Research ArticleGEOPHYSICS

Seismic anisotropy reveals crustal flow driven by mantle vertical loading in the Pacific NW

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Science Advances  08 Jul 2020:
Vol. 6, no. 28, eabb0476
DOI: 10.1126/sciadv.abb0476
  • Fig. 1 Regional and location maps for northeastern (NE) Oregon.

    (A) Regional map showing the broadband seismic stations used in this study (black inverted triangles). The dashed blue line depicts the Snake River Plain (SRP). WA, Washington; OR, Oregon; ID, Idaho. (B) Global map centered in NE Oregon. The thick black line encloses the region shown in (A). (C) Elevation map of the topographic bullseye region [red area in (A)]. The dashed larger ellipse is the outer limit of the bullseye, whereas the inner ellipse locates the Wallowa batholith.

  • Fig. 2 Example of beamformer outputs and final azimuthal anisotropy model.

    (A) Two-dimensional histograms over the azimuth-velocity space for the 3- to 17-s period band with the best-fitting anisotropy model (red dashed lines). The green bars on top of each panel indicate the number of noise cross-correlations available for each azimuth. (B) Azimuthal anisotropy model for the crust of the Pacific NW. Bar orientation gives the fast direction of azimuthal anisotropy, and bar length is proportional to anisotropy amplitude. The background color represents the intersection density of the anisotropy vectors assuming that they are of infinite length (i.e., projected to the bounds of the study region). The green and blue dots indicate the location of the two stations beamformed in (A).

  • Fig. 3 Comparison of seismic and geodynamic results with crustal anisotropy.

    (A) Azimuthal anisotropy model for the crust of the Pacific NW overlying a depth slice through the Vp tomography model at 250 km (16). The red dashed lines depict the Wallowa anomaly and the Siletzia slab curtain. (B) Modeled Moho stress and mid-lower crustal flow velocity for the Pacific NW. The colored contours represent the vertical stress at the Moho based on a global geodynamic model driven by density anomalies derived from the P-wave velocity structure. The black arrows denote the predicted mid-lower crustal flow velocity that results from the application of the modeled Moho stress to a viscously heterogeneous crust. The red bars represent the anisotropy measurements derived from this study.

  • Fig. 4 Station averaged shear wave splitting measurements for the Pacific NW.

    (A) SKS splitting measurements for the entire western United States (31). The red arrow depicts the relative motion between the North American plate (NA) and the hot spot reference frame (HS) (55). (B) SKS splitting measurements for our study region [red area in (A)]. The thick blue vectors depict the measurements of Niday and Humphreys (32), and the black vectors are from the database of Becker et al. (31). In both panels, the orientation of the vectors gives the angle of the fast polarization, and the length of the bars is proportional to the magnitude of the shear wave splitting. The white trajectories through the anisotropy field lines in (B) are used to represent the streamlines of the mantle flow assuming an east-oriented flow (56). The red circle in the background marks the location of the Wallowa anomaly. Note how mantle materials appear to flow smoothly around the lateral boundaries of the Wallowa anomaly.

  • Fig. 5 Schematic representation of the upside-down water bed model.

    The load of the mantle lithosphere is a force creating vertical stresses (σzz) on the Moho. The lithospheric load pulls down on the crust, which creates a lateral pressure gradient that drives Poiseuille flow in the ductile mid-lower crust. The asthenosphere flows independently (as evidenced by its independent anisotropy field; Fig. 4), creating a local Couette flow that is decoupled from the mid-lower crust by the mantle lithosphere.

  • Fig. 6 Crustal anisotropy and upper mantle velocity structure of California.

    The black vectors depict Lin et al.’s (33) surface wave anisotropy measurements for the 12-s period. Bar orientation gives the fast direction of azimuthal anisotropy, and bar length is proportional to anisotropy amplitude. The background color corresponds to a depth slice through the Vp tomography model at 195 km (16). The red dashed lines denote the two seismically fast and likely dense mantle anomalies.

Supplementary Materials

  • Supplementary Materials

    Seismic anisotropy reveals crustal flow driven by mantle vertical loading in the Pacific NW

    Jorge C. Castellanos, Jonathan Perry-Houts, Robert W. Clayton, YoungHee Kim, A. Christian Stanciu, Bart Niday, Eugene Humphreys

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