The effective elastic thickness of the lithosphere and the evolution of foreland basins

The elastic thickness of the continental lithosphere is strongly 'bi-modal'. Foreland basins reflect this bi-modality with narrow, deep basins (e.g. Apennines) and wide, shallow ones (e.g. Ganges). The bi-modal distribution cannot be explained by thermal models which describe the relationship between elastic thickness and plate age in the oceans, suggesting the involvement of factors other than secular cooling. The high values (80-90 km) are consistent with present-day temperature gradients of the cratons and the scatter within cratons may be explained by changes in the radiogenic heat production. The low values (10-20 km), however, are more difficult to explain. Foreland basins develop by flexure in front of thrust/fold loads as they advance over former passive margins and onto the cratons. Recent studies suggest that passive margins are underlain by highly attenuated crust and lithosphere which has a low elastic thickness that remains low for long periods (> 108 yr) of time. Foreland basins may inherit these low values as they migrate over a passive margin. Stratigraphic modelling suggests that the low elastic thicknesses would have a profound effect on the development of foreland basins predicting as they do the asymmetry, the pattern of onlap and, the transition from the 'underfilled' to 'overfilled' phase. Why stretched crust and lithosphere is so weak on long time-scales is enigmatic. Rifting, however, seems to proceed in such a way that the strong uppermost part of the crust is effectively 'de-coupled' from any support that it might otherwise receive from the strong underlying mantle