Lithosphere structure beneath the Phanerozoic intracratonic basins of North America

Four intracratonic basins of North America, the Hudson Bay, Michigan, Illin ois and Williston basins, have similar ages and are close to one another. Y et, they exhibit different subsidence histories characterised by different time-scales and sediment thicknesses. They can be explained by local lithos phere thinning and by the cooling of the induced thermal anomaly. Within th e framework of 1D thermal models for vertical heat transport, each basin re quires a different lithosphere thickness or a different boundary condition at the base of the lithosphere. Heat flow and seismic studies show that, be neath the North American craton, the lithosphere is too thick for the assum ption of purely vertical heat transfer to be valid. Thermal models are deve loped to account for finite thermal anomaly width and for two types of basa l boundary conditions, fixed temperature or fixed heat flux. Different subs idence 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' con vecting mantle. Localised lithospheric thinning, due for example to plume p enetration, results in the emplacement of compositionally denser mantle int o the lithosphere. This represents a load which drives permanent flexure. T he cooling time and the characteristics of flexure allow constraints on the dimensions of these deep lithospheric anomalies. There are no solutions fo r lithosphere thicknesses less than 170 km. The Williston and Illinois basi ns are associated with wide (similar to 200 km) and thin anomalies (similar to 100 km), whereas the Michigan and Hudson Bay are located on top of narr ow (similar to 100 km) and tall (similar to 200 km) anomalies. (C) 2000 Els evier Science B.V. All rights reserved.