Sedimentary Basins

Some sedimentary basins are bounded by near-vertical faults along which the subsidence has occurred. Others, however, have a smooth basement, and the subsidence is associated with the flexure of the elastic lithosphere. The horizontal dimensions of these sedimentary basins, about 400 to 1000 km, reflect the magnitude of the flexural parameter based on sediments of density ρs replacing mantle rock of density ρm, α = [4D/(ρm − ρs)g] 1/4.

Some sedimentary basins have a nearly twodimensional structure. They are caused by the loading of a linear mountain belt and are known as foreland basins. Examples are the series of sedimentary basins lying east of the Andes in South America and the Appalachian basin in the eastern United States lying west of the Appalachian Mountains. Depth contours of the basement beneath the Appalachian basin are given in Figure 3–36a. A basement profile is shown in Figure 3–36b. The depth w is the depth below sea level, and the coordinate −x is measured from the point where basement rocks are exposed at the surface. It is appropriate to model the structure of the Appalachian basin as a two-dimensional lithospheric plate under a linear end load. Thus the universal flexure profile given in Equation (3–159) is directly applicable. In order to fit the basement profile given in Figure 3–36b we take xb = 122 km and wb = 290 m (x0 = 0). Since the forebulge has been destroyed by erosion this choice of parameters is not unique. They can be varied somewhat, and a reasonable fit can still be obtained. However, these values are near the center of the acceptable range. From Equation (3–156) we find that they correspond to α = 155 km. As we have already noted, the flexural rigidity must be based on the density difference between the mantle and the sediments ρm − ρs. With ρm − ρs = 700 kg m−3 and g = 10 m s−2 we find D = 1024 N m. From Equation (3–72) with E = 70 GPa and ν = 0.25 we find that the thickness of the elastic continental lithosphere is h = 54 km. This is somewhat larger than the values we obtained for the thickness of the elastic oceanic lithosphere. Flexure studies of other sedimentary basins give similar values of elastic thickness.