Flattening of the oceanic topography and geoid: thermal versus dynamic origin

The origin of the flattening of the seafloor bathymetry and geoid at old ag es is still a matter of debate. Is this departure from the square-root-of-a ge law induced by dynamic uplift of the lithosphere linked to the flow in t he mantle or by cooling of the lithosphere different from that predicted fo r an infinite half-space? The amplitude of the dynamic uplift should strong ly decrease at short wavelengths and should not be correlated with the seaf loor age on a local scale. Therefore, in order to eliminate the 'dynamic si gnal', the cooling of the lithosphere is studied here by means of depth-age and geoid-age relationships computed from high-pass filtered bathymetric, geoid and age fields, or from local dtopography/d root age and dgeoid/dage slopes. The geoid-age and depth-age relationships obtained here are in good agreement with those previously deduced from global non-filtered data. At all observation scales, a significant departure from the halfspace solution is found. However, the data are not sufficiently accurate to discriminate between the Plate and CHABLIS (Constant Heat flow At the Base of the Lithos pheric ISotherm) models. Our results support the existence of small-scale c onvection supplying heat at the base of the old oceanic lithosphere.