Free thermal convection beneath intracratonic basins: thermal and subsidence effects

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Abstract
Two long‐standing discrepancies between observations and predictions from geophysical models for the evolution of intracratonic basins are: (1) thermal indicators, such as organic maturity, document higher basin temperatures than predicted by thermal conduction; and (2) periods of rapid/slow subsidence which deviate from the exponentially decreasing subsidence rate consistent with thermal contraction. A possible explanation for these problems is free thermal convection in the continental crust beneath the basin. Using a finite difference model for coupled fluid flow and heat transport, the Keweenawan rift beneath the Michigan basin is simulated as a plug of fractured igneous rock 10 km thick and 45 km wide. Overlying the igneous body and adjacent impermeable basement rocks in the model is 4 km of Proterozoic and Palaeozoic sediments of intermediate permeability. Model results indicate that during short‐lived (a few million years) periods of free thermal convection in the igneous body, temperature gradients in the overlying sediments can nearly double and rapid heat loss causes additional subsidence at the centre of the basin. Locally, additional tectonic subsidence can be more than 25 m. After fractures are sealed and free thermal convection slows or stops, basin subsidence is anomalously slow as the basement reheats back to conductive equilibrium.