High-pressure relics studied in many collisional mountain belts are overprinted by subsequent Barrovian metamorphism that may reach migmatite grade in the central parts of such orogens. We propose that this evolution is linked to the development of a narrow tectonic accretion channel (TAC) at the subducting plate boundary. Geologic evidence from the “Southern Steep Belt” of the Central Alps and the tectono-metamorphic record of this orogen guided us in constructing numerical models of a TAC. Simulations indicate that the accretion of crust enriched in radioactive elements to mantle depth provides a mechanism to obtain P-T-t (pressure-temperature-time) trajectories in reasonable agreement with observations in the Alps. Similarly, the generation of granitoid melts predicted by the model during late-orogenic exhumation of the TAC is in line with the Alpine record. This case study suggests that accretion of upper-crust fragments to mantle depth, by underplating along a subduction fault, and subsequent extrusion of parts of the TAC along that same fault, may be fundamental processes in the dynamic evolution of many collisional orogens.

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