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

Abstract

[1] 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.Citation: Hines, J. M., and M. I. Billen (2012), Sensitivity of the short-to intermediate-wavelength geoid to rheologic structure in subduction zones,

The zone of influence of the subducting slab in the asthenospheric mantle

“…Weak slabs with shallow to moderate dips generate more vertical flow due to the greater torque on the shallow slabs that tend to rotate the slab toward the vertical and the reduced ability of a weak slab to resist this bending. Thus, the results demonstrate that, in general, weaker slabs and slabs with a shallow to intermediate dip generate more vigorous flow in the asthenospheric mantle surrounding the slab, both in the mantle wedge and in the subslab mantle, consistent with previous models [ Jadamec and Billen , ; Stadler et al , ; Hines and Billen , ].…”

“…Thus, the flow magnitudes and velocity gradients will commonly be greater in a model with a composite viscosity upper mantle because of the reduced asthenospheric viscosity that develops surrounding the slab. This occurs in 2‐D models [ Andrews and Billen , ; Jadamec , ], 3‐D regional models [ Jadamec and Billen , ; Hines and Billen , ; Jadamec and Billen , ], and global models [ Stadler et al , ; Alisic et al , ].…”

Dynamic topography, gravity and the role of lateral viscosity variations from inversion of global mantle flow

“…Although most parts of the slab are still strong, they are weakly connected to the surface, yielding dynamic topography and gravitational fields similar to that of a homogeneous weak slab (Hines and Billen, 2012). Considering lateral grain size variations may further reduce the lateral viscosity variations (Glišović et al, 2015).…”

“…In the first case, there could be several factors lowering LVVs compared to those predictions from purely temperature, such as ionic diffusion (Ammann et al, 2010), grain size (Glišović et al, 2015) and stress and yielding induced weakening (Hines andBillen, 2012, Jadamec, 2015). When yielding and dislocation creep regimes are considered, some part of the slab might be weakened.…”

A Visual Survey of Global Slab Geometries With ShowEarthModel and Implications for a Three‐Dimensional Subduction Paradigm

“…Furthermore, along‐strike pressure gradients due to 3‐D slab dynamics and the presence of slab discontinuities have implications for interpreting atypical arc volcanism (Castillo, ; Defant & Drummond, ; Durance et al, ; Heyworth et al, ; Hoernle et al, ; Price et al, ; Yogodzinski et al, ). The contorted shapes of slabs as well as their lateral and depth discontinuities have implications for the strength of slabs (Billen & Hirth, ; Cizkova et al, ; Gerya et al, ; Hines & Billen, ; Jadamec, ; Liu & Stegman, ) and for lateral variations in the viscous support from the mantle (Billen & Jadamec, ; Jadamec, ; Jadamec & Billen, ). Moreover, the examination of the 3‐D connectivity of slabs can help to address the question of what actually defines the lateral limits of a subduction zone and distinguishes one subduction zone from the adjacent subduction zone (Bird, ; Li et al, ).…”