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The average median valley of the Mid-Atlantic Ridge between 12° and 18° N is described as a smooth depression flanked on both sides by a high. This applies both to the bathymetry and to the gravity anomalies. This picture of the median valley and its walls was obtained by stacking profiles across the valley in 50- to 70-km-wide bands. The reduced median valley can then be interpreted as the result of the parting of the lithosphere and the response of the asthenosphere as a viscous layer to repeated unloading. Fluid dynamic equations show that the response is in general broader than the original load disturbance. We describe this as a viscous lag of the shorter wavelength components. A steady-state solution was reached by numerical methods, showing a depression accompanied by a high on both sides. For the asthenosphere under the Mid-Atlantic Ridge at these latitudes a value followed for the kinematic viscosity of 1.5 x 1019 stokes. The model can be extended to other parts of the mid-ocean ridge system by adapting the time-dependent constants (viscosity and spreading rate). If the viscosity is a factor 5 lower, no median valley results. Rising to isostatic equilibrium of a light body under the floor of the median valley then accounts for the existence of a median ridge like found at Reykjanes Ridge and at the East Pacific Rise. The coefficient of viscosity under the East Pacific Rise would be about 0.4 x 1018 stokes. The concept of a viscous lag of the short-wavelength components replaces Sleep's (1969) original notion of a 'loss of head'. The secondary valleys and ridges found in the median valley and on the flanks of the Mid-Atlantic Ridge crest cannot be explained by the model. They represent essentially a non-continuum process, in which presumably an episodic jumping of the inner valley plays an important role. Additional faulting occurs at the hinge line between the floor and the walls of the median valley.
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