Foreland basins

Abstract

The characteristic pattern of the subsidence, deformation, uplift and erosion of foreland basins, or exogeosynclines, has long been recognized by geologists as an integral part of the geosynclinal cycle yet only descriptive models have been proposed. This evolution is quantified in terms of a model of regional isostatic adjustment of the lithosphere under the mass load of the adjacent migrating fold-thrust mountain belt. It is shown that the foredeep, which received the clastic wedge of detritus from the core of the orogen and the fold-thrust belt, owes its existence to downward flexure of the lithosphere by the fold-thrust belt. Additional depression occurs in response to the infilling sediment. The scale of the basin therefore reflects the degree of shortening in the orogen and the rheological properties of the underlying lithosphere. Subsidence reflects mass that accretes at a faster rate than mass wasting from the orogen, whereas regional uplift and erosion of the basin is attributed to the large-scale erosion of the fold-thrust belt that has been depressing the lithosphere. There is a complete coupling between the evolution of the foreland basin and its adjacent mountain belt through the lithospheric flexure. Lateral migration of the fold-thrust belt accounts for the progressive overriding and disruption of some foreland basins. The proposed model is tested by comparison with the structure of sections across the Alberta Foreland Basin of Western Canada. It is shown that a model in which mass loads continuously advance on to the craton in a series of pulses between the Upper Jurassic and the Eocene is consistent with observations. No successful models were found when the lithosphere was modelled as a uniform thin elastic plate. However, when the plate was allowed to relax stress according to a viscoelastic (Maxwell) rheology good agreement between observed and theoretical cross-sections was obtained. These results suggest that suitable values for the flexural rigidity, D, and relaxation time constant, τ, are 1025 Nm and 27.5 Myr. Models with significantly different D-τ parameter values failed to predict satisfactory results. Predictions of basin erosion, coal and shale compaction, and thermal metamorphism of coal are shown to be in agreement with observations. A significant result is that no change in geothermal gradient, with respect to that presently observed, is needed to explain coalification. This result confirms the view that subsidence of the basin was not thermally controlled, unlike Atlantic type marginal basins, and explains the high value of the flexural rigidity as that appropriate to an old, cool, thick lithospheric plate.