Strain localization in the root of detachment faults at a melt‐starved mid‐ocean ridge: a microstructural study of abyssal peridotites from the Southwest Indian Ridge

Type Article
Date 2021-05
Language English
Author(s) Bickert ManonORCID1, Cannat M.ORCID1, Tommasi A.ORCID2, Jammes S.ORCID3, Lavier L.ORCID4
Affiliation(s) 1 : Marine Geosciences, Institut de Physique du Globe de Paris UMR 7154 ‐CNRS Université de Paris, France
2 : Géosciences Montpellier, CNRS & Université de Montpellier, France
3 : Department of GeographyTexas State University San Marcos Texas 78666, USA
4 : Department of Geological Sciences, Institute for GeophysicsJackson School of Geosciences, The University of Texas at Austin, United States
Source Geochemistry Geophysics Geosystems (1525-2027) (American Geophysical Union (AGU)), 2021-05 , Vol. 22 , N. 5 , P. e2020GC009434 (29p.)
DOI 10.1029/2020GC009434
WOS© Times Cited 11

Detachment faults that exhume mantle peridotites to the seafloor play a major role in the accommodation of plate divergence at slow‐spreading ridges. Using 99 samples of partially serpentinized peridotites dredged from a nearly amagmatic segment of the eastern part of the Southwest Indian Ridge, we characterize the deformation processes active in the root zone of detachment fault systems. The deformation is heterogeneous even at the sample scale and combines both brittle and crystal‐plastic mechanisms. Strain localization is initially controlled by strength contrasts at the grain scale between olivine and orthopyroxene and between variably oriented olivine crystals. Orthopyroxene deformation is primarily brittle (microfractures), but kink bands and dynamic recrystallization are locally observed. In contrast, olivine deforms primarily by dislocation creep with dynamic recrystallization under high deviatoric stresses (80‐270 MPa). Olivine grains poorly oriented to deform by dislocation glide display kink bands and localized microfractures. Dynamic recrystallization controlled by strain and stress concentrations produce anastomosing zones of grain size reduction (GSR). GSR zones contain limited late to post‐kinematic amphibole, suggesting the presence of small volumes of hydrous fluids. Plagioclase, when present, is post‐kinematic. This heterogeneous high‐stress deformation is observed, with variable intensity, in every sample investigated, suggesting that it was pervasively distributed in the root region of axial detachments. Abyssal peridotite samples from more magmatically robust slow mid‐ocean ridges do not show this pervasive high stress deformation microstructure, implying magma, when present, tends to localize most of the strain at the root of axial detachment systems.

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Bickert Manon, Cannat M., Tommasi A., Jammes S., Lavier L. (2021). Strain localization in the root of detachment faults at a melt‐starved mid‐ocean ridge: a microstructural study of abyssal peridotites from the Southwest Indian Ridge. Geochemistry Geophysics Geosystems, 22(5), e2020GC009434 (29p.). Publisher's official version : , Open Access version :