THE 3D CRUSTAL STRUCTURE OF THE ALPS OF EASTERN SWITZERLAND AND WESTERN AUSTRIA INTERPRETED FROM A NETWORK OF DEEP-SEISMIC PROFILES

A 3D crustal model of the Alps of eastern Switzerland and western Austria was constructed. A total of twenty two-dimensional deep-seismic reflection profiles recorded between 1986 and 1991 were two-dimensionally interpreted based on depth-migrated line drawings and using correlation with surface geology, refraction-derived velocity-depth models, and correlation with intersecting or neighboring profiles. The network of crustal cross sections was then unified into 3D surfaces. This analysis was restricted to four crustal discontinuities: (1) the top of the European basement (=first European basement unit including the foreland encountered going downward from the topographic surface); (2) the top of the lower crust; (3) the top of the European Moho transition zone; and (4) the European Moho. Simulation of the seismic profiles using 3D normal-incidence and shot ray-tracing enabled the quantification and correction of distortions caused by out-of-plane reflected energy. The inferred 3D crustal configuration is likely the result of the latest phase of Alpine shortening during the late Tertiary. It depicts a smooth SSE-dipping European Moho overlain by a concordant lower crust in the external Alpine domain and a presumably wedge-shaped body of lower crust in the internal Alpine domain. In contrast to the lower crustal discontinuities, the top of the European basement reveals a highly structured 3D geometry dominated by thrust tectonics. The pronounced structural disharmony with respect to the lower crust implies an intracrustal decoupling mechanism during the formation of the external basement uplifts, with the zone of decoupling possibly located near the top of the lower crust. Comparison of the top-basement topography with the mapped surface geology indicates a close relation between the basement geometry and the shape of the Austroalpine basal thrust delineating the Prattigau halfwindow in the west and the Engadine window in the east.