This study presents a new analytical modeling approach to estimate the magnitudes of regional thrust load−controlled flexural subsidence and dynamic subsidence related to mantle flow within a retro-foreland basin, thereby providing a prediction of the present-day basin geometry. The Western Canada Sedimentary Basin, with a wealth of well data and previous studies, was used to constrain the approach. In contrast to previous studies, this model uses laterally variable lithospheric flexural rigidities. Constant flexural rigidity cannot produce acceptable matches to observed basin geometries, and it is geologically unreasonable in settings extending across several crustal terranes and thermal regimes.
Dynamic subsidence estimates derived from this model agree with those based on mantle convection modeling of the subducting Farallon plate beneath North America. It is this dynamic component that led to regional accommodation more than 250 km beyond the thrust front in the Western Canada Sedimentary Basin, because the flexural component does not contribute to subsidence this far into the foreland. It follows then that initial subsidence in retro-foreland basins can be attributed to dynamic processes, and that the thrust load−related flexural component only contributes in areas within a few hundred kilometers of the thrust front, depending on the flexural rigidity of the lithosphere.
The modeling approach developed here can be used in other retro-foreland basins to estimate overall basin geometries where minimal data are available. This has direct implications for predicting the regional distribution of facies in poorly constrained basins, since basin geometry is one of the fundamental controls. Additionally, regional subsidence models can be subtracted from the observed basin geometries in well-constrained basins to yield estimates of local accommodation.