Abstract
Oceanic transform faults that offset mid-ocean ridges slip through earthquakes and aseismic creep. The mode of slip varies with depth and along strike, with some fault patches that rupture in large, quasi-periodic earthquakes at temperatures <600 °C, and others that slip through creep and microearthquakes at temperatures up to 1,000 °C. Rocks from both fast- and slow-slipping transforms show evidence of interactions with seawater up to temperatures of at least 900 °C. Here we present a model for the mechanical structure of oceanic transform faults based on fault thermal structure and the impacts of hydration and metamorphic reactions on mantle rheology. Deep fluid circulation is accounted for in a modified friction-effective pressure law and in ductile flow laws for olivine and serpentine. Combined with observations of grain size reduction and hydrous mineralogy from high-strain mylonites, our model shows that brittle and ductile deformation can occur over a broad temperature range, 300–1,000 °C. The ability of seawater to penetrate faults determines whether slip is accommodated at depth by seismic asperities or by aseismic creep in weak, hydrous shear zones. Our results suggest that seawater infiltration into ocean transform faults controls the extent of seismicity and spatiotemporal variations in the mode of slip.
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Data availability
OTF mylonite and OBS deployment locations are provided in Supplementary Information Table 1. OTF mylonite compositional data is available at EarthChemLibrary48. The Gofar seismic data is available from the IRIS Data Management Center (https://www.fdsn.org/networks/detail/ZD_2007/). Source data are provided with this paper.
Code availability
The codes used to generate the thermal models, deformation mechanism maps and strength–depth profiles can be accessed at https://github.com/ahkohli/OTFs.
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Acknowledgements
We thank C. Teyssier, S. Birner, K. Kumamoto, L. Hansen, M. Boettcher, M. Behn, J. McGuire and G. Hirth for helpful discussions. This work was supported by a NSF Graduate Research Fellowship to A.K. and NSF grants EAR-1347696, EAR-1619880 and OCE-1832868 to J.M.W.
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Contributions
A.K., C.P. and J.M.W. performed analyses of the mylonite samples. M.W.-S. built the thermal models. C.P. conducted the geochemical measurements. A.K. performed the rheology calculations. All the authors discussed the results and contributed to writing the manuscript.
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Extended data
Extended Data Fig. 1 Deformation mechanism maps for Shaka transform fault.
Maps were generated using the flow law parameters in Extended Data Table 2. a, Protolith. b, HT mylonite. c, MT mylonite. d, LT mylonite.
Extended Data Fig. 2 Deformation mechanism maps for Gofar transform fault.
Maps were generated using the flow law parameters in Extended Data Table 2. a, Protolith. b, HT mylonite. c, MT mylonite. d, LT mylonite.
Extended Data Fig. 4 Modified friction-effective stress relationship for olivine and serpentine.
Modified friction-effective stress relationship for (a) olivine and (b) serpentine calculated using the Shaka geotherm (Fig. 2a) and hydrostatic pore fluid pressure (λ=0.4). The pore fluid factor, α, decreases from 1 at the surface to 0 at the brittle-ductile transition as the normal stress on asperity contacts nears the yield strength.
Supplementary information
Supplementary Information
Supplementary Fig. 1, discussion and Tables 1–4.
Source data
Source Data Fig. 1a
Oceanic transform fault locations, lengths, and slip rates.
Source Data Fig. 1e
Mineral chlorine concentrations in SWIR mylonites.
Source Data Fig. 2c
Calcium in orthopyroxene thermometry data.
Source Data Fig. 3b
Earthquake times, locations, and magnitudes from the 2008 Gofar OBS deployment.
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Kohli, A., Wolfson-Schwehr, M., Prigent, C. et al. Oceanic transform fault seismicity and slip mode influenced by seawater infiltration. Nat. Geosci. 14, 606–611 (2021). https://doi.org/10.1038/s41561-021-00778-1
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DOI: https://doi.org/10.1038/s41561-021-00778-1
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