Numerical analysis of deformation in the upper part of subduction zones
The stresses and deformation in an accretionary prism, and the crystalline basement behind and beneath it, have been modelled using finite element analysis, assuming a visco-elastic Maxwell rheology for the rocks involved. A new method for finding the effect of lateral variations in density and body forces on the deformation in such models has been developed, so that the balance between weight and basal shear in the accretionary orism, and the associated displacement and stress distributions, could be modelled. Analysis shows that there is an equilibrium basal stress that supports the weight of the accreted sediments. Above this stress the accretionary prism is built higher, and below it subsides and spreads up the basal slope. The average value for this stress was found to be 12 MPa for the Middle America subduction zone and 5 MPa for the central Aleutians. Models of these two subduction zones show important differences in surface displacement and stress distribution, due to the slope and extent of the overriding basement rocks. In the island arc model, it was concluded that the igneous crust extended beneath the Aleutian terrace to the edge of the inner trench slope, while in the case of the Middle America subduction zone the continental basement is cut back at depth and parts of it are underlain by accreted sediments. Displacement boundary conditions were applied to the basal thrust to investigate the effects of coupling and decoupling on it, and in this way the repeat time for earthquakes, at a depth of c. 15 km in the Middle America subduction zone, was predicted to be c. 250 yr, or less. Finally, the results for a simple accretionary wedge, applied to the mechanics of a thrust sheet, show that the basal gradient is an important controlling factor, and that gravitational forces alone cannot cause thrust motion up a basal slope, unless the thrust wedge is supported (at the end lower down the basal slope) by stresses which are lithostatic or greater.