CRUSTAL STRUCTURE BENEATH THE EASTERN SWISS ALPS DERIVED FROM SEISMIC-REFRACTION DATA

The eastern part of the Swiss Central Alps is densely covered by a net work of seismic refraction and wide-angle reflection profiles recorded mostly parallel to the tectonic strike of the Alps and along the newl y acquired N-S-oriented European Geotraverse (EGT). To obtain a well-c onstrained crustal transect along the EGT an initial model was constru cted from information based mainly on the coincident reflection seismi c profile of the Swiss National Research Program NFP20 and short-range observations of the EGT data for the shallow structure and on the alo ng-strike wide-angle profiles for the deep structure. The refinement o f the initial model by 2-D raytracing during the subsequent interpreta tion of the EGT data leads to a detailed P-wave velocity distribution of the crustal cross-section beneath the Central Swiss Alps and its ad jacent areas. In general, the distinctly layered crustal structure bel ow the Alpine foreland thickens considerably as the Alps are approache d, reaching a maximum thickness of nearly 60 km below the Insubric Lin e. 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 veloc ity of 6.5-6.6 km/s is found. Below the Penninic nappes the lower crus t 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 Af rican plate. A clear vertical offset between the smoothly south-dippin g European and the more rapidly rising Adriatic crust-mantle boundarie s is found. The complex structures of the upper crust beneath the Alps caused by the nappe tectonics can only be partly resolved by the refr action seismic data. A south-dipping high-velocity zone within the Pen ninic nappe pile and a reflector beneath the northern front of the Aar Massif can possibly be interpreted as incomplete images of the shallo w heterogeneous 3-D structure.