A geological model for the structure of ridge segments in slow spreading ocean crust

First-order (transform) and second-order ridge-axis discontinuities create a fundamental segmentation of the lithosphere along mid-ocean ridges, and in slow spreading crust they commonly are associated with exposure of subvolcanic rust and upper mantle. We analyzed available morphological, gravity, and rock sample data from the Atlantic Ocean to determine whether consistent structural patems occur at these discontinuites and to constrain the processes that control the patterns. The results show that along their older, inside-comer sides, both firstand semond-order discontinuities are characterized by thinned crust and/or mantle exposures as well as by irregular fault patterns and a paucity of volcanic features. Crust on young, outside-comer sides of discontinuities has more normal thickness, regular fault patems, and common volcanic forms. These patterns are consistent with tectonic th'mning of crust at inside comers by lowangle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981] and Karson [1990]. Volcanic upper crust accretes in the hang'rag wall of the detachment, is stripped from the inside-comer footwall, and is carried to the outside comer. Gravity and morphological data suggest that detachment faulting is a relatively continuous, long-lived process in crust spread'rag at<25-30 mm/yr, that it may be intermittent at intermediate rates of 2540 mm/yr, and that it is unlikely to occur at faster rates. Detachment surfaces are dissected by later, high-angle faults formed during crustal uplift 'into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by seismic reflection proffi'mg over Cretaceous North Atlantic crust. Off-axis variations in gravity anomalies 'indicate that slow spreading crust experiences cyclic magmatic/amagmatic extension and that a typical cycle is about 2 m.y. long. During magmatic phases the footwall of the detachment fault probably exposes lower crustal gabbros, although these rocks locally may have an unconformable volcanic carapace. During amagmafic extension the detachment may dip steeply through the crust, providing a mechanism whereby upper mantle ultramafic rocks can Ix: exhumed very rapidly, perhaps in as littic as 0.5 m.y. Togc•cr, detachment faulting and cyclic magmatic/amagmatic extension create strongly heterogeneous lithosphere both along and across isochrons in slow spreading ocean crust.