Some remarks on subduction zones

The rate of subduction is larger than the convergence rate along W-directed subduction zones, whereas it is smaller along Eor NE-directed subduction zones. Along Wdirected slabs, the subduction rate is the convergence rate plus the slab retreat rate, which tends to equal the backarc extension rate. Along Eor NE-directed slabs, the subduction rate is decreased by the shortening in the upper plate. The two opposite kinematics predict a subduction hinge migrating away from (W-directed subduction), or toward the (E-NE-directed subduction) upper plate. Relative to the mantle, the W-directed slab hinges are fixed, whereas they move west or southwest along Eor NE-directed subduction zones. With a given convergence rate, the amount of shortening in the orogens related to Eor NE-directed subduction zones is inversely proportional to the viscosity of the upper plate, i.e., higher the viscosity, smaller the shortening and faster the subduction rate. The convergence/shortening ratio in this type of orogens can be a useful tool for mapping upper plate viscosity values. In the central Andes the ratio is 2.2. 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. The slab pull is questioned as not a first order force for plate motions. Mantle convection is also inadequate to explain the Earth’s surface kinematics. The Earth’s rotation seems to be the simplest candidate to explain the asymmetry along subduction zones. Corresponding author: C. Doglioni; Dipartimento di Scienze della Terra, Università La Sapienza, P.le A. Moro 5, 00185 Roma, Italy. Phone +39 0649914549, e-mail: [email protected]