Geological and Geophysical Parameters of Mid-Plate Volcanism [and Discussion]

Although volcanism occurs mainly at oceanic ridges and subduction zones, it also takes place within continental plates. Mid-plate Cainozoic volcanism is widespread in Africa where it is alkaline in composition, is characteristically associated with uplift and faulting, and is almost completely restricted to non-cratonic areas. Cratons in Africa are characterized by low elevations, heatflow less than 45 mW m$^{-2}$, lithospheric thicknesses greater than 200 km and higher than average seismic velocities. In contrast, non-cratonic regions stand higher and have greater heatflow and lower lithospheric thicknesses and seismic velocities. There is a clear time correlation between the pause at ca. 45 Ma in the African apparent polar wander path and the outbreak of volcanism at ca. 35 Ma. Accepting that sub-lithospheric heat input is regionally variable, geophysical modelling indicates that the spatial and temporal distribution of uplift-associated mid-plate volcanism can be almost entirely explained in terms of plate thickness and velocity. We examine, quantitatively, conductive models that determine the thermal disturbances in the lithosphere explicitly in terms of perturbation strength, plate thickness and plate velocity. For example, for a plate over 200 km thick, a relatively modest movement (> 2 cm a$^{-1}$) will suppress the upward propagation of most sub-lithospheric thermal anomalies, thereby precluding mid-plate volcanism. Where such thermal anomalies are confined to the base of the lithosphere, uplift alone, without surface volcanism, would result. The thinning of the lithosphere predicted by the theoretical models can be accomplished by deep, strong thermal perturbations or by upward migrating perturbations of lesser strength. The latter is consistent with configurations for the East African-Ethiopian lithothermal systems derived from geophysical and geochemical data. Mesozoic volcanism of Gondwanaland was more widespread and voluminous, and erupted through both cratons and non-cratons, thereby suggesting that the causative thermal anomalies were much more vigorous than those in the Cainozoic.