The upper part of three deep seismic lines running across the Penninic
Swiss Alps of Valais have been studied. Numerous reflectors illustrate
the nappe structure of this internal part of the orogen. These
reflectors, even at great depths (20-25 km), can be correlated with
outcropping geological features and are most likely produced by
lithological boundaries rather than by mylonites zones, which are hardly
reflective in such an environment. Our interpretations, largely
constrained by projections of the outcropping geology, have improved our
knowledge of the deep structure of this segment of the Alpine belt,
enhancing the importance of the backfolding and the crustal scale
deformation phase which produced the Rawil-Valpelline depression and the
Aar-Toce culmination. Furthermore we have here the possibility of
correlating seismic patterns produced by ductile folds with the
outcropping structures.