Episodicity in foredeep basins

Abstract

The stratigraphic record contained in several foredeep basins is incompatible with the formation of these basins by flexure of a homogeneously strong foreland lithosphere. For example, the stratigraphy of the Early Proterozoic Kilohigok foredeep basin of northern Canada indicates that the inner part of the basin deepened in time without migration of the flexural bulge toward the foreland, and that the flexural bulge remained fixed in time relative to the downgoing plate. One mechanism that may be responsible for the localization of the flexural bulge in the Kilohigok and similar basins is that of lateral strength variations in the flexing lithosphere. Our results show that lateral strength variations within the foreland lithosphere can produce temporal variations in basin geometry and also serve to localize the position of the flexural bulge relative to the foreland for extended periods of time. This produces basin episodicity on a time scale of millions to tens of millions of years. Perfectly elastic models require a large strength contrast (about 50% in elastic plate thickness), but a brittle-elastic-ductile lithosphere rheology reduces the magnitude of the strength contrast required for episodicity to about 10%-15% of the initial effective elastic plate thickness. We suggest that basin episodicity may induce comparable episodicity in the coeval thrust belt, because theoretical studies of thrust mechanics suggest that the deformation in thrust belts is highly sensitive to the dip of the basal thrust angle. Through mechanisms such as this, the laterally heterogeneous continental lithosphere may provide a natural filter that transforms the presumably continuous and smoothly varying processes of the mantle into the spatially irregular and temporally episodic deformation observed in the crust at time scales of tens of millions of years.