OCEANIC TOPOGRAPHY AND HEATFLOW - INDICATIONS FOR A SILENT DISCHARGE OF COLD ROCK INTO THE CONVECTING EARTH

The 2.5 km mean relative elevation of mid ocean ridges is arguably con sistent with a steady state thickness of approximate to 100 km for the oceanic lithosphere. Here we explore some consequences of this partic ular value. Based on seismic models of the uppermost oceanic mantle, w e infer that no significant heat is added to normal asthenosphere foll owing accretion at spreading centers, Therefore we adopt the view that kinematic thinning moderates the conductive thickening to yield the s teady state thickness of approximate to 100 km. We devise a method for estimating mean rates of thinning directly from topography-age data, with no assumptions on the detailed dynamics of delamination. We find that delamination delivers a significant fraction (>20%) of the global downwelling discharge. This large non subducting discharge of cold li thospheric material provides an explanation for the most robust featur es of upper-mantle tomography, and should be considered in studies of mantle cooling and mixing.