South-American plate advance and forced Andean trench retreat as drivers for transient flat subduction episodes

Abstract

At two trench segments below the Andes, the Nazca Plate is subducting sub-horizontally over -1/4200-300 km, thought to result from a combination of buoyant oceanic-plateau subduction and hydrodynamic mantle-wedge suction. Whether the actual conditions for both processes to work in concert existed is uncertain. Here we infer from a tectonic reconstruction of the Andes constructed in a mantle reference frame that the Nazca slab has retreated at -1/42 cm per year since -1/450 Ma. In the flat slab portions, no rollback has occurred since their formation at -1/412 Ma, generating - €' horse-shoe' slab geometries. We propose that, in concert with other drivers, an overpressured sub-slab mantle supporting the weight of the slab in an advancing upper plate-motion setting can locally impede rollback and maintain flat slabs until slab tearing releases the overpressure. Tear subduction re-establishes a continuous slab and allows the process to recur, providing a mechanism for the transient character of flat slabs.