Constraints on the deep structure and dynamic processes beneath the Alps and adjacent regions from an analysis of gravity anomalies

Gravity anomalies, including terrain corrections, over the western Alps and the Molasse Basin are within 30 mgal of those calculated assuming local Airy isostatic equilibrium, but isostatic anomalies over regions NW and SE of the Alps exceed 50 mgal. Therefore, mass excesses and deficits that must be supported by the strength of the lithosphere or by dynamically induced stress in the asthenosphere appear to be greater beneath the areas surrounding the Alps than beneath the Alpine chain itself. Attempts to account for the gravity gradient over the Molasse Basin and sub-Alpine chains and the small Bouguer anomalies measured over the elevated areas of the Vosges and Black Forest by the flexure of an elastic plate require an absurd set of parameters. The high elevations of the Vosges and the Black Forest are presumably due to high temperature in the underlying uppermost mantle, but the deviations from isostatic equilibrium, as well as the high temperature, probably result from active upwelling in the asthenosphere beneath this area. The absence of large isostatic anomalies over the Alps and the failure of an elastic model to account for the gravity anomalies over the Molasse Basin suggest that the dynamic processes that flexed the European plate down to form the Molasse Basin and that built the Alpine chain have waned. The late Cenozoic uplift of the Molasse Basin and the Alps might be a consequence of a diminution or termination of downwelling of mantle material beneath the Alps.