Fluid magma should tend to rise up to the level of local isostatic equilibrium at an axis of plate spreading. The magma and underlying partial melt accretes to the side of the separating plates and gets denser as it cools and freezes. This load bends down the lithosphere and additional accretion forms a plate with concave upward curvature. The topography resulting from a curved plate passively moving off axis can be described analytically for accretion on the vertical side of uniform lithosphere when its flexural response is that of a thin elastic plate. Model predictions are consistent with the shape of the axial highs at fast spreading centers (typically similar to 400m high and similar to 20 km wide) as well as related gravity anomalies. This model requires at least several kilometers of lithospheric thickening within a few kilometers of the ridge axis. Seismic data appear consistent with this lithospheric structure. The magnitude and lateral distribution of faulting on the flanks of axial highs is crudely consistent with formation during plate unbending as lithosphere moves away from the axis. According to this model abandoned fast spreading centers should retain an elastically supported axial high, while for previous models abandoned highs should collapse.
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