Geophysics

Hitching a Ride into the Mantle

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Science  12 Oct 2012:
Vol. 338, Issue 6104, pp. 172
DOI: 10.1126/science.338.6104.172-d

The movement of Earth's crust relies on the underlying mantle to behave viscously and flow as a response to convective forces. As a consequence, the upper mantle just below the oceanic crust may get pulled along with the crust as it is driven to depth at subduction zones. Alternatively, the mantle may decouple from the subducting crust as a result of mantle flow in another direction. One way to track the flow of the mantle is through monitoring how seismic waves respond to the orientation of certain mineral grains, which preferentially align according to flow directions and induce anisotropic splitting of seismic waves. Song and Kawakatsu examined this seismic anisotropy below the global oceanic asthenosphere—the viscous and ductile upper layer of the mantle—below subduction zones as a function of subducting slab dip and incident angles. They found that the times and direction of seismic shear waves split into fast and slow components in a way that is consistent with about 100 km of the asthenosphere flowing as though it is coupled to the subducting slab even with buoyancy forces resisting its downward motion. Appreciable subduction of oceanic asthenosphere over long time scales would require the revision of mantle mixing models.

Geophys. Res. Lett. 39, L17301 (2012).

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