Simple Kinematics of Subduction Zones

Abstract

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 subduction 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 southwest 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.

The Calabrian Arc subduction complex in the Ionian Sea: Regional architecture, active deformation, and seismic hazard

“…Despite the very slow modern‐day plate convergence rates observed by GPS, subduction may still be active in the CA, and subduction rate will depend both on the convergence rate and the velocity of the subduction hinge [ Doglioni et al , 2006, 2007]. GPS sites in Calabria (relative to Apulia) show systematic residuals directed toward the Ionian Sea, suggesting active crustal compression and an outward motion of the CA as a result of still active subduction that accounts for the shortening taken up in the accretionary wedge, eventually accommodated by long‐term slip on the subduction interface [ Gutscher et al , 2006; D'Agostino et al , 2008].…”

When and how did plate tectonics begin? Theoretical and empirical considerations

“…Beneath the interior of an oceanic plate, asthenosphere moves towards the ridge [7] , whereas around the plate margins the asthenosphere flows into or away from the basin, depending whether the ocean is growing (Atlantic-type) or shrinking (Pacific-type) [8] . Some workers infer an "eastward drift" of asthenosphere [9] but east-moving plates do not move faster than west-moving plates. There may be some basal drag affecting the thick lithospheric keels of continents [10][11][12] , but this flow impedes as well as encourages the motions of continent-bearing plates.…”

Plate subrotations

“…Apart the forces that move the lithosphere, every time a plate experiences a subrotation, there would be an additional force, as for example a couple of forces, generating the two-rotation motion. Regardless the kinematic implications, the absolute subrotations could indicate a differential mantle drag at the base of the lithosphere, and support a passive behavior of the lithospheric motion [Cruciani et al, 2005;Doglioni et al, 2006].…”