SUBDUCTION OF THE LITHOSPHERE AND BACK-ARC DYNAMICS - INSIGHTS FROM PHYSICAL MODELING

Physical modeling of the subduction of an elastico-plastic lithosphere underlained by a low-viscosity asthenosphere and driven by both horizontal compressional force and gravitational sinking of the subducting plate has shown the subduction regime to be strongly affected by DELTArho (DELTArho = rho(l)-rho(a) where rho(a) and rho(l) are the asthenosphere and lithosphere densities). At DELTArho = 0, a steady state process develops which requires a horizontal lithosphere compression of sigma(h) = (0.3-0.4)sigma(s) almost-equal-to 10(8) Pa, where sigma(s) is the lithosphere yield limit. The central part of the leading wedge of the overriding plate is nonisostatically elevated in this case, especially in the region of the frontal arc which corresponds to a free air gravity anomaly maximum of 200-400 mGal. A decrease in DELTArho (DELTArho<0) results in an increase of compression, growth of the frontal arc, and flattening of the Benioff zone, and, when compression is very intensive, a failure of the lithosphere and the formation of a new subduction zone either at the back arc zone abutting the volcanic arc, directly at the volcanic arc, or at the area just behind the outer rise, depending on the specific conditions. At DELTArho>0, compression of the overriding plate is considerably less from the onset of subduction and then decreases during subduction. At some point, compression changes into tension. Tensional nonhydrostatic stresses grow during the process to a value near sigma(h) = (0.1-0.2)sigma(s) almost-equal-to 0.4 x 10(8) Pa. Both subsidence of the overriding wedge below the isostatic equilibrium level and an increase in the Benioff dip angle accompany this process. The gravity anomaly maximum is displaced into the volcanic arc in this subduction regime, where it is normally less than 150 mGal. If some weaknesses are present in the overriding plate, tensional stresses result in a failure of the lithosphere at these locations and the broken lithosphere segment (the arc plate) starts to move counter to the subduction plate. The arc plate slides along the underthrusting plate's sulface without loss of contact and causes back arc opening. Hydrostatic suction prevents the plates from being separated. This effect is caused by the peculiarity of a mechanical system whereby a low-viscosity asthenosphere and a plastic lithosphere which effective yield limit is considerably lower than the hydrostatic stresses in the plate. If the overriding lithosphere is not weakened enough to fail under the existing tension, then the hanging slab breaks down under its own weight when it reaches a specific length. Both interacting plates undergo isostatic recovery in this case, and the whole cycle is repeated again. The hydrostatic suction effect also provides back arc tension under other conditions, such as when asthenospheric flow forces the subducted plate to move away from the island arc.