J. Ansorge

Crustal structure beneath the eastern Swiss Alps derived from seismic refraction data

Abstract The eastern part of the Swiss Central Alps is densely covered by a network of seismic refraction and wide-angle reflection profiles recorded mostly parallel to the tectonic strike of the Alps and along the newly acquired N-S-oriented European Geotraverse (EGT). To obtain a well-constrained crustal transect along the EGT an initial model was constructed from information based mainly on the coincident reflection seismic profile of the Swiss National Research Program NFP20 and short-range observations of the EGT data for the shallow structure and on the along-strike wide-angle profiles for the deep structure. The refinement of the initial model by 2-D raytracing during the subsequent interpretation of the EGT data leads to a detailed P-wave velocity distribution of the crustal cross-section beneath the Central Swiss Alps and its adjacent areas. In general, the distinctly layered crustal structure below the Alpine foreland thickens considerably as the Alps are approached, reaching a maximum thickness of nearly 60 km below the Insubric Line. The upper and middle crust has velocities between 6.0 and 6.2 km/s. Except for the area below the southern part of the Molasse Basin and the Helvetic nappes a distinct lower crust with a relatively low velocity of 6.5–6.6 km/s is found. Below the Penninic nappes the lower crust thickens remarkably, merging probably with the high-velocity zone of 6.6 km/s at a depth of about 21 km, which has been interpreted as the top of the indenting lower crust of the Adriatic promontory of the African plate. A clear vertical offset between the smoothly south-dipping European and the more rapidly rising Adriatic crust-mantle boundaries is found. The complex structures of the upper crust beneath the Alps caused by the nappe tectonics can only be partly resolved by the refraction seismic data. A south-dipping high-velocity zone within the Penninic nappe pile and a reflector beneath the northern front of the Aar Massif can possibly be interpreted as incomplete images of the shallow heterogeneous 3-D structure.

Crustal structure beneath the Rhinegraben from seismic refraction and reflection measurements

Abstract A model of the crustal structures beneath the Rhinegraben rift system has been derived based on results of seismic refraction and reflection measurements. Observations along two lines parallel to the strike of the graben and along two overlapping profiles traversing the rift normal to its axis provided the necessary data. They were supplemented by results of deep seismic reflection experiments carried out within the graben and in its adjacent regions. Important findings are the existence of two velocity inversions within the crust and of a characteristic “rift cushion” with a compressional velocity of 7.6–7.7 km/sec and of a correspondingly high shear velocity. This body is considered to be the driving mechanism of the rifting process. Immediately below the pronounced low-velocity channel in the upper crust an intermediate layer with lamellar structure and of presumably basic composition has been found. The most conspicuous feature of the crustal cross-section is its asymmetry. It seems as if the rifting process must have started at the eastern margin of the graben. Complete bilateral symmetry of the structure has not yet been achieved.

Important findings expected from Europe's largest seismic array

An international, interdisciplinary project, which 2 years ago deployed the largest dense seismic antenna ever in Europe, expects in the next 2 years to present important findings on the lithosphere and asthenosphere of a portion of the Trans-European Suture Zone (TESZ). Final processing is currently under way of the data from the array of 120 seismographs along a 900-km-long by 100-km-wide strip from Gottingen, Germany, in the south, through Denmark, to Stockholm, Sweden in the north, across the northwestern part of the TESZ (Figure 1). Project Tor is a teleseismic tomography experiment with interdisciplinary data exploitation. It extends across the broad TESZ boundary between two markedly different lithospheric domains.These are (1) Proterozoic Europe, with Precambrian crust in Sweden and eastern Europe, and (2) Phanerozoic central Europe, with most of the crust influenced by the Caledonian and Variscan orogenies and only small areas of relic Precambrian crust. The project is designed to investigate the deep lithosphere traces of the broad-scale geology of the TESZ area, including the Tornquist Zone, from which Project Tor has its name. It is part of EUROPROBE, a major Earth science program of the European Science Foundation, which is run by a regional committee of the International Lithosphere Program.

Deep crustal reflections from a Vibroseis® survey in northern Switzerland☆

Abstract In 1982 a Vibroseis ® survey comprising 180 km of reflection profiles was run in northern Switzerland in order to investigate the suitability of the crystalline basement for the deposition of highly radioactive waste. A configuration was chosen with 144 channels, 25 m of geophone spacing, 20 s sweeps ranging from 11 to 61 Hz and stacking of 4 or 8 sweeps of 3 simultaneous vibrators at twice the geophone spacing. The listening time was generally 4 s and at 4 sites it was extended to 11s for the detection of deeper crustal reflectors. This survey unravelled the complicated fault and thrust system beneath the Swiss folded Jura mountains. The stack from 4 s to 11 s reveals clearly a strong sloping reflector between 3.0 and 3.5 s which is strong evidence for a pronounced differentiation in the upper crust. A series of reflections is observed between 5.8 and 7.2 s the top of which can be correlated with the Conrad discontinuity. A strong “layered” signal between 9.0 and 9.5 s is interpreted as reflections from the M-discontinuity. The main features are compatible with results from nearby refraction surveys in the southern Rhinegraben rift system which show a distinct velocity increase of about 0.5 km/s in the lower crust at a depth ranging from 15 to 20 km, followed by an inversion zone or a laminated structure before reaching the Moho at about 27 km depth. The correlation of the field recordings with the first 10 s of the up-sweep only, shows some loss of resolution in the uppermost 3 s because of the lower frequency content of the signal. However, the lower parts of the sections are nearly identical. The fact that the deeper reflectors in the sections can consistently be traced laterally is a strong argument for using this processing technique. Thus high-coverage Vibroseis surveys utilizing up-sweep can be processed for deep crustal reflections even if the recording time is restricted to the standard 4 s, provided the surface static corrections are carried out with high precision.

Deep seismic sounding experiments in Spain

Abstract Since 1974, under the Geodynamic Project and with the collaboration of institutions of several countries, a long-range program of deep seismic sounding experiments has been carried out in Spain. In the first three years of the program the following profiles have been made: 1974—Cartagena—Cadiz, Alboran Sea (two N-S and one E-W); 1975—Adra—Cartagena, Adra—Ubeda, Lanzarote (Canary Islands); 1976—Balearic Islands, Pyrenees and the Iberic system. For 1977 complementary lines are programmed in the Betica region, Alboran Sea and Canary Islands. Interpretation of the data has resulted in some preliminary models of the crust under South Spain and the Alboran Sea. Strong dips and lateral variations of the structures are present. Thickness of the crust varies from 16 km in the Alboran Sea to 38 km under the Sierra Nevada.