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Mylonitic deformation at the Kane oceanic core complex: Implications for the rheological behavior of oceanic detachment faults

Control ID: 1787860

Title: Mylonitic deformation at the Kane oceanic core complex: Implications for the rheological behavior of oceanic detachment faults (Invited)

Authors (First Name, Last Name): Lars N Hansen1, Michael J Cheadle2, Barbara E John2, Susan Swapp2, Henry J Dick3, Brian E Tucholke3, Maurice Tivey3

Institutions (All):

  1. Stanford University, Stanford, CA, United States.
  2. University of Wyoming, Laramie, WY, United States.
  3. Woods Hole Oceanographic Institution, Woods Hole, MA, United States.

Abstract Body: The depth extent, strength, and composition of oceanic detachment faults remain poorly understood because the occurrence of rocks with evidence for high-temperature deformation varies widely among sampled oceanic core complexes. We address this issue through petrographic, crystallographic and thermometric analysis of 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 the scarp of a normal fault that cuts the detachment surface and exposes the dominantly peridotite interior of the most prominent dome. With these samples, 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. Analysis of the morphology of the complex in conjunction with recent thermochronology suggests that deformation initiated at depths of ~7 km along a >45° dipping fault. Geothermometry of deformed gabbros demonstrates that crystal plastic deformation was occurring at temperatures >700°C. Thus we suggest that the detachment system extended into or below the brittle-plastic transition (BPT). Microstructural and crystallographic 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 during deformation that are consistent with those predicted by dry-plagioclase flow laws. The temperature (~750 °C) and stress (~100 MPa) at the BPT determined from the intersection of Byerlee’s Law and the plagioclase flow law 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 composition, thermal structure, and water content of the newly forming oceanic lithosphere.

Keywords: 3902 MINERAL PHYSICS Creep and deformation, 8159 TECTONOPHYSICS Rheology: crust and lithosphere, 8120 TECTONOPHYSICS Dynamics of lithosphere and mantle: general, 8150 TECTONOPHYSICS Plate boundary: general.