Fluid-assisted grain size reduction leads to strain localization in oceanic transform faults

Manon Bickert (), Mary-Alix Kaczmarek, Daniele Brunelli (), Marcia Maia, Thomas F. C. Campos and Susanna E. Sichel
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Manon Bickert: Geo-Ocean, Univ Brest, CNRS, IFREMER, UMR6538
Mary-Alix Kaczmarek: CNRS-CNES-IRD-Université Toulouse III, Observatoire Midi Pyrénées
Daniele Brunelli: Università di Modena e Reggio Emilia
Marcia Maia: Geo-Ocean, Univ Brest, CNRS, IFREMER, UMR6538
Thomas F. C. Campos: Rio Grande do Norte Federal University
Susanna E. Sichel: Federal Fluminense University

Nature Communications, 2023, vol. 14, issue 1, 1-13

Abstract: Abstract Oceanic Transform Faults are major plate boundaries representing the most seismogenic part of the mid ocean ridge system. Nonetheless, their structure and deformation mechanisms at depth are largely unknown due to rare exposures of deep sections. Here we study the mineral fabric of deformed mantle peridotites - ultramafic mylonites - collected from the transpressive Atobá ridge, along the northern fault of the St. Paul transform system in the Equatorial Atlantic Ocean. We show that, at pressure and temperature conditions of the lower oceanic lithosphere, the dominant deformation mechanism is fluid-assisted dissolution-precipitation creep. Grain size reduction during deformation is enhanced by dissolution of coarser pyroxene grains in presence of fluid and contextual precipitation of small interstitial ones, leading to strain localization at lower stresses than dislocation creep. This mechanism potentially represents the dominant weakening factor in the oceanic lithosphere and a main driver for the onset and maintenance of oceanic transform faults.

Date: 2023
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DOI: 10.1038/s41467-023-39556-5

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