Highly variable petrophysical properties in felsic high-pressure rocks of the continental crust

Abstract

Seismic wave velocities and anisotropy can potentially discern lithologies and their structural configuration at depth. An increasing amount of data exists that indicates seismic anisotropy can be a valuable marker to identify internal deformation within deeply buried crust. Here, we present petrophysical properties of variably deformed felsic rocks that have been metamorphosed at blueschist and eclogite-facies conditions, representative of the bulk composition of the mid continental crust. The samples were collected from the Monte Mucrone area (Sesia Zone, Western Alps) and the Serra di Pigno, Farinole, and Tenda areas (Corsica). The mineral assemblages of all samples indicate that fluids were abundant during re-equilibration at HP conditions. Petrophysical properties were derived from ultrasonic measurements as well as calculated from crystallographic preferred orientations obtained by neutron texture goniometry. The results show that crystallographic preferred orientations are weak and yet the magnitude of seismic anisotropy produced by metamorphism of the felsic continental crust is highly variable, ranging from 1 to 12% for P waves and 1–14% for S waves depending on the strength of deformation fabrics. At the same time, absolute wave velocities remain low compared to more mafic rocks that have equilibrated at HP conditions. These results demonstrate that, other than their mafic equivalents, felsic HP rocks cannot be distinguished from their protoliths by wave velocities because their increase during HP metamorphism is too small. However, fluid availability during metamorphism and deformation causes strong variations in the seismic anisotropy of such lithologies, that might be distinguishable at depth.