Drilling during ODP Leg 209, dredging, and submersible dives have delineated an anomalous stretch of the Mid-Atlantic Ridge north and south of the 15 degrees 20'N Fracture Zone. The seafloor here consists dominantly of mantle peridotite with gabbroic intrusions that in places is covered by a thin, discontinuous extrusive volcanic layer. Thick lithosphere ( 10 - 20 km) in this region inhibits magma from reaching shallow levels beneath the ridge axis, thereby causing plate accretion to be accommodated by extensional faulting rather than magmatism. The bathymetry and complex fault relations in the drill-core suggest that mantle denudation and spreading are accommodated by a combination of high-displacement, rolling-hinge normal faults and secondary lower-displacement normal faults. These extensional faults must also accommodate corner flow rotation ( up to 90 degrees) of the upwelling mantle within the shallow lithosphere, consistent with remnant magnetic inclinations in denuded peridotite and gabbro from Leg 209 core that indicate up to 90 degrees of sub-Curie-temperature rotation.
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