Visco-elastic finite element analysis of subduction zones

Abstract

A new visco-elastic finite element method is developed and applied to some of the processes in subduction zones. The effects of phase changes are simulated by deriving an equation of state for the mantle under mineralogical equilibrium. Using the elastic parameters determined from this equation, the stresses due to the contraction of the descending slab as it changes phase at the olivinespinel transition are shown to be about 8 x 10 (^8) n/m (^2). The phase changes are also shown to play an important role in the flexure and bending of the lithosphere from the earth's surface to plunge at 45 – 60o into the asthenosphere. The phase changes effectively reduce the bulk modulus and so the lithosphere bends more easily. The major bending is at 30 - 60 km depth where the stresses due to bending extend the area of phase transitions so that it extends throughout the thickness of the descending slab. Phase changes and fracture combine to reduce the flexural parameters of the lithosphere to that estimated from the shape of the outer-rise. Thin plate theory, however, is shown to be inapplicable to this region. Tensional stresses aligned parallel to the dip of the descending slab are shown to be necessary to maintain the large negative gravity anomaly associated with subduction zones. This applies in all subduction zones, and local stresses must be responsible for the earthquakes indicating down dip compression in the upper part of the descending slab. The shear zone between two converging plates can be adequately modelled in visco-elastic finite element analysis by a row of elements whose viscosity is 2-3 orders of magnitude lower than the surrounding rocks.