Plan‐view curvature of foreland basins and its implications for the palaeostrength of the lithosphere underlying the western Alps

Abstract

In zones of continental collision, three methods can be used to calculate the strength of the underthrust lithosphere: (1) a forward model approach to the Bouguer gravity field, (2) an inverse model of the gravity and topography using admittance techniques, or (3) a forward model of the stratigraphic infill of the foreland basin to estimate the cross‐sectional profile of the downflexed plate. The use of reconstructed stratigraphy has the potential to yield values for the equivalent elastic thickness (Te) of the cratonic lithosphere at varying slices in geological time, and hence enable an insight into the longer term (10–50 Myr) mechanical behaviour of the continental lithosphere.Calculations of Te based on isopachs of foreland basin stratigraphy use sea level as a reference line to estimate the basement deflection, and therefore are limited to using stratigraphy which records shallow marine or coastal sedimentation. A new empirical approach is applied to evaluating ancient Te values using the reconstructed palaeocurvature of the basin in plan view. The radius of curvature of 12 curvilinear foreland basins is plotted against their documented Te values and shows a linear relationship. The maximum Te value for a given radius of curvature can also be plotted as a straight line. The palaeocurvature of reconstructed basins can then be compared with the plots, and estimates of likely maximum Te values may be obtained.During Eocene times, the underfilled foreland basin of the Alps was characterized on its cratonic edge by the deposition of Nummulite‐rich limestones. Palaeogeographical reconstructions of the Nummulitic Limestones enable estimates of the palaeocurvature of the cratonic margin of the Alpine foreland basin during the Eocene. By comparing this value with the curvature of documented basins, it is possible to suggest that the European lithosphere underlying the western Alps had an effective elastic thickness of no greater than 17 km during the Eocene. It has been suggested that the transition in the depositional state of the Alpine foreland basin from an underfilled to a filled state during middle Oligocene times was linked to a thickening of the continental lithosphere associated with the effective ramp of the Tethyan passive margin. The Te value of less than 17 km during the underfilled stage combined with a value of 10±5 km for the later filled stage at 17 Ma does not lend support to this hypothesis.