Lithosphere structure beneath the Phanerozoic intracratonic basins of North America

Abstract

Four intracratonic basins of North America, the Hudson Bay, Michigan, Illinois and Williston basins, have similar ages and are close to one another. Yet, they exhibit different subsidence histories characterised by different time-scales and sediment thicknesses. They can be explained by local lithosphere thinning and by the cooling of the induced thermal anomaly. Within the framework of 1D thermal models for vertical heat transport, each basin requires a different lithosphere thickness or a different boundary condition at the base of the lithosphere. Heat flow and seismic studies show that, beneath the North American craton, the lithosphere is too thick for the assumption of purely vertical heat transfer to be valid. Thermal models are developed to account for finite thermal anomaly width and for two types of basal boundary conditions, fixed temperature or fixed heat flux. Different subsidence histories are explained by deep lithospheric anomalies of different sizes. The stability of thick continental roots requires the mantle part of the lithosphere to be compositionally buoyant with respect to ‘normal’ convecting mantle. Localised lithospheric thinning, due for example to plume penetration, results in the emplacement of compositionally denser mantle into the lithosphere. This represents a load which drives permanent flexure. The cooling time and the characteristics of flexure allow constraints on the dimensions of these deep lithospheric anomalies. There are no solutions for lithosphere thicknesses less than 170 km. The Williston and Illinois basins are associated with wide (∼200 km) and thin anomalies (∼100 km), whereas the Michigan and Hudson Bay are located on top of narrow (∼100 km) and tall (∼200 km) anomalies.