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Mid-ocean ridge unfaulting revealed by magmatic intrusions

Nature volume 628, pages 782–787 (2024)Cite this article

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Abstract

Mid-ocean ridges (MORs) are quintessential sites of tectonic extension1,2,3,4, at which divergence between lithospheric plates shapes abyssal hills that cover about two-thirds of the Earth’s surface5,6. Here we show that tectonic extension at the ridge axis can be partially undone by tectonic shortening across the ridge flanks. This process is evidenced by recent sequences of reverse-faulting earthquakes about 15 km off-axis at the Mid-Atlantic Ridge and Carlsberg Ridge. Using mechanical models, we show that shallow compression of the ridge flanks up to the brittle failure point is a natural consequence of lithosphere unbending away from the axial relief. Intrusion of magma-filled fractures, which manifests as migrating swarms of extensional seismicity along the ridge axis, can provide the small increment of compressive stress that triggers reverse-faulting earthquakes. Through bathymetric analyses, we further find that reverse reactivation of MOR normal faults is a widely occurring process that can reduce the amplitude of abyssal hills by as much as 50%, shortly after they form at the ridge axis. This ‘unfaulting’ mechanism exerts a first-order influence on the fabric of the global ocean floor and provides a physical explanation for reverse-faulting earthquakes in an extensional environment.

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Fig. 1: Swarms of normal and reverse seismicity at two slow-spreading MORs.
Fig. 2: Tectonic strain and the bathymetric signature of unfaulting.
Fig. 3: The stress state of MOR shoulders.
Fig. 4: MOR unfaulting.

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Data availability

All bathymetric data used in this study are from the published literature as referenced 50,67,68 or openly available in the GMRT (https://www.gmrt.org/) and NOAA-NCEI (https://www.ncei.noaa.gov/maps/bathymetry/) repositories. The fault scarp dataset in ref. 24 is provided in Supplementary Table 4. Earthquake data are from the Global CMT catalogue (https://www.globalcmt.org/CMTsearch.html), except for earthquake relocations, which are provided in Supplementary Table 1.

Code availability

The simulation shown in Fig. 3a was run with the version of the FLAC code60 developed in refs. 30,31. This code and the corresponding visualization scripts are available from the corresponding author on reasonable request. The stress calculations shown in Fig. 3a were carried out with the code openly distributed with ref. 64.

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Acknowledgements

G.E. received support from the Consortium for Monitoring, Technology, and Verification under the Department of Energy National Nuclear Security Administration award number DE-NA0003920. Z.L. was supported by the JLU Science and Technology Innovative Research Team programme (no. 2021TD-05). M.B. was supported by the ISblue project (Interdisciplinary Graduate School for the Blue Planet: ANR-17-EURE-0015) co-funded by a France 2030/Investissement d’Avenir grant from the French government. We thank our editor and the reviewers for their insightful feedback. We also thank I. Grevemeyer, S. Cesca, S. Solomon and A. Janin for helpful discussions. J. Chen and L. C. Malatesta provided valuable assistance with figure design. Finally, we thank S. Skolotnev and M. Ligi for providing the Mid-Atlantic Ridge 54° N bathymetric data.

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Authors and Affiliations

  1. Laboratoire de Géologie, CNRS - École Normale Supérieure - PSL University, Paris, France

    Jean-Arthur Olive & Javier Escartín

  2. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA

    Göran Ekström & W. Roger Buck

  3. College of Earth Sciences, Jilin University, Changchun, China

    Zhonglan Liu

  4. Geo-Ocean, Univ. Brest, CNRS, Ifremer, UMR6538, Plouzané, France

    Manon Bickert

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Contributions

J.-A.O. designed the study, conducted the bathymetric analyses with J.E. and M.B., carried out the elastic stress modelling and wrote the initial manuscript. G.E. compiled and analysed the earthquake data. W.R.B. and Z.L. designed the models of ridge flank flexure. All authors discussed and analysed the results and provided feedback on the manuscript.

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Correspondence to Jean-Arthur Olive.

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Extended data figures and tables

Extended Data Fig. 1 Examples of near-axis compression at MORs.

a, Bathymetric map of the Southeast Indian Ridge (SEIR) near 115° E, with focal mechanisms from the CMT catalogue. b, Bathymetric map of the Mid-Atlantic Ridge (MAR) south of the Marathon transform fault (TF), with focal mechanisms from the CMT catalogue. c, Bathymetric profile across the SEIR (dashed line in panel a), with coloured segments indicating fault scarps. d, Cumulative fault heave versus distance from the axis for the southern (red) and northern (yellow) sides of the SEIR, with best-fitting piecewise linear functions shown as dashed lines (T = 0.13, T′ = 0.05 on the south side; T = 0.09, T′ = 0.07 on the north side). e, Bathymetric profile across the western flank of the MAR (dashed line in panel b), with coloured segments indicating fault scarps. f, Cumulative fault heave versus distance from the axis for the western side of the MAR, with best-fitting piecewise linear function shown as dashed lines (T = 0.21, T′ = 0.08).

Extended Data Fig. 2 The 2022 northern Mid-Atlantic Ridge and 2014 Carlsberg Ridge seismic sequences.

Latitude and moment magnitude of earthquakes, colour-coded by mechanism (blue, reverse faulting; red, normal faulting) and location, throughout the 2022 Mid-Atlantic Ridge sequence (a–d) and 2014 Carlsberg Ridge sequence (e–h).

Extended Data Fig. 3 Examples of near-axis reverse-faulting earthquakes at MORs.

Bathymetric maps, reverse focal mechanism and profile used for strain analyses at the Mid-Atlantic Ridge 0° 50′ N near the St. Paul transform fault (TF)67,68 (a); Mid-Atlantic Ridge 7° 50′ S (b); Mid-Atlantic Ridge 34° 50′ N (c); Carlsberg Ridge 9° 45′ N (d); Mid-Atlantic Ridge 1° 20′ S (e); Southwest Indian Ridge 18° 30′ E (f); Mid-Atlantic Ridge 17° S (g); Southwest Indian Ridge 43° E near the Discovery II transform fault (TF) (h); and Southwest Indian Ridge 57° E (i). FZ, fracture zone.

Extended Data Fig. 4 Bathymetric signatures of unfaulting in the Atlantic Ocean.

Left, bathymetric cross-sections with fault scarps highlighted in colour. Right, plots of cumulative fault heave versus distance at selected sections of the Mid-Atlantic Ridge. Corresponding values of T, T′ and xc are listed in Supplementary Table 3.

Extended Data Fig. 5 Bathymetric signatures of unfaulting in the Indian Ocean.

Left, bathymetric cross-sections with fault scarps highlighted in colour. Right, plots of cumulative fault heave versus distance at selected sections of the Southwest Indian Ridge and the Carlsberg Ridge. Corresponding values of T, T′ and xc are listed in Supplementary Table 3.

Extended Data Fig. 6 Unfaulting along the intermediate-spreading Chile Ridge.

a, Bathymetric map of the Chile Ridge axis outlining its main segments, adapted from ref. 24. Insets detail bathymetry of segments N1 and N9N–N9S. Position (b) and amplitude (c) of the change in apparent T at the Chile Ridge: histograms of xc and (T − T′) for all transects across the Chile Ridge, excluding poor fits highlighted in grey in Supplementary Table 3. d, Average slope of axis-facing scarps versus distance to the ridge axis. Data are from the fault scarp compilation of ref. 24.

Extended Data Fig. 7 Tectonically accommodated strain along the Chile Ridge.

Cumulative fault heave versus distance from the axis along individual transects from each segment of the Chile Ridge, based on the fault scarp compilation of ref. 24. Red and yellow dots correspond to the western and eastern sides of the axis, respectively, with best-fitting piecewise linear functions shown as black lines. This excludes poor fits highlighted in grey in Supplementary Table 3.

Extended Data Fig. 8 Amplitude and position of the change in apparent T along the Chile Ridge.

a–i, Histograms of the amplitude of the change (T − T′) in apparent T in each bathymetric transect, grouped by segment. j–r, Histograms of the distance xc for which the change in apparent T occurs in each bathymetric transect. These plots exclude poor fits highlighted in grey in Supplementary Table 3.

Extended Data Fig. 9 Lack of unfaulting near an axial high.

a, Bathymetric map of the fast-spreading East Pacific Rise at 9° 30′ N, which—unlike every other ridge section studied here—features an axial high instead of an axial valley. White line indicates the location of the bathymetric transect. b, Bathymetric cross-sections with fault scarps highlighted in colour. c, Cumulative fault heave versus distance, with best-fitting piecewise linear function shown as black lines. In this case, T′ > T.

Extended Data Fig. 10 Mechanics of ridge shoulder unbending.

The yield stress of the lithosphere σy is defined as the difference between the vertical and horizontal stresses needed to produce fault slip. a, An idealized plate that was accreted with curvature and no bending stresses. σ0 is an assumed background stress difference. The simple case shown has no cohesion and parameters A and B depend strongly on the friction coefficient f and assumed pore pressure PP on the faults, as defined in the Supplementary Information. b, A case with reverse faults that are three times stronger than normal faults. c, Deepening of the neutral depth, D, when the reverse faults are assumed to be weaker than the normal faults. d, Analytical estimate of the neutral depth D, which marks the base of the compressive zone in an unbending ridge shoulder. The ratio of D to the layer thickness, H, is plotted versus the ratio of pore pressures on reverse versus normal faults assuming f = 0.75 and that the pore pressure on normal faults is one-third the lithostatic pressure (blue curve). Assuming a rock density of 3,000 kg m−3 and water density of 1,000 kg m−3, the left limit is for hydrostatic pore pressure on the reverse faults, whereas the right limit is for lithostatic pore pressure on the reverse faults. Red curve shows the effect on the neutral depth of a regional horizontal extensional stress difference equal to 20% of the extensional yield stress at the base of the layer. See Supplementary Information for details.

Supplementary information

Supplementary Information

Supplementary Methods describing the fault scarp dataset from the Chile Ridge and detailing a simple analytical model for the depth of the compressive zone taking into account the effect of pore fluid pressure.

Supplementary Table 1

Relocated earthquakes at the Mid-Atlantic Ridge and the Carlsberg Ridge. Event code follows the convention of Global CMT catalogue. Table also includes depth estimates for four events from the 2022 Mid-Atlantic Ridge 53º N earthquake swarm.

Supplementary Table 2

Unfaulting earthquakes. List of reverse earthquakes from the Global CMT catalogue that occurred in areas mapped with high-resolution shipboard echosounders. Events labelled JM_X are part of the compilation from ref. 20.

Supplementary Table 3

Compilation of unfaulting signatures in high-resolution seafloor bathymetry. Best-fitting slope T of cumulative fault offset versus distance to the ridge axis near the axis (T best, with maximum and minimum estimates) and away from the axis (T′), past critical distance xc. Each row is a bathymetric transect on a specified side of the ridge axis. Shaded rows are transects that did not yield a satisfactory fit with a piecewise linear function.

Supplementary Table 4

Normal faults along the Chile Ridge. Original dataset from ref. 24. Each fault is identified along a ridge-normal transect by the position of the bottom and top of its seafloor scarp (locations 1 and 2). Positive and negative distances to the ridge axis correspond to fault located west and east of the axis, respectively. Segments are numbered as follows (#1–#9): N1, N10, N5, N8, N9N, N9S, S5N, S5S and V4.

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Olive, JA., Ekström, G., Buck, W.R. et al. Mid-ocean ridge unfaulting revealed by magmatic intrusions. Nature 628, 782–787 (2024). https://doi.org/10.1038/s41586-024-07247-w

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