Simple kinematics of subduction zones

Two main types of subduction zones can be distinguished: (1) those where the subduction hinge migrates away from the upper plate; and (2) those in which the subduction hinge migrates toward the upper plate. Apart from a few exceptions, this distinction seems to apply particularly for W-directed subduction zones and E- or NE-directed subduction zones, respectively. Moreover, the rate of sub- duction is larger than the convergence rate along W-directed subduction zones, whereas it is smaller along E- or NE-directed subduction zones. Along W-inclined slabs, the subduction rate is the convergence rate plus the slab retreat rate, which tends to equal the backarc extension rate. Along E- or NE-dipping slabs, the shortening in the upper plate decreases the subduction rate, and no typical backarc basin forms. Relative to the mantle, the W-directed slab hinges are fixed, whereas they move west or south- west along E- or NE-directed subduction zones. The single vergent accretionary prism related to W-directed subduction zones is formed at the expenses of the shallow layers of the lower plate. The orogens generated by E- and NE-dipping subductions are double vergent since the early stages, and upper plate rocks mostly compose them until continental collision takes place, eventually involving the rocks of the lower plate more extensively. The convergence/shortening ratio in this type of orogens is higher than 1 and is inversely proportional to the viscosity of the upper plate. The higher the viscosity, the smaller the shortening and faster the subduction rate. The convergence/shortening ratio along W-directed subduction zones is instead generally lower than 1. W-directed subduction zones provide larger volumes of lithospheric return into the mantle than the opposite E- or NE-directed subduction zones. The latter may contribute to refertilize the shallow upper mantle. These observations suggest that subduction zones are passive features with respect to the far-field velocity of plate motions and the relative "eastward" mantle flow, implicit with a net "westward" rotation of the lithosphere.