3-D SEISMIC VELOCITIES CALCULATED FROM LATTICE-PREFERRED ORIENTATION AND REFLECTIVITY OF A LOWER CRUSTAL SECTION - EXAMPLES OF THE VAL SESIA SECTION (IVREA ZONE, NORTHERN ITALY)

To quantify the seismic properties of lower crustal rocks and to bette r constrain the origin of the lower crustal seismic reflectivity, we d etermined the complete 3-D seismic properties of a lower crustal secti on. Eight representative samples of the main lithologic and structural units outcropping in the Val Sesia (Ivrea zone) were studied in detai l. The seismic velocities were calculated using the single crystal sti ffness coefficients and the lattice preferred orientation (LPO) of eac h mineral in all samples. The 21 stiffness coefficients characterizing the elastic behaviour of each rock are determined. Mafic and ultramaf ic rocks such as pyroxenite and pyroxene-bearing gabbros display compl ex shear wave properties. These rocks are weakly birefringent (maximum 0.1 km s-1) and it is difficult to find consistent relationships betw een the seismic properties and the rock structure. On the other hand, seismic properties of deformed felsic rocks are essentially controlled by mica. They display strong S-wave birefringence (0.3 km s-1) and re latively high V(p) anisotropy (7.6 per cent). Amphibole also strongly influences the rock birefringence patterns. For both kind of rocks, th e foliation is highly birefringent and the fast polarized shear wave i s systematically oriented parallel to the foliation. We show that the number of mineral phases in the rock strongly controls the anisotropy. The seismic anisotropy has a complex role in the P-wave reflectivity. Compared to the isotropic case, anisotropy enhances the reflection co efficient for about 60 per cent of the possible lithological interface s. For 40 per cent of the interfaces, the reflection coefficient is mu ch lower when one considers the medium as anisotropic.