HORIZONTAL LITHOSPHERE COMPRESSION AND SUBDUCTION - CONSTRAINTS PROVIDED BY PHYSICAL MODELING

Physical modeling of the subduction is performed with a two-layered mantle model (elastico-plastic lithosphere and low-viscosity asthenosphere) and is governed by the criteria of similarity. Compression of the lithosphere in the area of a passive continental margin has been shown to produce a buckling instability in the oceanic plate with wavelengths of 200 km on the average. Later, a localization of deformation occurs in sagging at some distance from the margin where a strongly dislocated linear ridge is formed due to the thrusting. The plate then experiences a failure along the inclined zone, and subduction starts. The inner trench slope which forms therefore has a scraped structure and include a block of crushed and dislocated oceanic crust and sediments in the lower section. If there is an old inclined fault striking across the compression of an oceanic lithosphere, it is on that fault that a subduction zone is initiated. The inner trench slope has then a different structure and forms, due to normal faulting, in the frontal part of the overriding plate. The formation of a subduction zone requires a compression that is smaller than that in the preceding case by a factor of 2 or 3. The subducting plate experiences an elastico-plastic bending and (under specific conditions) thrusting along the zone, dipping from under the overriding plate oceanward and crossing the entire lithosphere. The best agreement between generalized relief in subduction zone in the model and nature is achieved when a shear yield limit tau(s) = 1.3 x 10(8) Pa, modulus of elasticity E about a few times 10(11) Pa, and a thickness H = 60 km, are adopted for the real lithosphere.