Three-dimensional subduction models of overriding plate deformation and mantle flow

Subduction zones form along convergent plate boundaries where an oceanic plate is thrust underneath an overriding plate into the Earth's mantle. Some overriding plates are characterized by extension and backarc basins, such as found in the Southwest Pacific, while others are characterized by shortening and mountain belts like the Andes in South America. The variability in overriding plate deformation at different subduction zones has been attributed to different factors such as subducting plate age, overriding plate velocity and subduction zone friction. A global statistical investigation of active subduction zones, however, indicates that none of these parameters can explain the wide variety of overriding plate deformation as observed in nature, as all of these parameters show correlations that are statistically insignificant at the 95% confidence level. The only correlation of statistical significance is the one that relates trench migration and overriding plate deformation. This correlation shows that relatively rapid trench retreat corresponds to backarc extension, while a relatively stationary trench or a slowly advancing trench corresponds to backarc shortening. This correlation implies that the negative buoyancy force of the slab is the primary driver of trench migration, backarc basin formation and subduction zone orogeny. Notwithstanding this statistical support for the slab's involvement in controlling overriding plate deformation, a physical basis for this hypothesis is lacking. The kinematics and dynamics of the subduction process have been investigated extensively in three-dimensional models of progressive subduction over the last ten years, but many of these models lack an overriding plate. 3D subduction models that do include an overriding plate generally have applied kinematic boundary conditions, and are thus not fully dynamic, or they are instantaneous models and thus do not provide insight into the progressive evolution of subduction and deformation. Here we present fully dynamic laboratory and numerical experiments of progressive subduction in 3D space illustrating how subduction zone slabs deform and interact with the overriding plate and the ambient mantle at depth. The models involve an overriding plate and a subducting plate that are both free at the trailing edge. The models show that narrow slabs retreat and that plate velocities and trench velocity are not significantly affected by the presence of an overriding plate as long as significant decoupling occurs at the plate boundary interface. The overriding plate is stretched considerably in the direction normal to the trench with several hundred km of extension after ~2000 km of subduction. The slab, trench and overriding plate develop an arc shaped geometry that is concave towards the mantle wedge. For subduction models with a narrow slab subduction is accommodated predominantly by trench retreat (~60%) and to a lesser extent by trench-directed subducting plate motion (~40%).