What moves slabs?

When considering a migrating subduction hinge, the kinematics of convergent geodynamic settings shows that subduction zone rates can be faster or slower than convergent rates as a function of whether the subduction hinge migrates away or toward the upper plate. This opposite behaviour occurs in particular along W-directed and E- or NE-directed subduction zones respectively. Along W-directed 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-directed slabs, the subduction rate is decreased by the shortening in the upper plate. Relative to the mantle, the W-directed slab hinges are fixed, whereas they move west or southwest along E- or NE-directed subduction zones. Therefore, subduction zones appear as passive features controlled by the far field plate velocities and their motion relative to the underlying �eastward� mantle flow rather than by the negative buoyancy alone of the downgoing plate. Several observations cast doubts about the efficiency of the slab pull alone in triggering plate motions. For example, kinematically, the slab is moving out of the mantle along some E- or NE-directed subduction zones, i.e., moving in the direction opposite to the one predicted by the pull of the slab. Mantle convection is also inadequate to explain the Earth�s surface kinematics. Plate motions driven by the Earth�s rotation seem to be the simplest explanation for the asymmetry along the subduction zones and the aforementioned incongruence.