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Ultralow velocity zone and deep mantle flow beneath the Himalayas linked to subducted slab

Nature Geoscience volume 17, pages 302–308 (2024)Cite this article

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Abstract

The origins of ultralow velocity zones, small-scale structures with extremely low seismic velocities found near the core–mantle boundary, remain poorly understood. One hypothesis is that they are mobile features that actively participate in mantle convection, but mantle flow adjacent to ultralow velocity zones is poorly understood and difficult to infer. Although deep mantle anisotropy observations can be used to infer mantle flow patterns, ultralow velocity zone structures are often not examined jointly with these observations. Here we present evidence from seismic waves that sample the lowermost mantle beneath the Himalayas for both an ultralow velocity zone and an adjacent region of seismic anisotropy associated with mantle flow. By modelling realistic mineral physics scenarios using global wavefield simulations, we show that the identified seismic anisotropy is consistent with horizontal shearing orientated northeast–southwest. Based on tomographic data of the surrounding mantle structure, we suggest that this southwestward flow is potentially linked to the remnants of the subducted slab impinging on the core–mantle boundary. The detected ultralow velocity zone is located at the southwestern edge of this anisotropic region, and therefore potentially affected by strong mantle deformation in the surrounding area.

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Fig. 1: Source–receiver configuration used in this study.
Fig. 2: Summary of ULVZ and anisotropy results.
Fig. 3: Comparison between observations and model results for a Ppv elastic tensor with a dominant slip system of [100](010).
Fig. 4: Summary of results and interpretations.

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Data availability

The data used in this study are freely available and were downloaded from the following data centres: Bundesanstalt für Geowissenschaften und Rohstoffe (http://eida.bgr.de), GEOFOrschungsNetz (http://geofon.gfz-potsdam.de), INGV Istituto Nazionale di Geofisica e Vulcanologia (http://webservices.ingv.it), Incorporated Research Institutions for Seismology (http://service.iris.edu), Kandilli Observatory and Earthquake Research Institute (http://eida.koeri.boun.edu.tr), Ludwig-Maximilians-Universität München (http://erde.geophysik.uni-muenchen.de), National Institute for Earth Physics (http://eida-sc3.infp.ro), Observatories and Research Facilities for European Seismology (http://www.orfeus-eu.org), Résif (http://ws.resif.fr) and Swiss Seismological Service (http://www.seismo.ethz.ch/en/research-and-teaching/products-software/waveform-data/), as further specified in the Supplementary Information.

Code availability

The synthetic seismograms for this study were computed using AxiSEM3D31,32, which is publicly available at https://github.com/AxiSEMunity.

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Acknowledgements

This work was funded by Yale University and by the US National Science Foundation via grant no. EAR-2026917 to M.D.L. and grant no. EAR-2027181 to D.A.F. We are grateful for helpful conversations with C. Martin and S. Cottaar.

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Authors and Affiliations

  1. Department of Earth and Planetary Sciences, Yale University, New Haven, CT, USA

    Jonathan Wolf & Maureen D. Long

  2. School of the Earth, Ocean and Environment, University of South Carolina, Columbia, SC, USA

    Daniel A. Frost

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Contributions

Conceptualization was carried out by J.W. and M.D.L. Data analysis was performed by J.W. and D.A.F. Synthetic modelling was carried out by J.W. J.W., M.D.L. and D.A.F. were responsible for methodology. Visualization was carried out by J.W. J.W. and M.D.L. wrote the paper.

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Correspondence to Jonathan Wolf.

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Nature Geoscience thanks Samantha Hansen, Barbara Romanowicz and Sebastian Rost for their contribution to the peer review of this work. Primary Handling Editors: Alireza Bahadori and James Super, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Synthetic and real transverse component (left column) and radial component (right column) velocity seismograms for event 1.

Seismograms are stacked linearly in 1.5° azimuth bins, after alignment to their maximum transverse amplitudes. Stacks are shown in black and individual seismograms in gray. Red lines indicate approximate arrival times. a AxiSEM3D synthetics using the 3D tomography model GyPSuM for Sdiff waveforms. Predicted radial amplitudes are very small, especially for large azimuths. Seismograms are normalized to maximum transverse amplitudes. b Real Sdiff waveforms with the same plotting conventions are in panel a. c Real SKS waveforms for a different event (2016-06-05) with a more favorable initial polarization for SKS analysis. Pink bars indicate the azimuths at which Sdiff is strongly split while SKS is not (compare panels b and c). Seismograms are normalized to maximum radial amplitudes.

Extended Data Fig. 2 Splitting diagnostic plots from SplitRacer that show an example of differential SKS-SKKS splitting recorded at seismic station BAR2 for an event that occurred on May 23, 2013.

a SKS splitting; top panel: Radial (R) and transverse (T) component waveforms. The PREM-predicted SKS arrival time is shown as a green line, and the start/end of the 30 randomly chosen measurement time windows with orange lines. Bottom left: Elliptical SKS particle motion. b Same as panel a for the SKKS phase. The SKKS particle motion is closer to linear than for SKS. Therefore, SKS-SKKS splitting is discrepant.

Extended Data Fig. 3 Beamforming results.

a Beamformed transverse velocity seismograms for event 1, showing the raw Sdiff beams as a function of azimuth. Beams are bandpass-filtered to retain periods between 7 and 20 and aligned with respect to the maximum Sdiff amplitudes. b As panel a but beams are stacked in the same way as single-station data, in 1.5° azimuthal intervals. Beams are shown in gray and stacks in black. In both panels, postcursors are visible. c Example subarray for which we conduct beamforming. Stations, shown as inverted triangles (see legend), are located in Italy. The backazimuth from which the Sdiff wave is predicted to arrive is shown at the central station as a black line. d Upper panel: F-Trace amplitude as a function of backazimuth and arrival time (see legend). The maximum F-Trace value is shown as a green circle. The time window for which beamforming was performed is indicated by dashed violet lines. Lower panel: Single station seismograms are shown in as black lines and the beam as a pink solid line. e Plotting conventions are the same as in panel d but beamforming was performed for the postcursor, which arrives from a slightly different backazimuth than the main Sdiff arrival (panel d). To amplify the weak postcursor, the color scale in panel e is saturated by 10 times relative that in d. The postcursor arrives from a more northerly backazimuth than the main Sdiff phase, as expected for a ULVZ in our suggested location.

Supplementary information

Supplementary Information

Supplementary Text, Supplementary Figs. 1–24, and Supplementary Tables 1 and 2.

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Wolf, J., Long, M.D. & Frost, D.A. Ultralow velocity zone and deep mantle flow beneath the Himalayas linked to subducted slab. Nat. Geosci. 17, 302–308 (2024). https://doi.org/10.1038/s41561-024-01386-5

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