Reconciling surface plate motions with rapid three-dimensional mantle flow around a slab edge

Journal name:
Nature
Volume:
465,
Pages:
338–341
Date published:
DOI:
doi:10.1038/nature09053
Received
Accepted

The direction of tectonic plate motion at the Earth’s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally, suggesting large-scale coupling between the mantle and the surface plates1, 2. The fit is typically poor at subduction zones, however, where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to3, 4, 5, 6, 7 and may be several times faster than6 plate motions. Here we present three-dimensional numerical models of buoyancy-driven deformation with realistic slab geometry for the Alaska subduction–transform system and use them to determine the origin of this regional decoupling of flow. We find that near a subduction zone edge, mantle flow velocities can have magnitudes of more than ten times the surface plate motions, whereas surface plate velocities are consistent with plate motions8 and the complex mantle flow field is consistent with observations from seismic anisotropy5. The seismic anisotropy observations constrain the shape of the eastern slab edge and require non-Newtonian mantle rheology. The incorporation of the non-Newtonian viscosity9, 10 results in mantle viscosities of 1017 to 1018Pas in regions of high strain rate (10-12s-1), and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates. These results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab-pull force available to drive subducting plates.

At a glance

Figures

  1. Schematic of full model domain and slab geometry.
    Figure 1: Schematic of full model domain and slab geometry.

    Outline of the high-resolution mesh region (dashed grey line); the portion of the slab geometry that is varied (short-dashed black line); the PBSZ and southern mesh boundary shear zone (SMSZ) (thick dark-grey lines); Juan de Fuca Ridge (JdFR, double black line); and the locations of the cross-sections shown in Fig. 3 (AA′ and BB′, black). NAM, North American plate; PAC, Pacific plate; T, temperature. See Methods and Supplementary Information.

  2. Maps of flow field.
    Figure 2: Maps of flow field.

    ac, Surface velocity field and viscosity (colour scale) for three models (σy = 500MPa; PBSZ viscosity, 1020Pas). The observed NUVEL-1A Pacific motion vector, assuming North America to be fixed8, is indicated using a white arrow. AVO, Alaska Volcano Observatory (Supplementary Information). df, Mantle velocity field at 100-km depth and vertical velocity magnitude (colour scale). The implied strong vertical gradient in velocity illustrates the significant decoupling of the overriding plate from the mantle flow.

  3. 3D mantle flow field and viscosity structure.
    Figure 3: 3D mantle flow field and viscosity structure.

    Calculated using the model with slabE115 and composite rheology (σy = 500MPa; PBSZ viscosity, 1020Pas). Figures show subset of modelled domain. a, Isosurface and cross-sections (AA′ and BB′) through composite viscosity show the strong slab and the low-viscosity regions in the mantle wedge and beneath the slab. Low-viscosity regions correlate with regions of high strain rate. b, Isosurface of viscosity showing an oblique, cross-sectional, radial slice through the velocity field. The cross-section BB′ shows poloidal flow and along-strike flow. The plan view shows anticlockwise toroidal flow and an upward component of flow east of the slab edge.

  4. Velocity and ISA orientations at 100-km depth.
    Figure 4: Velocity and ISA orientations at 100-km depth.

    a, Velocity field shows a trench-parallel component of flow near the slab nose for slabE115 with composite viscosity (σy = 500MPa; PBSZ viscosity, 1020Pas). b, The Newtonian viscosity has a damping effect on the toroidal component of flow. c, There is no toroidal flow above the slab nose for slabE325. df, ISA orientations coloured by the lag parameter (Π, colour scale). Superimposed are SKS fast-axis directions (blue) back-projected along the ray path to 100-km depth5. ISA orientation provides a good estimate of LPO for Π<1.0.

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Author information

Affiliations

  1. Department of Geology, University of California, Davis, California 95616, USA

    • Margarete A. Jadamec &
    • Magali I. Billen
  2. Present address: School of Mathematical Sciences & School of Geosciences, Monash University, Clayton, Victoria 3800, Australia.

    • Margarete A. Jadamec

Contributions

Both authors contributed equally to the overall development of the project, model design considerations, analysis and interpretations. M.A.J. performed all of the numerical modelling, except for the ISA calculations, which were done by M.I.B.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

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Supplementary information

PDF files

  1. Supplementary Information (2.7M)

    This files contains Supplementary Notes comprising Model design; Slab structure; Thermal structure; Rheology; Model results; Pacific plate motion and Comparisons of ISA and SKS; Supplementary Figures 1-8 with legends; Supplementary Tables 1-4 and References.

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