Thermal Contraction and Flexure of Mid-Continent and Atlantic Marginal Basins

Abstract

Summary Thermal contraction of the lithosphere is a probable cause of the gradual subsidence indicated by sediments of mid-continent basins and Atlantic continental shelves. The subsidence is complicated by time dependent regional isostatic compensation since adjacent parts of the lithosphere are mechanically coupled, and since creep in the lithosphere may relieve accumulated stress. Thus, if more subsidence occurs at point A than nearby point B, point A would be buoyed up and point B dragged down. Relaxation of this coupling during a later period of more gradual subsidence would produce uplift at B and downwarp at A. The absence of younger beds over local minima of subsidence such as the Florida arch, the flanks of the Michigan basin, and the Atlantic coastal plain (USA) can thus be explained. Variations in the subsidence rate due to exponential decay of the thermal anomaly or to starved basin-evaporite depositional sequences can produce observable effects. Analytic models of the Michigan basin and the Atlantic coast (USA) are compatible with previously estimated parameters: thermal decay time of the lithosphere, 50 My; flexural parameter of the lithosphere beneath air, 200 km; and viscosity of the lithosphere, 1025 poise. The effects of flexure are not clearly evident in Silurian evaporite deposition in the Michigan basin and it is probable that an extended time was required for the evaporite sequence to accumulate. The cause of the thermal heating event which precedes subsidence is unclear for mid-continent basins although bulk replacement of the uppermost mantle is necessary. The heating events may be associated with periods of slow sea-floor spreading (when slabs exert a tensional force on the lithosphere) and hence low eustatic sea level. There is little direct evidence that an initial heating event actually occurred in the Michigan basin immediately before the start of subsidence.