Seismic Anisotropy and Mantle Flow from Ridge to Trench Below the Gorda-Juan de Fuca Plate System

Abstract:

Tectonic plates are underlain by a low viscosity layer of the mantle, the asthenosphere, which flows. Flow in the asthenosphere may be induced by motion of the overriding plate, or by deeper mantle convection. Measurement of seismic anisotropy, the directional dependence of seismic wave speed, is an important tool in understanding mantle structure and dynamics, and is often used to infer information about asthenospheric flow geometry. However, isolation of asthenospheric signals is challenging because most seismometers are located on the continents, whose complex structure influences seismic waves en-route to the surface. Thus the challenge is to record seismic data on oceanic lithosphere, which is much thinner and simpler. We present stacked shear wave splitting results from phases 1 to 3 of the Cascadia Initiative: An ambitious, large-scale deployment of offshore seismometers across the Gorda and Juan-de-Fuca plates. Fast splitting directions (FSD) can approximate mantle flow geometry, thus for the first time allowing an interpretation of flow beneath an ocean basin from ridge to trench. The Juan-de-Fuca plate appears able to influence mantle flow: FSD rotate towards the absolute plate motion direction (APM) with increasing distance from the ridge. In contrast, Gorda FSD align with the motion of the adjacent Pacific plate rather than Gorda APM. These observations suggest that drag caused by motion the Pacific plate controls asthenospheric flow beneath Gorda, and thus that the Gorda plate may be decoupled from the asthenosphere. We also construct a simple geodynamic model of this situation, which supports its plausibility. Our findings imply that tectonic plates must reach a minimum size and speed before they are able to exert influence on asthenospheric flow.