Publication Date:
2022-10-26
Description:
© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Casini, L., Maino, M., Sanfilippo, A., Ildefonse, B., & Dick, H. J. B. High-temperature strain localization and the nucleation of oceanic core complexes (16.5 degrees N, Mid-Atlantic Ridge). Journal of Geophysical Research: Solid Earth, 126(9), (2021): e2021JB022215, https://doi.org/10.1029/2021JB022215.
Description:
Extension at slow to ultraslow midoceanic ridges is mostly accommodated by large detachment faults that expose mantle peridotite and/or lower-crustal rocks forming Oceanic Core Complexes (OCC). It is commonly accepted that OCC at slow spreading ridges form during the early stage of crystallization of the magmatic crust, when rocks are still close to their solidus temperature. This observation poses significant problems, as nucleation of detachment faults requires significant weakening, which instead is more easily obtained at low temperature. The RV Knorr cruise 210 Leg 5 on the 16.5°N OCC of the Mid-Atlantic Ridge recovered a narrow shear zone from the plutonic footwall of a mature detachment fault. Troctolites preserve a continuous transition from proto-mylonite to mylonite and ultra-mylonite equilibrated at temperature between 1100° and 900°C. EBSD analysis highlights increased phase mixing and weaker crystallographic fabrics in the ultra-mylonite with respect the mylonitic domains. While host troctolites were completely solidified at the deformation incoming, high-strain zones preserve evidences of syn-kinematic melt-related textures. Fabric patterns combined with plagioclase and olivine grain size piezometry and 1D rheological modeling indicate that the development of ultra-mylonite requires a switch from dislocation creep to melt-enhanced grain-boundary sliding. Activation of this mechanism was promoted by the occurrence of hydrous melt possibly produced by selective re-melting of plagioclase + Ti-pargasite microdomains in response to strain localization at subseismic strain rates. This study highlights the importance of hydrated magmatic phases to promote the onset of detachment faulting in OCC.
Description:
L. Casini thanks to Regione Autonoma della Sardegna for partly supporting this research (RASSR14473), and Università di Sassari (FAR2019). Funding for H. Dick was provided by US National Science Foundation grant No. 1935837. Open access funding enabled and organized by Projekt DEAL.
Keywords:
Detachment faulting
;
EBSD
;
Grain boundary sliding
;
Melt-present deformation
;
Plagioclase rheology
Repository Name:
Woods Hole Open Access Server
Type:
Article
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