MAPPING UPPER MANTLE DEPLETION: THE INFLUENCE OF RIDGE MIGRATION AND SEAFLOOR SPREADING HISTORIES ON SEAFLOOR DEPTH

The time-dependent extraction of asthenospheric material by the spreading process affects seafloor topography. Mid-ocean ridge migration and spreading rates determine the extent and duration of partial melting that occurs in the upper mantle beneath mid-ocean ridges. We compute global relative upper mantle depletion using ridge migration and seafloor spreading rates calculated over the past 140 My at 1 My intervals as proxies for ridge residence times and melt extraction rates. This is used to investigate broad-scale geophysical differences between regions that have experienced substantial melt extraction over the past 140 My and those that have not. The preferential removal of dense incompatible elements through partial melting results in depleted anomalously low-density residual upper mantle. The East Pacific region exhibits relatively high melt depletion due to slow westward migration of the East Pacific Rise accompanied by relatively fast seafloor spreading rates. We suggest that this has led to anomalously low seismic uppermost mantle velocities in the south of the East Pacific region, as observed in recent seismic tomographic images. A comparison between maps of mantle depletion and residual basement depth in the East Pacific suggests that anomalously high mantle depletion also correlates with anomalously deep basement close to present-day mid-ocean ridges. We attribute this to melt extraction-driven mass deficits in the upper mantle that have not yet been extensively replenished. Therefore there is a “top-down” effect on seafloor topography and crustal thickness driven by the plates themselves. The slower a ridge migrates and the faster it spreads the more severely the mantle becomes depleted, resulting in anomalously deep seafloor due to the effect of withdrawing asthenospheric material – an transient effect – and also possibly reducing crustal thickness. Large portions of the East Pacific region affected by excessive asthenospheric depletion correlate with anomalously thin crust based on a published Moho depth map. The supply of new asthenospheric material by mantle plumes acts to counterbalance this effect – hence we would not expect to see such depletion anomalies close to plumes, a hypothesis broadly supported by our results.