LATERAL FLOW OF HOT PLUME MATERIAL PONDED AT SUBLITHOSPHERIC DEPTHS

Local buoyancy forces drive the flow of ponded plume material along th e base of the lithosphere. This processes complicates the geological e ffects of plumes beyond those of simple passage of plates over point h eat sources. Variations in the depth to the base of the lithosphere si gnificantly affect lateral flow. The flow rate is modified to a modera te extent by locally thin lithosphere beneath midplate hotspots, such as Hawaii. The effect is relevant when numerical models are rescaled t o better fit the observed uplift and the observed distance from the ho tspot to the nose of the swell. Variations in the depth to the base of the lithosphere have a much stronger effect on flow near ridge axes, effectively forming an upside-down drainage pattern. Plume material fl ows upslope toward ridge axes from off-axis plumes and dong ridge axes from on-axis plumes at sufficiently low spreading rates. Thinner lith osphere exists above plume material than above normal asthenosphere. T he edges of normal lithosphere adjacent to plume material act to some extent as levees which limit the lateral spreading of plume material. Pressure release melt is associated with the upslope flow of plume mat erial. It is expected to cause volcanism away from the plume and to so metimes supply melt-depleted plume material td ridge axes that are ups lope from the region of voluminous melting.