Geophysical investigation programme Northern Switzerland. Regional seismic refraction measurements

In the framework of the geophysical survey of Switzerland the depth to the basement under the northeastern parts of the Swiss Jura mountains was determined by means of the seismic refraction method. This included an assessment of the seismic velocities within this crystalline basement at depth as well as for its outcrop in the nearby southern part of the Black Forest. The Swiss Geophysical Commission (SGPK) and the National Cooperative for the Storage of Radioactive Waste (NAGRA) were joint sponsors of this survey. The Institute of Geophysics of the Swiss Federal Institute of Technology in Zurich (ETH-Z) organized the fieldwork and performed the interpretation of the data. On a contour map covering the area of the northeastern Jura and the adjacent fringe of the Molasse basin the basement dips down to the south-southeast under the Mosozoic and Cenozoic sediments. This dip is more pronounced under the tabular Jura mountains and east of the Aare river than further south where the relief flattens out considerably. Pronounced east-west striking variations overlie the general southward oriented slope. Two small scale troughs seem to exist near Moenthal and Frick west of the Aare valley. To the east of the Aare valley the crystalline basement forms a regional trough with a maximum depth of 2100 m below mean sea level. This unexpectedly large trough is likely to be filled with Permo-Carboniferous sediments. The eastern ranges of the folded Jura mountains again are underlain by a depression in the basement which is much less marked, however. To the south of this area, between Wildegg and Mellingen, a relative high of 200 m is superimposed on the generally SSE dipping basement. The unexpected low velocities of the upper basement indicate severe weathering. Possibly the uppermost rocks are altered to such a degree that the location of the top of the basement is relatively inaccurate