The variation of crustal thickness across the Swiss Alps based on gravity and explosion seismic data

Summary Recently determined gravity anomalies along the NW-SE oriented Swiss Geotraverse from Basel to Bellinzona are used in combination with seismic refraction data to deduce a crustal section across the Swiss Alps. Topographic, Bouguer, free air, isostatic and geological corrections were applied to the data. Geological features considered in the corrections are the Swiss Molasse basin filled with sediments and the Ivrea body of high-density material. The resultant Bouguer anomaly over the Gotthard massif is 130 mgal lower than the Bouguer anomaly at the northern end of the profile near Basel. The Alpine region is associated with negative isostatic anomalies down to –20 mgal. The crustal thickness is found to increase gradually from the northern end of the profile (thicknessH=30 km) towards the Helvetic nappes at the northern margin of the Alps (H=38 km) and more rapidly towards the Gotthard massif (H=50 km) and further south to Biasca down to a depth of 58 km. From Biasca southward the crustal thickness thins quite rapidly to reach a depth of 30 km at the southern end of the profile near Bellinzona. Thus the Alps have a distinct asymmetric crustal root whose maximum thickness is almost twice the average crustal thickness in Central Europe. With the Mohorovii-discontinuity deduced from seismic observations an average constant density contrast of –0.33 gcm^–3 is found between the lower crust and upper mantle underneath the Alps.Institut für Geophysik, ETH Zürich, Contribution No. 130.