Subsidence of the Earth’s surface often results in the formation of sedimentary basins. We can explain the subsidence history of many sedimentary basins by essentially the same model that we used to understand the cooling, thickening, and subsidence of the oceanic lithosphere. The model is illustrated in Figure 4–49. Consider a region of the Earth that is hot, either because of seafloor spreading or extensive volcanism. Initially (t = 0) there is no sediment, and the basement has a temperature T1 and a density ρm.
Surface cooling causes subsidence as the basement rocks cool and contract. We assume that sediments fill the basin caused by the subsidence; that is, the region 0 < y < ySB. This assumption requires an adequate supply of sediment to prevent the formation of a deep ocean basin.
As long as the thickness of the sediment ySB is much smaller than the lithosphere thickness yL, we can carry over the results of the cooling oceanic lithosphere calculation. Therefore the depth of the sedimentary basin is given by Equation (4–209) with ρs, the density of the sediments, replacing ρw, and t, the age of the basin, replacing x/u0, the age of the oceanic lithosphere; the result is ySB = 2ρmαm(T1 − T0) (ρm − ρs) ( κmt π )1/2 , (4.220) where the subscripts m on α and κ emphasize that these properties refer to the mantle rocks and not the sediment (αm is αv for the mantle rocks). The depth of the sedimentary basin is proportional to the square root of time. The subsidence of the basin is caused primarily by the cooling of the basement or lithospheric rocks. The cooling of the sedimentary rocks is a negligible effect when ySB/yL ≪ 1.