988; Rey et al., 2001).

These discoveries called for a reassessment of the orogenic cycle in order to capture the variety of orogenic belts as a function of plate kinematics, the fate of the crust along convergent plate boundaries, and the thermal-mechanical evolution of the orogenic crust (Vanderhaeghe, 2009; Vanderhaeghe & Duchêne, 2010; Vanderhaeghe et al., 2012). Convergent plate boundaries are marked, at the lithospheric scale, by slab advance or retreat associated to crust/mantle mechanical coupling or decoupling. Slab advance is characterized by distributed deformation across sutures between former continental blocs and corresponds to indentation. In turn, slab retreat promotes subduction of the continental crust and HP/LT metamorphism, but also exhumation of these units, owing to their buoyancy, into the space induced by extension of the overriding plate. In this case, the orogenic wedge is predominantly constructed by tectonic accretion and vertical extrusion of terranes mechanically decoupled from the downgoing plate. After tectonic accretion, slab retreat induces concomitant thickening of the orogenic crust and thinning of the lithospheric mantle, which favor the construction of a hot, buoyant and weak orogenic crust. Partial melting and gravity-driven flow of the orogenic root control the transition from an orogenic wedge to an orogenic plateau. If plate kinematics changes and/or if the 3D geometry of the plate boundaries comprises a free boundary, lateral flow of the orogenic crust might result in gravitational collapse of the orogenic belt. These different stages of orogenic evolution are pictured examples in the Alpine and Variscan orogenic belts.