The interaction of subducting slabs and the 670 kilometer discontinuity

Citation

King, Scott David (1991) The interaction of subducting slabs and the 670 kilometer discontinuity. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pme7-gx03. https://resolver.caltech.edu/CaltechTHESIS:04132011-101632415

Abstract

The subduction of oceanic lithosphere plays a major role in the dynamics of the Earth. The dynamics of subduction are influenced both by variations in density (due to phase changes or compositional changes), and variations in viscosity encountered by the slab; the rheology of the slab and the coupling of the slab with the oceanic lithosphere also play important roles. Geoid and topography place fundamental constraints on subduction, and observations can be used to test various mantle models. The effects of the rheology of slabs are considered using finite element convection calculations with Newtonian (linear) and non-Newtonian (power-law) temperature-dependent rheology. Newtonian temperature-dependent fluids do not exhibit slab-like features without weakening the thermal boundary layer. Weak zones are imposed at the trench and ridge, and the effects of varying the size, location, and strength of the weak zones are studied. Non-Newtonian rheology provides a self-consistent mechanism for weakening the thermal boundary layer without imposing a weak zone at the trench. This self-consistency is not proof or confirmation of the importance of power-law deformation in the Earth. Even with non-Newtonian rheology, a weak zone at the ridge is necessary for plate-like behavior. The geoid and topography for slabs with a density discontinuity and a viscosity discontinuity are compared. Weak slabs deform rapidly by spreading out along the density discontinuity with little deformation of the boundary, while strong slabs deform slowly and locally depress the density boundary. However, the long wavelength components of the geoid and topography are independent of the lateral variations in viscosity from the slab. Finite deformations of a compositional boundary are compared with an undeformable boundary approximation; the long wavelength components of the geoid and topography are indistinguishable for boundary deformations up to several hundred kilometers. Subduction calculations are computationally intensive and high resolution is required to resolve deformation at the trench. The solutions are time-dependent, and a temperature-dependent rheology is required. Faster and more powerful numerical techniques are needed. A fast implementation of the finite element method is presented. Applied to creeping flow, this formulation allows large viscosity variations, but is still efficient on a vector supercomputer.

Thesis Files