Thermal contraction and flexure of intracratonal basins: a three‐dimensional study of the Michigan basin

Summary. Cooling of continental lithosphere by thermal conduction to the surface following a heating event imposes a load on the lithosphere and causes subsidence as the basement rocks contract. Accumulation of sediments in the resulting depression forms a sedimentary basin. Studies of the geometry of sedimentary basins with horizontal dimensions of a few hundred kilometres suggests the lithosphere responds to loads by regional flexure of a strong elastic or viscoelastic upper lithosphere. Two‐ and three‐dimensional models for flexure of the lithosphere owing to thermal contraction are applied to the Michigan basin. 2‐D flexural models are compatible with previously estimated rheological parameters. For an elastic lithosphere, an effective flexural rigidity of about 5 × 10 29 dyne cm can explain observed gravity anomalies and structural contours across the Michigan basin. A viscoelastic lithosphere requires an effective flexural rigidity of about 10 32 dyne cm for a viscosity of 10 25 poise. For a 3‐D elastic or viscoelastic lithosphere, effective flexural rigidity must be approximately an order of magnitude smaller: 2 × 10 28 and 10 31 dyne cm, respectively. Larger values leave an excessive remaining load at the centre of the basin, which is not indicated by observed gravity anomalies. The decrease in effective flexural rigidity is attributed to greater difficulty in equi‐dimensional bending of a 3‐D plate. The magnitude and spatial distribution of the computed thermal contraction load are consistent with the presumed mechanism. As the thermal anomaly of the initial heating event was present mainly at great depths in the lithosphere, the lack of geological evidence for an initial heating event prior to subsidence is not a fatal objection to the thermal contraction mechanism of subsidence. A driving load, presumably from subsurface processes including crustal stretching and emplacement of dense rocks into the crust, is necessary to explain the net subsidence of the basin from the time before the heating event until the lithosphere beneath the basin cooled. Without this driving load, the thermal expansion would produce uplift, but the surface would subside only back to sea‐level after the lithosphere cooled. Theoretical gravity results indicate that the driving load is centred at a depth of approximately 15 km. Deviations in subsidence curves from exponentials associated with thermal contraction can be explained by changes in sediment supply. Spatial variations in sediment load, caused by regional facies changes, produce migration of the centre of maximum deposition. Water and basement depths are determined for sequential time intervals during basin development. The predicted depositional environments are consistent with lithofacies maps of the Middle Devonian.