Mylonitic deformation at the Kane oceanic core complex: Implications for the rheological behavior of oceanic detachment faults. (28th August 2013)

• Record Type: Journal Article
• Authors: Hansen, Lars N.; Cheadle, Michael J.; John, Barbara E.; Swapp, Susan M.; Dick, Henry J. B.; Tucholke, Brian E.; Tivey, Maurice A.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 14:Number 8(2013)
• Page Start: 3085
• Page End: 3108
• Publication Date: 2013-08-28
• Subjects: oceanic core complex -- oceanic detachment fault -- mylonite -- rheology of the lithosphere; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/ggge.20184
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract : [1] The depth extent, strength, and composition of oceanic detachment faults remain poorly understood because the grade of deformation‐related fabrics varies widely among sampled oceanic core complexes (OCCs). We address this issue by analyzing fault rocks collected from the Kane oceanic core complex at 23°30′N on the Mid‐Atlantic Ridge. A portion of the sample suite was collected from a younger fault scarp that cuts the detachment surface and exposes the interior of the most prominent dome. The style of deformation was assessed as a function of proximity to the detachment surface, revealing a ∼450 m thick zone of high‐temperature mylonitization overprinted by a ∼200 m thick zone of brittle deformation. Geothermometry of deformed gabbros demonstrates that crystal‐plastic deformation occurred at temperatures >700°C. Analysis of the morphology of the complex in conjunction with recent thermochronology suggests that deformation initiated at depths of ∼7 km. Thus we suggest the detachment system extended into or below the brittle‐plastic transition (BPT). Microstructural evidence suggests that gabbros and peridotites with high‐temperature fabrics were dominantly deforming by dislocation‐accommodated processes and diffusion creep. Recrystallized grain size piezometry yields differential stresses consistent with those predicted by dry‐plagioclase flow laws. The temperature and stress at the BPT determined from laboratory‐derived constitutive models agree well with the lowest temperatures and highest stresses estimated from gabbro mylonites. We suggest that the variation in abundance of mylonites among oceanic core complexes can be explained by variation in the depth of the BPT, which depends to a first order on the thermal structure and water content of newly forming oceanic lithosphere. Key Points: The detachment fault is defined by a 450 m zone of mylonitic deformation The fault deformed viscously and rooted at depths >7 km and temperatures >700oC Variation among oceanic faults is due to water content and/or thermal structure