Extensional Faulting at 15\deg North on the Mid-Atlantic Ridge, ODP Leg 209

At slow spreading ridges, oceanic mantle can be uplifted and brought to the seafloor by extensional faulting, but the style of faulting and mechanisms by which this occurs remain poorly understood. We present data from faults observed in mantle peridotite and gabbro intrusions drilled at six sites on the Mid-Atlantic Ridge near the 15\deg20' Fracture Zone during ODP Leg 209. The data reveal that faulting and significant tectonic rotation can take place either via long-lived (detachment?) faults that are active through ductile and brittle regimes, or via successive series of short-lived (domino) faults. We recognize three styles of faults: a) High-temperature, mylonitic shear zones at Sites 1268, 1272 and 1274 that are overprinted by dominantly static greenschist facies alteration. These textures indicate that ductile faults were active at depth and later became inactive during denudation and are cut by brittle faults at shallow lithospheric levels. b) Brittle and semi-brittle faults that do not directly overprint ductile shear zones. These include partially- to non-cohesive serpentine mud fault gouge in zones ranging from 10 cm to several meters thick (Sites 1268, 1272 and 1274), and cohesive cataclasites and talc/tremolite schists (Site 1275). c) Ductile to brittle faults in peridotites from Sites 1270 and 1271. Strain in the peridotites at both sites appears to have been initially localized into gabbroic veins and dikes at granulite facies, and remained localized in these zones to sub-greenschist facies during long-lived faulting and denudation of peridotite. There is no apparent correlation between the faulting style and seafloor bathymetry. For example, bathymetry of Sites 1270 and 1275 indicate the presence of a detachment fault (oceanic core complex), yet 1270 displays type c faults and 1275 displays type b faults. Peridotite and gabbro from all six sites experienced significant tectonic rotation (50° to 90°) as interpreted from the inclination of the remnant magnetization vector (assuming horizontal axis rotations). This result is consistent with both rotation via long-lived (detachment) faulting, and/or multiple short-lived fault bounded domino systems. Variability in faulting styles and seafloor morphology along with significant tectonic rotations indicate that highly complex faulting may be typical at slow-spreading ridges.