DETERMINING COOLING HISTORIES AND DEPTH OF DETACHMENT FAULTING OF OCEANIC CORE COMPLEXES USING U-PB AND (U-TH)/HE THERMOCHRONOMETRY

Oceanic core complexes (OCCs) are domal uplifts of lower oceanic lithosphere and mantle peridotite denuded on long-lived oceanic detachment faults at slow and ultraslow mid-ocean ridges. Combined U-Pb zircon geochronometry, Ti-in-zircon thermometry, and (U-Th)/He zircon thermochronology can be used to determine the cooling histories of OCCs. Cooling rates are established using high temperature (700-750°C) U-Pb crystallization ages combined with low-temperature (180-230°C) (U-Th)/He zircon ages. Zircon thermochronometric data can also be used to estimate depths of detachment faulting.

We present 12 new (U-Th)/He zircon ages from the Kane OCC, located at 23°N just south of the Kane Transform Fault on the Mid-Atlantic Ridge (MAR). We examine lateral age variations of gabbroic rocks from a N-S strike-line parallel to the ridge axis and from a E-W flowline normal to ridge along the detachment fault surface. Samples cooled from ~740 to 200°C at rates ranging from 1000-1700°C/Myr before being denuded to the seafloor. The difference in age between the U-Pb geochronometer and the (U-Th)/He thermochronometer ranges from 0.3 to 0.5 Myr, and provides an estimate of the depth of detachment faulting. Assuming a constant fault slip rate of 16.5mm/yr and a rolling hinge subsurface fault geometry with a dip of 60°, this range of age differences suggest a variation in depth of faulting from ~8km near the transform fault to ~5km near the ridge segment center. The data also suggest shallowing of the detachment fault from ~8km to ~5.5km from west to east, likely associated with areas where gabbro dominates over peridotite, and hence may be related to increased magmatism and steepening of the geotherm.

We compare these data to U-Pb and (U-Th)/He zircon ages from four other OCCs (Atlantis Massif [MAR], two OCCs from the Fifteen-Twenty Fracture Zone [MAR], and Atlantis Bank on the Southwest Indian Ridge). All of the OCCs, except Atlantis Bank, show comparable differences in age between the chronometers of 0.3 to 0.5 Myr, and using similar assumptions, the oceanic detachment faults consistently root at depths between from 6.5 to 8.5 km. The majority of samples from Atlantis Bank have much larger age differences (2-4 Myr), suggesting this OCC has an anomalous cooling history, possibly explained by a late, off-axis magmatism (Schwartz et al. 2009).