Mylonitic Deformation at the Kane Oceanic Core Complex: Implications for the Rheological Behavior of Oceanic Detachment Faults

Barbara E. John, University of Wyoming
Lars N. Lars, Stanford University
Michael J. Cheadle, University of Wyoming
Susan M. Swapp, University of Wyoming
Henry J B Dick, Woods Hole Oceanographic Institution, Massachusetts
Brian E. Tucholke, Woods Hole Oceanographic Institution, Massachusetts
Maurice A. Tivey, Woods Hole Oceanographic Institution, Massachusetts

An edited version of this paper was published by AGU. Copyright 2013 American Geophysical Union. To view the published open abstract, go to http://dx.doi.org and enter the DOI.

Abstract

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.