COMPARISON BETWEEN GLOBAL RIDGE OUTPUT AND SLAB INPUT FOR THE 20 MA

We calculated the volume of global lithospheric output along all mid-ocean ridges in addition to slab volume input at subduction zones for the last 20 Ma to determine the role of lower mantle in global tectonics. Ridge output, oceanic underplating, and plate thickness volumes were calculated utilizing Google Earth with Muller's seafloor age plugin to determine spherical ridge area. Global slab input volume was calculated for the last 20 Ma based on the volume of lithosphere subducted above the 660km density barrier in the mantle. We determined slab lithosphere volume via tomography, subduction rates, and the length of subduction zones. Because the Earth is a closed system that is neither growing nor shrinking, outputs at the ridge must equal inputs at subduction zones over time. However, over short time periods the upper mantle may be isolated from the lower mantle leading to fluxing between two (i.e. periods in which the upper mantle contributes to the lower mantle and vice versa).

Our calculations have defined three possible outcomes for the last 20 Ma: 1) The global volume of lithospheric input equals the volumes of mid-ocean ridge output and the upper mantle can be assumed to be a closed system. 2) The global volume of lithospheric input is less than the mid-ocean ridge output in which case a lower mantle contribution would be needed to compensate for all ridge outputs. 3) The global volume of input is greater than mid-ocean ridge output, which would indicate that subducted slabs are going through the 660km barrier. Initial calculations indicate that it not a closed system and that the input (slab subduction) into the upper mantle is currently greater than the output, suggesting that there is a time lag between fluxes between the upper and lower mantle.