Laboratory measurements of seismic P-wave velocities on rocks from the Betic chain (southern Iberian Peninsula)

Abstract

Ultrasonic laboratory measurements of seismic compressional velocities ( V p) carried out on rock samples encompasing a wide range of metamorphic grades and lithologies from the Betic chain are presented. Measurements were performed at a high confining pressure on a total of 12 representative samples of the Alboran Domain (Internal Betics). P-wave mean velocities, referred to pressure conditions of 280±10 MPa, range from 5.82 to 7.13 km s −1, the maximum values corresponding to high amphibolite facies metapelites (garnet gneiss). Under these pressure conditions, V p anisotropies range from 1.2 to 16.1%, with the lowest values being associated with granitic rocks and the highest values associated with metapelites from the upper structural units. P-wave velocities were calculated for prevailing P– T conditions at depths of 10, 20, 30 and 40 km. We discuss the significance of the analyzed rock-types in terms of metamorphic grade and derived V p at different crustal levels. P-wave velocities and densities measured on low-to-middle and middle-to-high grade metamorphic rocks are compatible with values obtained from previous seismic and gravity studies for the upper and middle crust, respectively. At lower crustal levels, the integration of our measurements with other geophysical data suggests the presence of partial melting of high-grade metamorphic rocks. The estimation of the impedance contrast from the measured physical properties suggests that the reflective character of the mid-lower crust is associated with shear/mylonitic zones and/or fine-scale lithological layering enhanced by partial melting.