Cooling histories and depth of oceanic detachment faulting at the Mid-Atlantic Ridge: Constraints from thermochronometry

The cooling history of lithosphere exposed in the footwalls to large-slip detachment faults associated with oceanic core complexes (OCCs) can be used to help establish depths of detachment faulting and gabbro emplacement in slow spreading ocean crust. Here, we use thermochronometric data to constrain the cooling history of gabbroic rocks hosted in the footwalls of two OCCs along the Mid-Atlantic Ridge (MAR): the Atlantis Massif OCC at 30°N and the Kane OCC at 23°N.;U-Pb zircon crystallization combined with (U-Th)/He zircon cooling ages allows the determination of the cooling rates of gabbroic rocks sampled from these core complexes over a significant temperature interval. The crystallization temperature for U-Pb in zircons from oceanic gabbros is ∼750+/-50°C based on titanium-in-zircon thermometry; the closure temperature for the (U-Th)/He system in zircon is ∼190°C for these rapidly cooled rocks. Ages at intermediate temperatures (300--580°C) are constrained by multi-component remnant magnetization measurements. Thus, thermochronometry and geomagnetic studies help delimit the cooling history from ∼750°C to 190°C.;The Atlantis Massif oceanic core complex, is located at 30°N at the intersection of the MAR and the Atlantis Transform Fault. Samples were chosen from the 1415m deep Integrated Ocean Drilling Program (IODP) Hole U1309D located ∼15km west of the ridge axis. Rocks from the upper 600m of the core cooled rapidly (∼1710°C/Myr) from crystallization temperatures to ∼300°C, whereas deeper samples cooled more slowly (∼885°C/Myr). Using a fault slip rate of 20 mm/yr, the thermochronometric data combined with the multi-component remnant magnetic data are used to model the depths of the 190°C, 300°C, 450°C, 580°C, and 715°C isotherms in the footwall beneath the subsurface continuation of the detachment fault system. The depth of the 715°C isotherm constrains both the depth of detachment faulting within the brittle lithosphere and gabbro emplacement in the footwall to be 7.0 +/- 1 km beneath the rift valley floor. An apparent decrease in cooling rate between the 190 and 300°C isotherms immediately beneath the detachment fault may suggest temperature buffering by hydrothermal circulation of seawater along the detachment fault system.;The Kane Oceanic Core Complex, is located at 23°N, just south of the Kane Transform Fault, west of the MAR. Surface samples collected by dredge and ROV were chosen to investigate lateral variation in (U-Th)/He zircon age of the gabbroic rocks from an ∼40km long north to south strike-line parallel to the ridge axis, and from an ∼25km long east-west flow line normal to the ridge along the detachment fault surface. Cooling rates from 735 to 230°C are ∼1400°C/Myr during denudation of the entire oceanic detachment fault. Using combined U-Pb and (U-Th)/He zircon ages and assuming a constant slip rate of 16.5 mm/yr, the depth of the detachment fault varied from ∼8km in the north adjacent to the transform and shallows to ∼5km in the east and south near the ridge segment center. The shallowing of the depth of detachment faulting may correlate with areas of increased magmatic activity, and consequently an increased thermal gradient.