The unique anisotropy of the Pacific upper mantle

The development and interpretation of tomographic models of the Earth's mantle have usually proceeded under the assumption that fast and slow seismic velocity anomalies represent a spatially heterogeneous temperature field associated with mantle convection. Implicit in this approach is an assumption that either the effect of anisotropy on seismic velocities is small in comparison with isotropic thermal or compositional effects, or that the tomographic results represent the average isotropic heterogeneity, even if individual seismic observations are affected by anisotropic structure. For example, velocity anomalies in the upper portions of the oceanic mantle are commonly interpreted in terms of the progressive cooling(1,2) (and localized reheating(3)) of a mechanical and thermal boundary layer consisting of rigid oceanic lithosphere and an underlying, less viscous, asthenosphere. Here, however, we present results from a global three-dimensional tomographic model of shear-wave velocity which shows that the uppermost mantle beneath the central Pacific Ocean is considerably more complicated than this simple model. Over a broad area, with its centre near Hawaii, the seismic data reveal a regional anomaly in elastic anisotropy which produces variations of seismic velocities that are at least as large as those due to thermal effects. Because seismic anisotropy is an indicator of strain in Earth materials, our tomographic results can be used to put constraints on both buoyancy forces (thermal effects) and flow patterns in the upper mantle.