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Deep hydration and lithospheric thinning at oceanic transform plate boundaries

Abstract

Transform faults accommodate the lateral motions between lithospheric plates, producing large earthquakes. Away from active transform boundaries, former oceanic transform faults also form the fracture zones that cover the ocean floor. However, the deep structure of these faults remains enigmatic. Here we present ultra-long offset seismic data from the Romanche transform fault in the equatorial Atlantic Ocean that indicates the presence of a low-velocity anomaly extending to ~60 km below sea level. We performed three-dimensional thermal modelling that suggests the anomaly is probably due to extensive serpentinization down to ~16 km, overlying a hydrated, shear mylonite zone down to 32 km. The water is considered to be sourced from seawater-derived fluids that infiltrate deep into the fault. Below 32 km is interpreted to be a low-temperature, water-induced melting zone that elevates the lithosphere–asthenosphere boundary, causing substantial thinning of the lithosphere at the transform fault. The presence of a thinned lithosphere at transform faults could explain observations of volcanism, thickened crust and intra-transform spreading centres at transform faults. It also suggests that migration and mixing of water-induced melt with the high-temperature melt may occur beneath the ridge axis.

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Fig. 1: Bathymetry map of the equatorial Atlantic Ocean.
Fig. 2: Ultra-long offset seismic data recorded by OBS45.
Fig. 3: Velocity structures of the mantle beneath the Romanche transform fault.
Fig. 4: Thermal structure across the Romanche transform fault along the seismic profile.
Fig. 5: Nature of the lithosphere across the Romanche transform fault.

Data availability

The high-resolution bathymetry data, the multichannel seismic reflection data and the OBS data from OBS26 to OBS50 are available online (https://doi.pangaea.de/10.1594/PANGAEA.922331) under the condition of acknowledging Marjanović et al. 2020 (https://doi.org/10.1029/2020JB020275). The OBS data from OBS01 to OBS15 are available online (https://doi.pangaea.de/10.1594/PANGAEA.937195) under the condition of acknowledging Growe et al. 2021 (https://doi.org/10.1029/2021JB022456). The OBS data from OBS16 to OBS25 are available online (https://doi.org/10.1594/PANGAEA.946565).

Code availability

The tomography code used in this work is available on request from the corresponding author.

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Acknowledgements

We are grateful to the officers, crew and scientific technicians of the 2018 ILAB-SPARC cruise for their hard work. Discussions with D. Brunelli and B. Romanowicz were extremely useful. We thank S. P. Hicks, J. M. Warren and B. Urann for their useful comments and suggestions. Z.W. thanks I. Grevemeyer and H. Carton for their help during the progress of this project. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC Advance Grant agreement no. 339442_TransAtlanticILAB (SCS). We thank the Computational Infrastructure for Geodynamics (geodynamics.org), which is funded by the National Science Foundation under awards EAR-0949446 and EAR-1550901 for supporting the development of ASPECT. We are grateful to L. H. Rüpke for his help on thermal modelling using ASPECT and to G. Moguilny and M. Roharik for their help on the installation of ASPECT. The thermal modelling is performed on the S-CAPAD platform of IPGP, France. This is an IPGP contribution number 4267.

Author information

Authors and Affiliations

Authors

Contributions

Z.W. processed the OBS data, performed thermal modelling and wrote the paper. S.C.S. developed the project, led the data acquisition, supervised the data processing and interpretation and wrote the paper. C.P. participated in the calculations and interpretation. E.P.M.G. and M.M. participated in the data acquisition and data processing. All authors discussed results and contributed to the article.

Corresponding author

Correspondence to Satish C. Singh.

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The authors declare no competing interests.

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Nature Geoscience thanks Ross Parnell-Turner and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Louise Hawkins, Simon Harold and Stefan Lachowycz, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 OBS data at intermediate offsets and comparisons of picked and modelled travel times.

a, Bathymetry map showing the location of the Romanche TF, the seismic profile (black line) and the locations of OBSs (red stars). b,c, Seismic sections and the picked Pn travel times, marked in red. OBS29 (b), OBS16 (c). The seismic data are plotted at travel time reduced at a reduction velocity of 8.0 km/s. The effect of bathymetry variations is removed by subtracting the propagating time in water, assuming a water velocity of 1.5 km/s. The cyan ellipses on the seismic plots indicate the location of the Romanche transform zone. A travel time delay and amplitude weakening are observed on the seismic sections where the seismic waves enter the Earth within the Romanche transform zone. d,e, Comparisons of picked travel times (in red) and modelled travel times (in blue). OBS29 (d), OBS16 (e). The red error bars indicate the picking uncertainty. The zero offset corresponds to the OBS location.

Extended Data Fig. 2 Final tomographic model and the density of ray coverage.

a, Final tomographic model and b, the density of ray coverage. The black contours and the numbers indicate the velocity in km/s. The maximum ray sampling depth is ~60 km in the final inverted model. The grey circles represent the locations of OBSs. The inverted triangles show the location of the Romanche transform zone. The portion of the tomographic velocity model within 200–650 km profile distance range is shown in Fig. 3a. The zero distance starts from the south along the profile shown in Fig. 1.

Extended Data Fig. 3 Vertical smearing tests.

af, Smearing effect tests for a low-velocity anomaly (LVA) in the mantle extending down to 30 km (a and b), 40 km (c and d) or 50 km (e and f) depth beneath the Romanche TF. The width of the inserted LVA is 30 km in a, c and e, and 50 km in b, d and f. The black dashed lines represent the side boundaries of the inserted LVAs, and the red solid lines show the base of the LVAs. The numbers labelled on the contours represent velocity reduction in percentage after the inversion.

Extended Data Fig. 4 Optimised synthetic mantle velocity model.

a, Optimised synthetic mantle velocity structure including a low-velocity anomaly with the final estimated width and velocity (Extended Data Table 2). b, Tomographic result of the travel times modelled using the synthetic model in a. The inverted black triangles indicate the location of the Romanche transform valley. The black contours and the numbers indicate the velocity in km/s.

Extended Data Fig. 5 Mantle serpentinization estimate.

The mantle P-wave velocity as a function of the degree of mantle serpentinization. The grey zone represents the estimated mantle velocity within the Romanche transform zone above 16 km depth. The squares and dots represent the mantle velocities at the Moho and at 16 km depth, respectively. The bars indicate the uncertainty bounds. P, pressure. T, temperature.

Extended Data Fig. 6 Mantle hydration and mylonitization estimates.

ad, The mantle P-wave velocity (black line) as a function of degree of hydration (a,b) or degree of mylonitization (c,d); for 16 km depth (a,c) and for 32 km depth (b,d). The thick red solid lines indicate the velocities at 16 km and 32 km and thin red lines are uncertainty bounds. The red dashed lines are bounds in %.

Extended Data Fig. 7 Melt fraction estimation.

P-wave velocity variation as a function of melt fraction at 32 km (a) and 60 km (b) depths below sea level. The aspect ratio of melt r = 50 is used in the estimations. The thick red solid lines indicate the velocity at each depth and the thin red lines are uncertainty bounds. The thick red dashed lines indicate the estimated melt fractions, and the thin dashed lines are uncertainty bounds. r = 1 for spherical melt inclusions.

Extended Data Fig. 8 Solidus of mantle.

a,b, Solidus of mantle with different bulk of water at 32 km depth (a) and 60 km depth (b).

Extended Data Table 1 Parameters and misfits of synthetic models
Extended Data Table 2 Estimated width and velocity reduction on the controlling nodes using different misfit thresholds

Supplementary information

Supplementary Information

Supplementary Figs. 1–11 and Tables 1 and 2.

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Wang, Z., Singh, S.C., Prigent, C. et al. Deep hydration and lithospheric thinning at oceanic transform plate boundaries. Nat. Geosci. 15, 741–746 (2022). https://doi.org/10.1038/s41561-022-01003-3

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