Sensitivity of the short- to intermediate-wavelength geoid to rheologic structure in subduction zones

Abstract

The rheologic structure of subduction zones determines how the negative buoyancy of sinking lithosphere (slabs) couples to the surface to cause surface deformation and plate motion. However, the rheologic structures determined using different methods are inconsistent. Analysis of the geoid and some dynamical models conclude that slabs must be weak, whereas other dynamical models and experimentally determined rheology predict that slabs are strong. Using idealized three-dimensional models of a subduction zone, we show that lateral variations in viscosity (LVVs) within a slab caused by yielding render the geoid insensitive to the viscosity structure outside of the bending region of the slab. Spectral analysis of the geoid up to harmonic degree 360 (features at wavelengths down to 110 km) shows that LVVs cause significant mode coupling with changes to the long-wavelength geoid (>1000 km) caused by short-wavelength (<500 km) changes to dynamic topography. Models with either a uniformly weak slab, or a strong slab with yielding, provide a good match to the observed geoid spectrum and characteristic features of the geoid over subduction zones. Therefore, observations other than the geoid are needed to constrain the strength profile of subducting slabs. Copyright 2012 by the American Geophysical Union.