Experimental study of the seismic properties of the Eastern Alps (Italy) along the Aurina-Tures-Badia Valleys transet.

In order to better constrain the interpretation and the nature of the seismic reflectors, experimental measurements at high confining pressure (up to 300 MPa) and room temperature of the compressional wave velocity (Vp) on 10 samples representative of the most common lithologies along the Aurina (Ahrntal), Tures (Tauferer Tal), and Badia (Abtei Tal) Valleys profile (Eastern Alps, Italy) have been performed. For each sample, the speed of ultrasonic waves was measured in three mutually perpendicular directions, parallel and normal to the rock foliation and lineation. The main results are: (a) Good agreement between the calculated vs. measured modal compositions of the considered rocks, indicating that they were presumably equilibrated at the estimated P-T conditions; therefore, the seismic properties are representative of the crustal level indicated by the thermobarometry. (b) Measured and calculated average Vp are in good agreement, and are typical of mid-crustal level (6.0-6.5 km/s). Only the amphibolites show Vp typical of the lower crust (7.2 km/s). (c) The seismic anisotropy of metapelites is very high (12-27%), both with orthorhombic and transverse isotropy symmetry; amphibolites are transversely isotropic with an anisotropy of 8%; orthogneisses and granitoids are isotropic or weakly anisotropic. (d) The contacts between amphibolites and all other rock types may generate good reflections, provided they are not steeply inclined. Although the metamorphic foliation remains steeply inclined, discordant buried sub-horizontal igneous contacts may be detected.