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Oceanic transform fault seismicity and slip mode influenced by seawater infiltration

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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Fig. 1: Map and microstructures of OTF mylonites.
Fig. 2: Thermal models and hydrous mineralogy of OTFs.
Fig. 3: Strength–depth profiles for slow- and fast-slipping OTFs.
Fig. 4: Vertical and along-strike variations in seismicity on OTFs.

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.

Author information

Authors and Affiliations

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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.

Corresponding authors

Correspondence to Arjun Kohli or Jessica M. Warren.

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The authors declare no competing interests.

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Peer review information Nature Geosciences thanks the anonymous reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Stefan Lachowycz.

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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. 3 Composite deformation mechanism maps for (a) Shaka and (b) Gofar transform faults.

The total strain rate is the arithmetic sum of the strain rates from each deformation mechanism (that is, Ol total in Extended Data Figs. 1, 2). The ranges for each curve represent uncertainty in the pressure-temperature estimates for deformation.

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.

Extended Data Table 1 Material properties and boundary conditions for fault thermal models (Fig. 2a,b)
Extended Data Table 2 Flow laws and constitutive parameters used in deformation mechanism maps (Extended Data Figs. 1, 2) and strength-depth profiles (Fig. 3). *The value of A has been adjusted from the original references to account for revised estimates of the water content in the experimental samples. GBS - Grain boundary sliding; LTP - Low temperature plasticity

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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