A seismic reflection-refraction experiment across the exposed lower crust in Calabria (southern Italy): first results

Exposed crystalline basement of the Serre Mountains in Calabria presents a tilted block of a nearly complete section of the Hercynian continental lower crust (HCLC). In addition to petrological and structural data from surface mapping, and petrophysical data from the laboratory, a seismic reflection-refraction experiment was conducted in May 1990. This consisted of a 40 km long N-S profile crossing the HCLC and of four short transverse profiles, each recorded using 3-component receivers with an 80 m spacing and explosive sources.
The reflectivity of the outcropping lower crustal units is lower than theoretically predicted from the observed compositional layering. A low-velocity zone, outcropping in the north, and dipping to the south, marks the contact between the HCLC and the underlying Alpine metamorphic units. Below this zone, the deeper crust appears well-structured by strong and continuous, discrete reflections down to 6.5–8 s t.w.t. (presumably the crust-mantle boundary at 19–24 km depth) with a dominant dip toward the south.
Analysis of refracted-wave velocities reveals values systematically lower by up to 30% than laboratory data on rock samples or calculated data from modal analysis. This discrepancy can only partly be explained by the effect of microcracks (10%), the underestimation of the amount of leucosomes (2–5%) and the effect of seismic anisotropy (0–5%). The remaining discrepancy must be attributed to large scale alteration of the rocks due to Apennine tectogenetic events.