ASYMMETRIC SUBDUCTIONS IN AN ASYMMETRIC EARTH

The subduction rate is V_S=V_H-V_L, where V_H and V_L are the velocities of the subduction hinge and the lower plate respectively with respect to the upper plate. The velocity between the upper and the lower plates (i.e., the convergence rate) is partitioned into the subduction rate and the shortening in the accretionary prism or orogen. In general, the convergence/shortening ratio along subduction zones is >1 along E- or NE-directed subduction zones, whereas it tends to be <1 along the W-directed subduction zones (Doglioni et al., 2007, ESR, 83, 125). Along E- or NE-directed subduction zones, the convergnce/shortening ration is higher during oceanic subduction (1.7-1.9), whereas it decreases during continental subduction (1.3-1.5). The difference between the W-directed and E- or NE-directed subduction zones fits with the net rotation of the lithosphere, i.e., the so-called W-ward drift. This rotation has the pole of rotation at about -56° Latitude and 137° Longitude, with an estimated angular velocity ω of 1.2°/Ma (Crespi et al., 2007, GJI, 168, 491). The "westerly" directed decoupling of the lithosphere relative to the underlying mantle can account for the asymmetry in the kinematic behavior and geometries between the opposite subduction zones and the first order tectonic differences among the related prisms. The main signatures are in order low structural and morphological elevation, shallow decollement (top lithosphere) and single verging prism, backarc basin, steep slab, and subduction rate faster than the convergence rate along the W-directed subduction zones. Along the opposite E- or NE-directed subduction zones, there rather are higher structural and morphologic elevation, shallower slab, deep decollements (whole crust and LID) and double verging orogen, and the subduction rate is slower than the convergence rate. Along the W-directed subduction zones the prism is mostly composed by lower plate rocks, whereas the orogen is dominantly composed by shortening of the upper plate during oceanic subduction, eventually involving thick sections of the lower plate during continental subduction (i.e., collision). A geographically sensitive asymmetric pattern has been suggested also for oceanic basins (Panza et al., 2010, Geology, 38, 59). All these asymmetries point for a global tuning of plate tectonics, i.e., a "westerly" directed force acting on the whole lithosphere generated by the tidal despinning of the Earth (Riguzzi et al., 2010, Tectonophysics, 484, 60).