Abstract:
Abstract: 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°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 °) 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 ° of sub-Curie-temperature rotation.
Keywords: seafloor spreading; ocean drilling program; nonvolcanic mid-ocean ridges; extensional faulting.
