Abstract

Foreland basins are linked flexurally through the lithosphere to their adjacent overthrust belts. Consequently, their stratigraphy records the temporal and spatial emplacement of tectonic loads. A quantitative, geodynamic model for continental margin evolution is constrained with the observed stratigraphy of the Alberta and Molasse basins to estimate the tectonic loading histories across the southern Canadian Cordillera and the Swiss Alps, respectively.

Our model follows margin evolution from rifting, through thermal decay and passive margin development, to eventual active margin emplacement of overthrusts. Because we explicitly include the configuration of the passive margin prior to overthrust emplacement, we can predict the topography of the overthrust wedge through time. In each of the Cordilleran and Alpine cases we infer a period of very rapid tectonic loading. In the Cordillera, this corresponds with the development of the Purcell Anticlinorium located near the inherited passive margin hingeline. In the Alps, this period preceeds the development of the external basement massifs by approximately 10 My.

The incorporation of pre-existing passive margin bathymetry enables emplacement of large tectonic loads with little or no topographic expression. This eliminates the apparent need for a “sub-surface load” hypothesized by other workers.

Computed terrain-corrected gravity anomalies across the models suggest that the low-high Bouguer anomaly couple commonly observed across orogens is in part a signature from the tectonically overridden and downwarped, pre-existing passive margin. The inheritance of pre-existing structure by an overthrust belt is therefore reflected in the gravity data as well as the topography.