Overview
This study presented a tomographic model of the upper mantle beneath the Juan de Fuca plate. We used data from the offshore component of the Cascadia Initiative, an experiment that deployed ocean bottom seismometers from the Washington, Oregon, and Northern California coast to out past the Juan de Fuca and Gorda Ridges.
Perturbations to shear-wave velocity at different depths in the preferred model
BUOYANCY driven Upwelling
We observe an asymmetric, a sharply demarcated, and a sinuously shaped low velocity zone beneath the Juan de Fuca Ridge. These observations are most consistent with models of buoyancy driven upwelling, which occurs in geodynamic models when the viscosity of the upper mantle is low enough for the buoyancy of the mantle to enhance the rate of upwelling.
Variations in asymmetry
One of the remarkably findings was how the shape of the low velocity zone varies along the axis of the Juan de Fuca Ridge. On the southern part of the ridge (near 45N), the low velocity zone is relative symmetric, perhaps shifted to the east some. But, there is pronounced asymmetry to the west near 47 N. Gravity anomalies suggest the same asymmetry in the density of the mantle and helium isotopes reaches peak He3 concentrations near the maximum asymmetry. All of this data suggest a high temperature anomaly in the mantle near the peak in asymmetry.
(left) Cartoon showing the inferred melting region (shades of orange) and mantle flow lines (right) Comparison between the degree of asymmetry from the tomography model (positive indicates a westward shifted low Vs region), helium isotopes, and asymmetry in crustal thickness-corrected gravity anomalies (positive is weaker gravity west of the ridge) Remarkably the data all seems to correlate.