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Internal deformation of the subducted Nazca slab inferred from seismic anisotropy

Nature Geoscience volume 9, pages 56–59 (2016)Cite this article

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Abstract

Within oceanic lithosphere a fossilized fabric is often preserved originating from the time of plate formation. Such fabric is thought to form at the mid-ocean ridge when olivine crystals align with the direction of plate spreading1,2. It is unclear, however, whether this fossil fabric is preserved within slabs during subduction or overprinted by subduction-induced deformation. The alignment of olivine crystals, such as within fossil fabrics, can generate anisotropy that is sensed by passing seismic waves. Seismic anisotropy is therefore a useful tool for investigating the dynamics of subduction zones, but it has so far proved difficult to observe the anisotropic properties of the subducted slab itself. Here we analyse seismic anisotropy in the subducted Nazca slab beneath Peru and find that the fast direction of seismic wave propagation aligns with the contours of the slab. We use numerical modelling to simulate the olivine fabric created at the mid-ocean ridge, but find it is inconsistent with our observations of seismic anisotropy in the subducted Nazca slab. Instead we find that an orientation of the olivine crystal fast axes aligned parallel to the strike of the slab provides the best fit, consistent with along-strike extension induced by flattening of the slab during subduction (A. Kumar et al., manuscript in preparation). We conclude that the fossil fabric has been overprinted during subduction and that the Nazca slab must therefore be sufficiently weak to undergo internal deformation.

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Figure 1: Tectonic setting of southern Peru, showing the distribution of stations and events, and comparing the splitting results with the slab geometry.
Figure 2: Comparison of the regional splitting pattern with seismic signatures of the subducting slab.
Figure 3: Summary of the modelling results illustrating the orientation of olivine that would best reproduce the slab splitting fast directions.

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Acknowledgements

The PULSE deployment was facilitated by the PASSCAL program of IRIS (Incorporated Research Institutions for Seismology) and the data was accessed through the IRIS Data Management Center (DMC). We thank all those from Yale University, University of North Carolina—Chapel Hill, University of Arizona and the Instituto Geofísico del Perú (IGP) who participated in the fieldwork. We thank R. Clayton and P. Davis for providing access to data from PeruSE stations. We acknowledge helpful discussions and suggestions by S. Karato on modelling anisotropy within the slab. The PULSE experiment was supported by National Science Foundation grants EAR-0943962 (M.D.L.), EAR-0944184 (L.S.W.), and EAR-0943991 (S.L.B.).

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Authors and Affiliations

  1. Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA

    Caroline M. Eakin & Maureen D. Long

  2. University of Southampton, National Oceanography Centre, Southampton SO14 3ZH, UK

    Caroline M. Eakin

  3. Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

    Alissa Scire, Susan L. Beck & George Zandt

  4. Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington DC 20015, USA

    Lara S. Wagner

  5. Instituto Geofísico del Perú, Lima 15400, Peru

    Hernando Tavera

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  1. Caroline M. Eakin
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Contributions

C.M.E. conceived the paper topic, made the measurements, and carried out the modelling in collaboration with M.D.L.; A.S. and G.Z. provided the tomographic images and slab contours; M.D.L., S.L.B., L.S.W. and H.T. were principal investigators on the PULSE deployment; C.M.E. and M.D.L. co-wrote the paper with feedback and input from all co-authors.

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Correspondence to Caroline M. Eakin.

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The authors declare no competing financial interests.

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Eakin, C., Long, M., Scire, A. et al. Internal deformation of the subducted Nazca slab inferred from seismic anisotropy. Nature Geosci 9, 56–59 (2016). https://doi.org/10.1038/ngeo2592

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