OBSERVATIONAL HINTS FOR A PLUME-FED, SUBOCEANIC ASTHENOSPHERE AND ITS ROLE IN MANTLE CONVECTION

An asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection. In this study we discuss geochemical, seismic, and geoid/depth evidence for lateral flow within this type of asthenosphere. In particular, we suggest that there are large-scale layered, horizontal flow structures that connect upward plume input beneath hotspots to near-ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere. Lateral asthenosphere flow is also shaped by oceanic subduction zones, with a partial return flow from trenches, and by deep continental roots that are migrating barriers to asthenosphere flow. This alternative paradigm offers relatively simple explanations for several puzzles about mantle convection, for example, the low mantle heat flow beneath continents. It also offers an explanation for why mid-ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large-scale mantle flow, while in contrast, trenches appear to be strongly coupled to mantle-thick regions of fast (colder) seismic velocity anomalies. We also discuss several implications of this paradigm that should be testable in future studies, such as the prediction of cogenetic off-axis seamount volcanism that is created between an off-axis hotspot and its neighboring ridge axis.