Reconciling surface plate motions with rapid three-dimensional mantle flow around a slab edge

Abstract

The direction of tectonic plate motion at the Earth's surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally, suggesting large-scale coupling between the mantle and the surface plates. The fit is typically poor at subduction zones, however, where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to and may be several times faster than plate motions. Here we present three-dimensional numerical models of buoyancy-driven deformation with realistic slab geometry for the Alaska subduction-transform system and use them to determine the origin of this regional decoupling of flow. We find that near a subduction zone edge, mantle flow velocities can have magnitudes of more than ten times the surface plate motions, whereas surface plate velocities are consistent with plate motions and the complex mantle flow field is consistent with observations from seismic anisotropy. The seismic anisotropy observations constrain the shape of the eastern slab edge and require non-Newtonian mantle rheology. The incorporation of the non-Newtonian viscosity results in mantle viscosities of 10(17) to 10(18) Pa s in regions of high strain rate (10(-12) s(-1)), and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates. These results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab-pull force available to drive subducting plates.

Using open sidewalls for modelling self-consistent lithosphere subduction dynamics

“…Jadamec and Billen, 2010;Piromallo et al, 2006;van Hunen et al, 2011) may proof beneficial. In 3-D, flow patterns have larger degrees of freedom as toroidal flow can be excited, e.g.…”

Three-dimensional simulations of geometrically complex subduction with large viscosity variations

“…The non-linear relationship between stress and strainrate for deformation by dislocation creep leads to a faster rate of deformation (higher strain-rates) for a given stress, which can be expressed as a reduction in the effective viscosity (i.e., η eff = σ/2ε). This suggests that where strain rates are high in the mantle wedge, the region between the overriding plate and the subducting plate sinking into the mantle, the viscosity can be reduced to as low as 10 17 Pa s. Therefore, in subduction zones, large viscosity variations can occur between the relatively stronger lithosphere and weaker mantle wedge, with a transition from 10 24 Pa s in the lithosphere to 10 17 Pa s in the center of the mantle wedge occurring over distances of less than 100 km [14,16].…”

“…For example, experimental studies and observations of seismic anisotropy suggest that dislocation creep is the dominant deformation mechanism in the upper mantle [14,21,16]. The non-linear relationship between stress and strainrate for deformation by dislocation creep leads to a faster rate of deformation (higher strain-rates) for a given stress, which can be expressed as a reduction in the effective viscosity (i.e., η eff = σ/2ε).…”

“…The rheology of the earth contains large viscosity variations occurring over short distances, and this poses a challenge for computational codes [25,24,30,22,10,6,16,9]. For example, experimental studies and observations of seismic anisotropy suggest that dislocation creep is the dominant deformation mechanism in the upper mantle [14,21,16].…”

“…Numerical models of subduction zone dynamics that incorporate both a subducting plate and an overriding plate typically treat the plate interface as either a discrete fault [18,34] or as a localized low viscosity zone [12,19,3,16]. Depending on the coupling between the plates, the rheological contrasts across this interface can be significant.…”