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Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere

Abstract

Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting1 and melt production has been inferred to be highly discontinuous2,3,4. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes5. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges6. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere–asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.

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Figure 1: Along-axis seismicity of ultraslow-spreading ridges.
Figure 2: Contrasting magmatic and amagmatic sections of Gakkel Ridge.
Figure 3: Conceptual sketch of the two lithosphere types at the Gakkel Ridge.

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Acknowledgements

This study was enabled by grants SCHL853/1-1 and SCHL853/3-1 of the German Science Foundation to V.S. Instruments were borrowed from the DEPAS pool. We acknowledge the efforts of the crews of RV Polarstern cruises ANT-XXIX/2+8 and ARK-XXIV/3, RV Meteor cruise M101 and RV Marion Dufresne.

Author information

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Authors

Contributions

V.S. planned and conducted the surveys, processed data for site 3 and wrote the paper. F.S. processed data from site 1. Both authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Vera Schlindwein.

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The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks S. M. Carbotte and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Earthquake location at survey site 1 including poorly located events.

a, Epicentres (circles) colour-coded by hypocentre depth. Earthquakes not projected onto the cross-section (white line) in c are shown by squares. The red inverted triangles show OBS locations and the dashed blue line shows the position of the refraction seismic line15 used to constrain velocities in the uppermost lithosphere. b, The final velocity model used for earthquake location is shown in red, and the velocity models used for the robustness tests are shown in blue (the fast end-member representing a thin crust with ultramafic rocks underneath) orange (a velocity reduction of 0.3 km s−1 relative to the final model) and purple (a velocity increase of 0.3 km s−1 relative to the final model). Velocities of 7.0–7.6 km s−1 (grey bar) are considered anomalously low for lithospheric mantle. The histograms show the distribution of hypocentre depths obtained for the different velocity models. Faster models result in fewer well-located events, but the depth distribution is similar (see Extended Data Table 2). c, Cross-section of the hypocentres projected onto the axis and colour-coded according to the distance from the profile. The topography of the seismicity band is not an artefact of projection because at all depth intervals the earthquakes from various off-axis distances are present. The aseismic regions remain devoid of seismicity even when all poorly located earthquakes are shown.

Extended Data Figure 2 Earthquake location at survey site 2 including poorly located events.

a, Epicentres (circles) colour-coded by hypocentre depth. Earthquakes not projected onto the cross-section (white line) in c are shown by the squares. The red inverted triangles show OBS locations and the dashed blue line indicates the position of the refraction seismic line34 used to constrain velocities in the uppermost lithosphere. b, The final velocity model used for the earthquake location is shown in red and the velocity models used for the robustness tests are shown in blue (the fast end-member representing a thin crust with ultramafic rocks underneath) orange (a velocity reduction of 0.3 km s−1 relative to the final model) and purple (a velocity increase of 0.3 km s−1 relative to the final model). Velocities of 7.0–7.6 km s−1 (grey bar) are considered as anomalously low for lithospheric mantle. The histograms show the distribution of hypocentre depths obtained for the different velocity models. Faster models result in fewer well located events, but the depth distribution is similar (see Extended Data Table 2). c, Cross-section of the hypocentres projected onto the axis and colour-coded according to the distance from the profile. The topography of the seismicity band is not an artefact of projection because for all depth intervals the earthquakes from various off-axis distances are present. The aseismic regions remain devoid of seismicity even when all poorly located earthquakes are shown.

Extended Data Figure 3 Earthquake location at survey site 3 including poorly located events.

a, Epicentres (circles) colour-coded by hypocentre depth. Earthquakes not projected onto the cross-section (white line) in c are shown by the squares. The red inverted triangles show OBS locations and the dashed blue line indicates the position of the refraction seismic lines35 used to constrain velocities in the uppermost lithosphere. b, The final velocity model used for the earthquake location is shown in red and the velocity models used for the robustness tests are shown in blue (the fast end-member representing a thin crust with ultramafic rocks underneath) orange (a velocity reduction of 0.3 km s−1 relative to the final model) and purple (a velocity increase of 0.3 km s−1 relative to the final model). Velocities of 7.0–7.6 km s−1 (grey bar) are considered as anomalously low for lithospheric mantle. The histograms show the distribution of hypocentre depths obtained for the different velocity models. Faster models result in fewer well-located events, but the depth distribution is similar (see Extended Data Table 2). c, Cross-section of the hypocentres projected onto the axis and colour-coded according to the distance from the profile. The topography of the seismicity band is not an artefact of projection because at all depth intervals the earthquakes from various off-axis distances are present. The aseismic regions remain devoid of seismicity even when all poorly located earthquakes are shown.

Extended Data Figure 4 Contrasting magmatic and amagmatic sections of western SWIR.

a, Teleseismic earthquake activity (open circles, scaled with magnitude) over bathymetry. The light yellow circles mark earthquake clusters of two or more events that are related in time and space. Data from ref. 5. b, Dredge lithology. Data from ref. 2. c, Magnetic anomalies. Data from ref. 31. Earthquakes from a are shown by the dots. The predominantly magmatic orthogonal supersegment shows more abundant and often clustered teleseismic earthquakes and a marked magnetic anomaly. The predominantly amagmatic oblique supersegment shows less seismicity and peridotite exposure. Areas of magmatic and amagmatic lithosphere within this segment are defined from the seafloor lithology and magnetic patterns. The differences in the event rates within segments (see Fig. 2) are not visible here owing to a large uncertainty in earthquake locations.

Extended Data Table 1 Location performance of the final one-dimensional velocity models and end-member models
Extended Data Table 2 Dependence of the hypocentre depths on the velocity model

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Schlindwein, V., Schmid, F. Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere. Nature 535, 276–279 (2016). https://doi.org/10.1038/nature18277

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