Processing and preliminary results of NFP/PNR20 seismic reflection profiles from the Western Swiss Alps

Abstract

Results of near-vertical seismic reflection profiling conducted in the western Swiss Alps are presented. All profiles were recorded using both high-fold Vibroseis and high-energy but low-fold dynamite sources, yielding two independent data sets. The Vibroseis seismic sections provide markedly superior resolution of upper crustal units down to the top of the crystalline basement. On these sections, high-impedance contrasts within the nappes are the source of strong reflections, and the basal thrust of the Penninic nappes (Penninic front) delineates a lateral change in reflection character. By comparison, the dynamite sections provide generally higher-quality images of the deep crust. In particular, they show prominent bands of strong reflections in the lower crust and a well-defined reflection Moho deepening from ~ 12 s (about 36 km) in the north to ~ 16 s (about 48 km) in the south. It is concluded that both types of data are required to resolve accurately shallow and deep structures in the Swiss Alps. Preliminary results of the dynamite survey from the most recently recorded line support proposals that shortening of the basement in the external domain has occurred, in agreement with interpretations of the ECORS-CROP seismic reflection line further south.