PLAN-VIEW CURVATURE OF FORELAND BASINS AND ITS IMPLICATIONS FOR THE PALAEOSTRENGTH OF THE LITHOSPHERE UNDERLYING THE WESTERN ALPS

In zones of continental collision, three methods can be used to calcul ate the strength of the underthrust lithosphere: (1) a forward model a pproach to the Bouguer gravity field, (2) an inverse model of the grav ity and topography using admittance techniques, or (3) a forward model of the stratigraphic infill of the foreland basin to estimate the cro ss-sectional profile of the downflexed plate. The use of reconstructed stratigraphy has the potential to yield values for the equivalent ela stic thickness (T-e) 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. Calculati ons of T-e based on isopachs of foreland basin stratigraphy use sea le vel as a reference line to estimate the basement deflection, and there fore are limited to using stratigraphy which records shallow marine or coastal sedimentation. A new empirical approach is applied to evaluat ing ancient T-e 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 T-e values and shows a lin ear relationship. The maximum T-e value for a given radius of curvatur e can also be plotted as a straight line. The palaeocurvature of recon structed basins can then be compared with the plots, and estimates of likely maximum T-e values may be obtained. During Eocene times, the un derfilled foreland basin of the Alps was characterized on its cratonic edge by the deposition of Nummulite-rich limestones. Palaeogeographic al reconstructions of the Nummulitic Limestones enable estimates of th e palaeocurvature of the cratonic margin of the Alpine foreland basin during the Eocene. By comparing this value with the curvature of docum ented basins, it is possible to suggest that the European lithosphere underlying the western Alps had an effective elastic thickness of no g reater than 17 km during the Eocene. It has been suggested that the tr ansition in the depositional state of the Alpine foreland basin from a n underfilled to a filled state during middle Oligocene times was link ed to a thickening of the continental lithosphere associated with the effective ramp of the Tethyan passive margin. The T-e value of less th an 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.