Seismically observable features of mature stagnant-lid convection at the base of the lithosphere: Some scaling relationships

Abstract

Small-scale convection transfers heat from the asthenosphere into the base of the lithosphere. Scaling constraints are obtained on features associated with such convection that may be resolved by seismic studies. Compact notation arises by simply representing rheology: viscosity changes by a factor of e with a temperature increase of Tη and the minimum viscosity in the asthenosphere is η0. The geothermal gradient in the lithosphere and the conductive heat flow are constrained by xenolith, seismic, and heat flow studies. Geoid studies constrain the viscosity ηUM in the deep asthenosphere to ∼1020 Pa s. Seismic studies have resolved cool downwellings at the edge of the Colorado Plateau. The total anomaly of these downwellings (temperature contrast times width) depends on the square root of the ambient viscosity at depth. The observed anomalies are reasonable if partial melt causes seismic velocity to vary rapidly with temperature. Convection produces scalloped relief on the lithosphere-asthenosphere boundary of ∼20 km beneath stable continents if Tη is small, less than 65 K. Shear associated with plate motions aligns convection cells. Convection is likely to strongly align cells if vigorous convection in fact exists beneath the Pacific plate. Modest alignment is predicted beneath slowly moving ocean and continental plates. Seismic studies have yet to resolve scalloping and cell alignment.