The Nature of Detachment faulting at the Kane Megamullion: Initial Results

On Knorr Cruise 180-2 in Nov.-Dec. 2004, we used Jason II, ABE, and dredges to survey and sample the Kane Megamullion oceanic core complex at 23°30'N on the Mid-Atlantic Ridge. Samples collected from twenty-three locations provide an extensive data set for analysis of the deformation associated with the main detachment fault. Dives and dredges were sited either on the main detachment surface or on younger N-S trending fault scarps that cut the detachment fault. Peridotites comprise 82% by weight, of the non-basaltic rocks recovered from the central domes (Cain and Abel Domes), and 40% and 29% respectively by weight of the non-basaltic rocks recovered from the both the northern and southern domes. Dredges and dives on the detachment surface at the margins of the main, central dome (Cain Dome) recovered high-temperature (>500°C), granulite/amphibolite-facies peridotite and gabbro with proto- to ultramylonite fabrics. High-temperature strain appears to be preferentially localized in gabbro and oxide gabbro intrusions and veins where present. This high-temperature deformation is extensively overprinted by low-temperature (<400°C) semi-brittle and brittle greenschist-facies cataclasis. Samples and dive observations from Jason dive 113 on the west edge of Cain Dome suggest that this later deformation was localized into shear zones around more resistant boudins or facoids at both the hand specimen (10-30cm) and outcrop (1-10m) scale. Strain was apparently localized onto the detachment fault by reaction-softening and fluid-assisted fracturing during greenschist- and subgreenschist-grade hydrothermal alteration of olivine, clinopyroxene, serpentine, and hornblende to tremolite, chlorite, and/or talc. Some, but not all, gabbros are weakly deformed below amphibolite facies (<500°C), suggesting that strain is commonly partitioned into altered/metamorphosed peridotite at low temperature. Analysis of the fault rock deformation data suggest that rocks from the detachment fault surface are commonly overprinted by brittle deformation, whereas those from high-angle fault scarps cutting the detachment appear to show less brittle deformation, suggesting that the semi-brittle and brittle shear zones associated with the detachment fault are concentrated in the top 10-100 meters below the surface of the dome. This result is consistent with the Schroeder and John (2004) analysis of the detachment fault system at Atlantis Massif. However, we cannot yet constrain the thickness of the high-temperature, crystal plastic deformation zone. The rare occurrence of both deformed and undeformed diabase within the detachment fault zone suggests that faulting was at least partly coeval with ridge magmatism.