Research ArticleGEOPHYSICS

Imaging paleoslabs in the D″ layer beneath Central America and the Caribbean using seismic waveform inversion

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Science Advances  29 Nov 2017:
Vol. 3, no. 11, e1602700
DOI: 10.1126/sciadv.1602700
  • Fig. 1 Target region.

    Waveforms from deep- and intermediate-focus earthquakes beneath South America (red stars) recorded at stations of the USArray, CNSN, CANOE, and other seismic networks (see text) (blue inverted triangles) provide dense raypath coverage of the target region 0 to 400 km above the CMB (yellow squares). Red curves show ScS raypaths that sample the target region, and black crosses show ScS bounce points at the CMB. The pink solid circle at 30°N and 110°W shows the location for the shallow structure trade-off test (fig. S3). The inset shows the location of the cross sections presented in Fig. 4 and the location of the Farallon plate boundary at 180 Ma (31).

  • Fig. 2 Model CACAR.

    The eight panels show the results of the inversion for the eight depth layers from 400 km above the CMB (upper left) to the CMB (lower right), with the lateral average of the 3D perturbation set to zero in each layer. The perturbation is relative to the initial 1D model PREM (37). Two distinct high-velocity regions at the CMB (lower right) suggest two distinct cold paleoslabs. A 3% velocity decrease beneath Mexico concentrated within 100 km of the CMB suggests the possible existence of chemically distinct material with enriched iron content (for example, basaltic composition). The location of high- and low-velocity anomalies is generally consistent with recently inferred topography of the D″ discontinuity (42). The Farallon plate boundary at 180 Ma (31) is shown in red in the lower right panel. Its location is consistent with the high-velocity anomaly beneath Venezuela but is ~1000 km away from the high-velocity anomaly beneath Central America. This might indicate past intra-oceanic subduction, or breaking or tearing of an ancient paleoslab in the upper mantle (22).

  • Fig. 3 Model CACAR′.

    Same as Fig. 2 but using PREM′ (fig. S2) as the initial 1D model. The velocity perturbations are shown with respect to PREM′, with the lateral average of the 3D perturbation set to zero in each layer. CACAR (Fig. 2) and CACAR′ are in good general agreement, which suggests that the inversion is robust. The strong low-velocity anomaly beneath Mexico observed in CACAR is still present, but the strongest low-velocity anomaly is beneath Ecuador. This suggests that strong low-velocity anomalies are present at several sites around the high-velocity anomalies.

  • Fig. 4 Cross sections.

    Left column: Cross sections through two previous models obtained by global waveform inversion (labeled FR2014) (7) and by regional finite-frequency travel-time tomography (labeled H+2005) (39). Middle column: Cross sections for our models CACAR and CACAR′. Right column: Whole-mantle cross section through the global model (7). The horizontal axis of each profile shows degrees along the corresponding great-circle cross section; vertical axis shows elevation above the CMB (in kilometers) for profiles in left and middle columns and depth (in kilometers) for the whole-mantle profiles in the right column. Locations of the cross sections are shown in the inset in Fig. 1. The leftmost part of the cross section for model H+2005 in the left column of (B) has been grayed out due to the lack of resolution. (Note that CACAR and CACAR′ also have little resolution in this region.) (A) The strong velocity contrasts within 100 km above the CMB in CACAR and CACAR′ (blue dashed line in the upper middle panel) suggest the presence of paleoslabs at the CMB and dense chemical heterogeneities. The paleoslab beneath Central America (labeled “CA”) is perched above a strong low-velocity anomaly (blue dashed line in the upper middle panel), which might suggest dense iron-enriched material at the CMB (for example, basaltic composition). Low-velocity vertically continuous structures (brown arrows with unfilled points in cross-section A-A′ in the middle column) at the edges of “CA” suggest upwelling from the possibly iron-enriched material at the CMB. (B) A low-velocity vertically continuous structure (red filled arrow in cross-section B-B′ in middle column) between two distinct paleoslabs beneath Central America (“CA”) and Venezuela (“VZ”), and connecting to a low-velocity region in the mid-mantle in a previous global waveform inversion model (7) (right column) suggests upsplashing of hot TBL material caused by two paleoslabs “CA” and “VZ” sinking to the CMB. Past location of the Farallon plate boundary at ~180 Ma (31) (green vertical dashed line labeled FA) is consistent with location of “VZ”, but is laterally ~1000 km away from “CA”.

  • Fig. 5 Possible geodynamical interpretations.

    Two high-velocity anomalies (“CA” and “VZ”, see Fig. 4) just above the CMB, beneath Central America and Venezuela, respectively, suggest the existence of two distinct paleoslabs that took different subduction paths to the lowermost mantle. Strong low-velocity anomalies in the lowermost 100 km of the mantle suggest dense iron-rich material (for example, basaltic composition). This dense low-velocity material at the CMB can also explain why paleoslab “CA” is perched above the strong low-velocity anomaly beneath Mexico. The low-velocity material is upwelling from the iron-rich anomalies and between the two slabs. The Farallon plate boundary at 180 Ma (red curve) is consistent with the location of the high-velocity anomaly beneath Venezuela, suggesting that the paleoslab “VZ” subducted at the western margin of Pangaea.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/11/e1602700/DC1

    Weighting factor

    Resolution and robustness tests

    fig. S1. Data statistics.

    fig. S2. Initial models for 3D inversions.

    fig. S3. Trade-off with shallow structure.

    fig. S4. Simultaneous inversion for D″ and shallow upper mantle structures.

    fig. S5. Possibility of contamination due to shallow structure.

    fig. S6. Checkerboard test.

    fig. S7. Checkerboard test with artificial noise.

    fig. S8. Block tests.

    fig. S9. Record section for the block test model.

    fig. S10. Nonlinear checkerboard test.

    fig. S11. Jackknife test.

    fig. S12. Inversion of western, central, and eastern data sets.

    fig. S13. Quality control stacks.

    fig. S14. Point-spread function.

    fig. S15. Crossing raypaths.

    table S1. Earthquakes used in this study.

    table S2. Variance and AIC.

  • Supplementary Materials

    This PDF file includes:

    • Weighting factor
    • Resolution and robustness tests
    • fig. S1. Data statistics.
    • fig. S2. Initial models for 3D inversions.
    • fig. S3. Trade-off with shallow structure.
    • fig. S4. Simultaneous inversion for D″ and shallow upper mantle structures.
    • fig. S5. Possibility of contamination due to shallow structure.
    • fig. S6. Checkerboard test.
    • fig. S7. Checkerboard test with artificial noise.
    • fig. S8. Block tests.
    • fig. S9. Record section for the block test model.
    • fig. S10. Nonlinear checkerboard test.
    • fig. S11. Jackknife test.
    • fig. S12. Inversion of western, central, and eastern data sets.
    • fig. S13. Quality control stacks.
    • fig. S14. Point-spread function.
    • fig. S15. Crossing raypaths.
    • table S1. Earthquakes used in this study.
    • table S2. Variance and AIC.

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