Mantle flow geometry from ridge to trench beneath the Gorda–Juan de Fuca plate system

Journal name:
Nature Geoscience
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Published online

Tectonic plates are underlain by a low-viscosity mantle layer, the asthenosphere. Asthenospheric flow may be induced by the overriding plate or by deeper mantle convection1. Shear strain due to this flow can be inferred using the directional dependence of seismic wave speeds—seismic anisotropy. However, isolation of asthenospheric signals is challenging; most seismometers are located on continents, whose complex structure influences the seismic waves en route to the surface. The Cascadia Initiative, an offshore seismometer deployment in the US Pacific Northwest, offers the opportunity to analyse seismic data recorded on simpler oceanic lithosphere2. Here we use measurements of seismic anisotropy across the Juan de Fuca and Gorda plates to reconstruct patterns of asthenospheric mantle shear flow from the Juan de Fuca mid-ocean ridge to the Cascadia subduction zone trench. We find that the direction of fastest seismic wave motion rotates with increasing distance from the mid-ocean ridge to become aligned with the direction of motion of the Juan de Fuca Plate, implying that this plate influences mantle flow. In contrast, asthenospheric mantle flow beneath the Gorda Plate does not align with Gorda Plate motion and instead aligns with the neighbouring Pacific Plate motion. These results show that asthenospheric flow beneath the small, slow-moving Gorda Plate is controlled largely by advection due to the much larger, faster-moving Pacific Plate.

At a glance


  1. Stacked splitting results determined by this study (red bars) and previous work (black bars; from refs ,).
    Figure 1: Stacked splitting results determined by this study (red bars) and previous work (black bars; from refs 4,28).

    The displayed tomography is a 100–400km vertical average through the DNA13 P-wave velocity model of ref. 29. This depth range corresponds to that part of the asthenosphere considered most likely to be the source of the observed anisotropy9. All splits are plotted at onshore seismometer/OBS locations. The splitting delay times are indicated by the length of the bars; example results with a delay time of 1.0s are shown in the legend (bottom left). Black lines indicate plate boundaries, and the red lines are slab depth contours spaced at 10km intervals30. Black arrows show the direction and magnitude of absolute plate motion (APM) in a hotspot reference frame23, and purple arrows show the subduction direction4. Inset maps show regions featuring a high concentration of splitting results.

  2. Two distinct patterns in the variation of FSDs with distance from the trench.
    Figure 2: Two distinct patterns in the variation of FSDs with distance from the trench.

    a, Results with latitudes between the MTJ and the southern tip of the Blanco Fracture Zone. b, Sites between latitudes of the southern and northern tips of the Juan de Fuca Ridge. In a, one population of FSDs lies west of the trench and is aligned with Pacific Plate motion, and another aligns with the subduction direction. Part b shows continuous variation in FSD with trench distance. Blue and red markers indicate offshore and onshore results, respectively. Error bars indicate the 95% confidence interval.

  3. Two-dimensional modelling to simulate mantle flow below the Gorda Plate as induced by motion of the Pacific Plate.
    Figure 3: Two-dimensional modelling to simulate mantle flow below the Gorda Plate as induced by motion of the Pacific Plate.

    The green plate is stationary while the red plate moves to the left at 60mmyr−1. This approximates the situation in profile perpendicular to the Gorda Ridge (see Methods for more detail). The set-up consists of an ‘asthenosphere’ from 50–150km and a ‘mesosphere’ below. a, In our preferred model, the viscosity of the mesosphere is 100 times that of the asthenosphere. b, Details of the model set-up, including the imposed periodic surface velocity field, region of interest and large-scale induced flow structure. The motion of the red plate is seen to generate flow beneath the adjacent stationary plate.


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  1. McCone Hall, Department of Earth and Planetary Science, UC Berkeley, California 94720, USA

    • Robert Martin-Short,
    • Richard M. Allen,
    • Eoghan Totten &
    • Mark A. Richards
  2. Department of Earth Science and Engineering, Royal School of Mines, Prince Consort Road, Imperial College London, London SW7 2BP, UK

    • Ian D. Bastow &
    • Eoghan Totten


This study was carried out and written up by R.M.-S., under supervision of R.M.A. I.D.B. assisted with data analysis and helped write the paper. E.T. and M.A.R. provided advice and minor modifications to the text.

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