FLATTENING OF THE SEA-FLOOR DEPTH AGE CURVE AS A RESPONSE TO ASTHENOSPHERIC FLOW

THE flattening of sea-floor depths from the square-root age dependence predicted by considering the cooling plate as a growing thermal boundary layer is a fundamental constraint on the evolution of oceanic lithosphere1. Previous explanations for the flattening have included reheating from convective instabilities that form beneath lithosphere older than approximately 80 Myr (ref. 2), thermal rejuvenation of lithosphere that passes over stationary hotspots3-5, or a whole-mantle flow forced by two converging plates6. We suggest here that the flattening of old ocean floors can perhaps best be explained as a dynamic phenomenon reflecting flow in asthenosphere underlying the oceanic lithosphere. Applying the model to the Pacific plate, we show that a solution for flow in an asthenosphere low-viscosity channel which is 'consumed' by plate accretion and the subduction of lithosphere at trenches, and replenished by near-ridge upwelling and near-ridge hotspots, generates the observed approximately 1 km of sea-floor flattening for asthenospheric viscosities of 2 x 10(18) Pa s and an absolute Pacific plate motion of 100 mm yr-1. Our model can also explain the asymmetric subsidence of the South American and African plates away from the Mid-Atlantic Ridge.