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High‐Temperature Strain Localization and the Nucleation of Oceanic Core Complexes (16.5°N, Mid‐Atlantic Ridge)
Author(s) -
Casini Leonardo,
Maino Matteo,
Sanfilippo Alessio,
Ildefonse Benoit,
Dick Henry J. B.
Publication year - 2021
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2021jb022215
Subject(s) - mylonite , detachment fault , geology , plagioclase , shear zone , olivine , geochemistry , grain boundary sliding , petrology , mantle (geology) , mid atlantic ridge , mineralogy , hydrothermal circulation , grain boundary , seismology , materials science , metallurgy , quartz , microstructure , paleontology , extensional definition , tectonics
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.