Abstract
Thirty-eight heat-flow measurements were carried out in 17 perialpine lakes to supplement the scanty information concerning the thermal conditions in the crust underlying central Europe. A probe of the corer type was built which allowed the measurement of 11 temperatures at equally spaced subbottom intervals to a maximum penetration of 10.5 m. Measurements of this type were made in Lake Constance, Greifensee, Lake Zurich, Lake Zug, Lake Lucerne, Baldeggersee, Lake Biel, Lake Murten, Lake Neuchâtel, Lake Geneva, Lac d'Annecy, Lac de Bourget, Lago Maggiore, Lago di Lugano, Lago di Como, Lago d'Iseo, and Lago di Garda.
After retrieval of the probe, a sediment core was collected for samples on which thermal conductivity was measured with the needle probe method after treating the material to remove exsolved gas pubbles. Continuous seismic-reflection and reflection profiles in all of these lakes provided a set of parameters for gradient correction due to heat-flow refraction. Additional corrections were applied on the basis of palynologically determined sedimentation rates and for the influence of the last period of glaciation. Final heat-flow values obtained in this manner are given. The lesser values are generally found in the more western lakes, north of the Alps; the higher values are found in the more eastern lakes, north and south of the Alps. The higher values are partially confirmed by earlier measurements in lakes but show considerable discrepancy with measurements from the central Alps. This could be explained by the underthrusting of a cold wedge under the Aar and Gotthard Massif in the late Miocene and early Pliocene, which then would lower the temperature gradient in the central Alps.
