Subduction dynamics and overriding plate deformation

The style of overriding plate deformation at subduction zones varies from backarc spreading and basin formation, as found in the Scotia Sea, to shortening and cordilleran orogeny, as observed in the Andes. Why this difference exists and why overriding plate extension occurs more frequently than shortening remains unexplained. In this contribution, various conceptual mechanisms of overriding plate deformation are reviewed and assessed. Time-evolving numerical models of buoyancy-driven subduction in three-dimensional space are presented to investigate the influence of subduction zone size and time on the style and rate of overriding plate deformation, and, more generally, on the geometry, kinematics and dynamics of subduction. The models demonstrate how time (temporal evolution, and its related parameter subduction depth) and subduction zone size (width, trench-parallel extent) control deformation style. Cordilleran orogeny occurs only for wide, mature subduction zones during subduction into the lower mantle, while narrow and intermediate-width subduction zones, and wide but young subduction zones, are characterized by overriding plate extension, even during lower mantle slab penetration for narrow and intermediate-width subduction zones and for subduction zone edges of wide subduction zones during early stage lower mantle penetration. This difference in deformation style is explained by the fact that subduction zone size, time and slab depth also control subduction kinematics, slab geometries, slab sinking directions, and mantle flow patterns. Narrow slabs, intermediate slabs, slab edges and short wide slabs generally have an important, slab-pull-driven, backward (oceanward) sinking component resulting in trench retreat and slab rollback, inducing strong local return flow patterns in the upper mantle close to the slab (toroidal for the narrow slabs, intermediate-width slabs and slab edges). The combination of strong local return flow near the slab and slab-pull-driven trench retreat induces deviatoric tension in the overriding plate through trenchward-increasing basal drag and subduction interface deviatoric tension, causing extension. In contrast, a wide slab, in particular that of an old subduction zone, generally sinks forward and subduction occurs mostly through trenchward subducting plate motion, inducing a strong whole-mantle asymmetric poloidal flow cell. The combination of strong poloidal flow-induced basal tractions at the base of the overriding plate that decrease trenchward and subduction interface deviatoric compression due to trenchward overriding plate motion and a relatively immobile subduction hinge induces deviatoric compression in the overriding plate, causing shortening, which is most pronounced in the centre of the wide subduction zone. Comparison between a global review of active subduction zones and the numerical modelling results shows consistent positive correlations for subducting plate velocity and negative correlations for trench velocity, overriding plate deformation rate and slab dip as a function of subduction zone size. Ultimately, the geodynamic models and subduction zone review provide a new explanation for the diversity of overriding plate deformation as exemplified by two endmembers, the small Scotia subduction zone (with backarc spreading) and large South American subduction zone (with shortening), for the non-uniform spatial and temporal occurrence of different deformation styles, and for the more frequent occurrence of active overriding plate extension compared to active overriding plate shortening on Earth.