Flattening of the sea-floor depth-age curve as a response to asthenospheric flow

Abstract

THE flattening of sea-floor depths from the √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 ∼80 Myr (ref. 2), thermal rejuvenation of lithosphere that passes over stationary hotspots^3–5, 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 ∼ 1 km of sea-floor flattening for asthenospheric viscosities of 2 × 10¹⁸ 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.