Seismic properties of rocks from an exposure of extended continental crust—new laboratory measurements from the Ivrea Zone

Abstract

Compressional wave velocities for new samples from the Ivrea-Verbano Zone (IVZ) and Strona-Ceneri Zone (SCZ) have been measured to 600 MPa in order to better understand the petrologic controls on velocity and to provide an expanded base for seismological models of the crust as represented by these terrains. These data, coupled with previous measurements, show that preferred orientation of mica controls both the velocity hysteresis with pressure and seismic anisotropy of SCZ and IVZ schists and gneisses. IVZ paragneisses form the basis of a linear velocity-density relationship for metagraywackes of various metamorphic grade. This relationship falls close to that expected for mafic rocks and overlaps velocity-density data for IVZ mafic gneisses. One-dimensional near-vertical reflection models, based on these data and recent structural reconstructions of the terrain, show that lower crustal reflectivity can be related to high reflection coefficients associated with interlayered sequences of upper amphibolite-granulite facies paragneiss, mafic gneiss and ultramafic rocks. Transparent upper crustal levels in the model result from the overall similarity of velocities for schists, gneisses and granites from the structurally and lithologically complex SCZ. A mean lower crustal velocity of 7.1 km/s is calculated for the part of the IVZ composed of interlayered mafic gneiss and paragneiss and for another area dominated by a mafic layered igneous complex. The velocity-density analysis coupled with this observation emphasize the ambiguity inherent in interpretation of crustal composition from P-wave velocity data in the absence of other constraints.