A model of velocity structure beneath Calabria, southern Italy, based on laboratory data

Abstract

Laboratory velocity measurements of representative rock samples from across a nearly complete crustal section from the Serre Mountains of southern Calabria have been carried out as an aid to interpreting in-situ seismic velocity data. The section is composed of interlayered acidic, mafic and pelitic rocks metamorphosed to granulite- and amphibolite- facies conditions. In addition, a suite of post-tectonic granitoids intrudes the section. The experimental data include the measurement of compressional wave ( V P) and shear wave ( V S) velocities at confining pressure ( < 600 MPa) and temperature ( < 700°C), the determination of the pressure and temperature derivatives of V P and V S, and the determination of the velocity anisotropy. The petrophysical data are correlated with the chemical and mineralogical characteristics of the relevant rocks. In conjunction with structural and stratigraphic field data the experimentally determined data are used to construct a seismic velocity model for the crust beneath Calabria. In the upper crustal levels, V P and V S vary in different manners. Below the uppermost amphibolite facies gneisses, a weak velocity inversion of V P is associated with the post-tectonic granites. At the granite/granodiorite and the tonalite/dioritic gneiss boundary V P increases in steps of about 200 and 300 m/s, respectively. In contrast, V S remains constant or even decreases slightly through this same sequence of rocks. This contrasting behaviour is primarily related to the quartz/feldspar ratio in the rocks. The most pronounced velocity jump occurs at the top of the granulite facies metapelite unit. Here, both V P and V S increase markedly, due to mineralogic changes that accompany increasing metamorphic grade and the greater abundance of mafic and garnet- bearing restites of partially melted pelite. The mafic rocks of the lowermost crustal layer have P- and S-wave velocities that are significantly lower than those in the overlying metapelites. The variation of Poisson's ratio with depth suggests that this parameter, which is a combination of V P and V S, is more sensitive to lithology than the P- and S-wave velocities alone. In rock units of upper and mid-crustal levels with average P-wave velocities < 6.5 km/s, the Poisson ratio can be used to discriminate quartz-rich rocks from feldspar-rich rocks.