letters to nature NATURE | VOL 389 | 2 OCTOBER 1997 479 not corroborated this high value [16][17][18] . Quinlan and Beaumont 16 matched the stratigraphy of the Appalachian basin by using a layered visco-elastic model that had an average EET of 67 km. Although they note that a pure elastic plate model does not adequately reproduce the stratigraphy, elastic plate models in which the EET decreases with curvature would produce offlapping stratigraphic patterns as seen in the observed stratigraphy. Recently, Stewart and Watts 17 re-estimated the EET of several mountain belts by using a variable-rigidity formulation. Converting their estimates to the values of E and u used in this paper gives a range of 50-88 km for the EET. These studies suggest that the high value found by Karner and Watts 1 is an overestimate and that a lower value averaging ϳ65-70 km is a better estimate. If so, our prediction of 60 km is within uncertainties.We have established a parametrization of flexural strength at continents based on the yield stress envelope that successfully predicts the EET of the continental lithosphere at foreland basins and mountain belts. We have also demonstrated the importance of sediment fill as parameter controlling flexural strength at continents. The sediment cover is most likely to control the value of EET in places where the lithosphere's crust is thin compared with an average 35-km-thick continental crust 5 , the age of the lithosphere is close to its thermal equilibrium and for which the sediment cover reaches thicknesses greater than 3-5 km. Accounting for the effect of sediments and crustal thickness should facilitate the evaluation of the flexural strength at other types of basins and continental margins. Ⅺ
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