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Technischer Bericht NTB 83-21

Geophysikalisches Untersuchungsprogramm Nordschweiz Regionale Refraktionsseismische Messungen

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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.

In the near-surface gneisses of the southern Black Forest the compressional wave velocities range from 2.2 km/s to 4.5 km/s. The velocity increases to about 5.0 km/s down to a depth of 300 m. It reaches values of about 5.5 km/s and 5.8 km/s at depths of 1 km and 2 km, respectively. At 3 km, which was the maximum depth reached, the velocity is still less than 6 km/s. No lateral changes or significant differences of velocity could be detected along or between the various profiles in the crystalline part of the Black Forest. This holds also for the surveyed gneissic and granitic areas.

The velocity - depth structure under the northeastern parts of the Swiss Jura and the adjacent parts of the Molasse basin is based on data from profiles which run from shotpoints near Villigen, the Hallwilersee and Kaisten in various directions. An average velocity of 3.7 to 3.8 km/ s was derived for the entire Mesozoic overburden. Underneath there is evidence for a heavily fractured transition zone in the crystalline basement with a gradual increase of velocity to about 5.2 km/s at a depth of 300 m below the base of the Mesozoic sediments. On the average a velocity of 6 km/s can be expected 2300 to 2500 m below this interface.

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 Mönthal 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.

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