Constraints on Oceanic Detachment Fault Formation: A Macro- and Microstructural Study of Fault Rocks from Atlantis Massif (MAR 30°N)

ISBN-10
0438831039
ISBN-13
9780438831032
Category
Fault zones
Pages
120
Language
English
Published
2018
Author
Andrew J. McPeak

Description

Macro- and microstructural observations, together with mineral thermometry, on gabbro and peridotite samples collected from the Atlantis Massif oceanic core complex (30°N, mid- Atlantic Ridge) indicate that detachment faulting was initiated under high temperature granulite conditions in the ductile regime, and continued down temperature through semi-brittle conditions into the brittle regime under sub-greenschist conditions as strain localized on the now exposed sub-horizontal fault surface. Footwall rocks host anastomosing high strain zones of mylonite which extend to >400 m below the denuded fault surface exposed at the seafloor; semi-brittle and brittle fabrics are concentrated in the ∼100m immediately below the principal slip surface of the detachment fault. Strain localization is present in samples that show high-temperature deformation and is marked by discrete, cm-scale zones of mylonite commonly enriched in Fe-Ti oxides and brown amphibole. Electron backscatter diffraction (EBSD) techniques show that high-temperature crystal plastic deformation of plagioclase hosted in gabbroic rocks is dominated by diffusion creep; the majority of samples demonstrate no well-defined crystallographic preferred orientation (CPO). Evidence of (001) [100] slip in two samples and one sample displaying (010) [100] slip suggests a transition from deformation via dislocation-creep processes to grain size-sensitive creep mechanisms including diffusion creep and grain-boundary sliding during strain localization. Interstitial, disseminated ilmenite in high-strain shear zones preserves a weak CPO indicating basal slip during fault slip. Paleopiezometry on fault rocks containing plagioclase and olivine allow estimates of differential stress at initiation of the fault system, between 30–100 MPa. Strain rates based on 2 wet and dry plagioclase, and dry olivine flow laws range from ∼10−10s to ∼10 −11s, consistent with plate spreading rates at the Mid-Atlantic Ridge. Plagioclase and olivine deformation and recrystallization mechanisms indicate that gabbroic samples deformed at peak temperatures of between ∼600-800°C; in contrast, one altered peridotite sample deformed at higher temperatures, at around ∼900°C. Together, these observations support a model for a distributed zone of deformation associated with the initiation of the detachment fault system at depths of ∼4.5-6km depth. The detachment system across Atlantis Massif steepens downward below the detachment termination and roots into an axial mush zone, or area with an elevated brittle-plastic transition (BPT). This elevated BPT would promote the extensive crystal-plastic deformation observed along the Southern Ridge of Atlantis Massif, and other OCCs along the global mid-ocean ridge system. This study supports the hypothesis that OCC related detachment systems localize in zones of weakened lithospheric conditions, likely due to higher water contents.

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