SEISMIC PROPERTIES OF HIGHLY STRAINED MARBLES FROM THE SPLUGENPASS, CENTRAL ALPS

Seismic reflection profiling across the Central Alps (NFP20) shows str ong reflectors, some of which correspond to a Mesozoic marble zone whi ch underlies the contact between the Tambo and Suretta nappes. By mean s of experimental velocity measurements at confining pressures up to 3 00 MPa on rock samples collected in the marble zone, we determined the seismic properties of these marbles and calculated their reflection c oefficients, in order better to constrain the seismic interpretation. The main findings are as follows: a) Marbles exhibit V-p anisotropy up to 9%, with orthorhombic or transverse isotropic (tetragonal) symmetr y due to the crystallographic preferred orientation of calcite. b) Com pressional waves, shear waves, and shear wave splitting are very sensi tive to the type of fabric (presence or absence of an r-axes concentra tion). c) Interfaces between marbles and amphibolites or orthogneisses could be good reflectors in normal incidence reflection. (C) 1998 Els evier Science Ltd.