University of Arizona, United States

Oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid-ocean ridges, but many questions persist about the thermal evolution and depth of faulting. We present new Pb/U and (U-Th)/He zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the Mid-Atlantic Ridge (Ocean Drilling Program (ODP) holes 1270D and 1275D near the 15 degrees 20'N Transform, and Atlantis Massif at 30 degrees N). Closure temperatures for the Pb/U ( approximately 800 degrees C-850 degrees C) and (U-Th)/He ( approximately 210 degrees C) isotopic systems in zircon bracket acquisition of magnetic remanence, collectively providing a temperature-time history during faulting. Results indicate cooling to approximately 200 degrees C in 0.3-0.5 Myr after zircon crystallization, recording time-averaged cooling rates of approximately 1000 degrees C-2000 degrees C/Myr. Assuming the footwalls were denuded along single continuous faults, differences in Pb/U and (U-Th)/He zircon ages together with independently determined slip rates allow the distance between the approximately 850 degrees C and approximately 200 degrees C isotherms along the fault plane to be estimated. Calculated distances are 8.4+ or -4.2 km and 5.0+ or -2.1 km from holes 1275D and 1270D and 8.4+ or -1.4 km at Atlantis Massif. Estimating an initial subsurface fault dip of 50 degrees and a depth of 1.5 km to the 200 degrees C isotherm leads to the prediction that the approximately 850 degrees C isotherm lies approximately 5-7 km below seafloor at the time of faulting. These depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the TAG hydrothermal field and high-temperature fault rocks recovered from many oceanic detachment faults.