Material models applied to pacific trench flexure

Abstract

The oceanic lithosphere adjacent to a subduction zone is modelled as an elastic beam with variable flexural rigidity or as a viscoelastic beam incorporating Mohr-Coulomb and tensile failure. The viscous flow equations include temperature and pressure dependent terms. As an oceanic plate approaches the axis of a deep trench, pervasive failure causes a large reduction in flexural rigidity over a distance of roughly 100 km. The same plate suffers little reduction in flexural rigidity in a shallow, low-curvature trench. In approaching the trench axis the upper part of the oceanic lithosphere model undergoes brittle failure, with normal faults extending to depths of up to 55 km. Below the zone of normal faulting elastic deformation prevails through a depth interval of 15 km with maximum differential stresses of 5–10 kb. In the 15 km below the elastic zone stresses drop by an order of magnitude as strain becomes viscous. The upper brittle-elastic zone of the lithospheric model has strength consistent with low-temperature experimental results for olivine. Thermally activated continuum deformation occurs only deep in the plate.