A GEOLOGICAL MODEL FOR THE STRUCTURE OF RIDGE SEGMENTS IN SLOW-SPREADING OCEAN CRUST

First-order (transform) and second-order ridge-axis discontinuities cr eate a fundamental segmentation of the lithosphere along mid-ocean rid ges, and in slow spreading crust they commonly are associated with exp osure of subvolcanic crust and upper mantle. We analyzed available mor phological, gravity, and rock sample data from the Atlantic Ocean to d etermine whether consistent structural patterns occur at these discont inuites and to constrain the processes that control the patterns. The results show that along their older, inside-comer sides, both first- a nd second-order discontinuities are characterized by thinned crust and /or mantle exposures as well as by irregular fault patterns and a pauc ity of volcanic features. Crust on young, outside er sides of disconti nuities has more normal thickness, regular fault patterns, and common volcanic forms. These patterns are consistent with tectonic thinning o f crust at inside comers by low-angle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981[ and Kars on [1990]. Volcanic upper crust accretes in the hanging wall of the de tachment, is stripped from the inside-comer footwall, and is carried t o the outside comer. Gravity and morphological data suggest that detac hment faulting is a relatively continuous, long-lived process in crust spreading at <25-30 mm/yr, that it may be intermittent at intermediat e rates of 25-40 mm/yr, and that it is unlikely to occur at faster rat es. Detachment surfaces are dissected by later, high-angle faults form ed during crustal uplift into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by sei smic reflection profiling over Cretaceous North Atlantic crust Off-axi s variations in gravity anomalies indicate that slow spreading crust e xperiences cyclic magmatic/amagmatic extension and that a typical cycl e is about 2 m.y. long. During magmatic phases the footwall of the det achment fault probably exposes lower crustal gabbros, although these r ocks locally may have an unconformable volcanic carapace. During amagm atic extension the detachment may dip steeply through the crust, provi ding a mechanism whereby upper mantle ultramafic rocks can be exhumed very rapidly, perhaps in as little as 0.5 m.y. Together, detachment fa ulting and cyclic magmatic/amagmatic extension create strongly heterog eneous lithosphere both along and across isochrons in slow spreading o cean crust.