Reinterpretation of a wide angle reflection “fan” across the Central Alps

Abstract

Within the framework of a new phase of crustal exploration in the Alpine range (by vertical reflection seismics) all existing seismic data (refraction-wide-angle reflection profiles and fans) have been reprocessed and reinterpreted. This study discusses the results of the reinterpretation of the fan ZE (recorded in 1986), which crosses the Central Alps east of the European Geotraverse (EGT). The geophones (3 components) were positioned 130 km east of the shot-point C (on the EGT); therefore, the “mirror” of the supercritical reflections from the Moho is located approximately to the north of the Iseo lake. Reprocessing included a spectral analysis (on noise, P m P and S m S signals), a polarization analysis (on 3-component reoriented recordings), and a reflectivity analysis (on the vertical component). A line-drawing was prepared and finally the depth of the correlated reflection segments was calculated using average velocities constrained mainly by the Sudalp 77 profile, which intersects the fan ZE. The analysis shows that the Penninic crust is more reflective than the South-alpine crust and that the transparency of the lower crust increases where the total thickness is greater, here, the frequency of the signal reflected from the Moho also seems to increase. The geometry shown in the depth cross-section is discussed also considering the resolution power of the fan. Furthermore, like in the Western Alps as well as along the EGT, the asymmetry of the lower crust clearly indicates that a trace of lithospheric subduction still exists in the present configuration.