SUBDUCTION DYNAMICS, CORDILLERAN MOUNTAINS AND BACKARC BASINS

The style of overriding plate deformation at subduction zones varies considerably. At one end of the spectrum are subduction systems that experience backarc spreading or overriding plate extension in the backarc and/or forearc, such as the Scotia Sea in the South Atlantic and the Aegean Sea bordering the Hellenic subduction zone. At the other end of the spectrum are subduction systems that experience overriding plate shortening that leads to cordilleran mountain building such as the Andes at the South American subduction zone. It remains unknown why a large number of overriding plates bordering subduction zones are characterized by extension/spreading, some experience shortening, and yet others show very little deformation. In the current contribution, advanced numerical models of progressive subduction in three-dimensional space will be presented that have been developed to investigate the role of subduction zone size (its trench-parallel extent, expressed as slab width W) and time on the style and rate of deformation in the overriding plate, as well as, more generally, on the style, geometry, kinematics and dynamics of subduction. The models demonstrate the crucial role that subduction zone size and time play in determining overriding plate deformation style. Cordilleran orogeny is only observed for subduction zones with a very wide slab (≥ ~4000 km) and commences only in a mature stage of subduction (after ~70 Myr) once the slab enters the lower mantle. In contrast, narrow slab subduction is characterized exclusively by overriding plate extension, even during subduction into the lower mantle, and results in very different subduction kinematics and slab geometries compared to wide slab subduction. Ultimately, the geodynamic models provide a new explanation for the diversity of overriding plate deformation at subduction zones as exemplified by two end member cases, namely the Scotia backarc basin in the South Atlantic and the Andes mountains in South America.