SEGMENTATION AND CRUSTAL STRUCTURE OF THE WESTERN MID-ATLANTIC RIDGE FLANK, 25-DEGREES-25'-27-DEGREES-10'N AND 0-29 M.Y

We conducted a detailed geological-geophysical survey of the west flan k of the Mid-Atlantic Ridge between 25 degrees 25'N and 27 degrees 10' N and from the ridge axis out to 29 Ma crust, acquiring Hydrosweep mul tibeam bathymetry, HAWAII MR1 sidescan-sonar imagery, gravity, magneti cs, and single-channel seismic reflection profiles. The survey covered all or part of nine spreading segments bounded by mostly nontransform , right-stepping discontinuities which are subparallel to flow lines b ut which migrated independently of one another. Some discontinuities a lternated between small right- and left-stepping offsets or exhibited zero offset for up to 3-4 m.y. Despite these changes, the spreading se gments have been long-lived and extend 20 m.y. or more across isochron s. A large shift (similar to 9 degrees) in relative plate motion about 24-22 Ma caused significant changes in segmentation pattern. The natu re of this plate-boundary response, together with the persistence of s egments through periods of zero offset at their bounding discontinuiti es, suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focus ed zones of rising melt. Within segments, there are distinct differenc es in seafloor depth, morphology, residual mantle Bouguer gravity anom aly, and apparent crustal thickness between inside-corner and outside- corner crust. This demands fundamentally asymmetric crustal accretion and extension across the ridge axis, which we attribute to low-angle, detachment faulting near segment ends. Cyclic variations in residual g ravity over the cross-isochron run of segments also suggest crustal-th ickness changes of at least 1-2 km every 2-3 m.y. These are interprete d to be caused by episodes of magmatic versus relatively amagmatic ext ension, controlled by retention and quasiperiodic release of melt from the upwelling mantle. Detachment faulting appears to be especially ef fective in exhuming lower crust to upper mantle at inside corners duri ng relatively amagmatic episodes, creating crustal domes analogous to ''turtleback'' metamorphic core complexes that are formed by low-angle , detachment faulting in subaerial extensional environments.