Mantle Bouguer gravity anomalies (MBA) and bathymetry on three profiles covering more than 1000 km along the axis of the Mid-Atlantic Ridge (MAR) are highly correlated, suggesting that along-axis topographic relief is locally compensated by variations in crustal thickness and/or mantle density structure. The quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin, narrow elastic strip, representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak, bounding normal faults. The paradox is that across-axis profiles show that the median valley is an uncompensated feature, apparently created by a dynamic mechanism. New, extensive off-axis coverage of the MAR from 31¿ to 36¿S shows that the high correlation does not persist outside the axial zone. We suggest that the on-axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along-axis variations in crustal thickness, both of which contribute to the MBA. If the mechanism is extension of a brittle-ductile lithosphere, the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis. A three-dimensional (3-D) thermal model incorporating passive mantle flow, hydrothermal circulation, the plate boundary geometry, and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography.

We model the expected topography using cross-axis sections of the 3-D thermal model and a 2-D finite element model of an extending lithosphere that incorporates temperature- and strain-dependent rheology, as well as the flexural response of a thickening plate. The predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry. At the 18 mm yr-1 spreading half-rate in our South Atlantic survey, the 2-D models also reproduce the observed rapid transition along the axis between rift valley/no rift valley morphology. We conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the Southern MAR and the along-axis correlation between MBA and bathymetry. ¿ American Geophysical Union 1993