Seismic velocities of Lewisian metamorphic rocks at pressures to 8 kbar: relationship to crustal layering in North Britain

Abstract

Summary P- and S-wave velocities on a set of rocks from the Archaean Lewisian metamorphic complex, north-west Britain, have been measured in the laboratory at confining pressures to 8 kbar. From estimates of velocities at crustal pressures and temperatures, it is concluded that the composition of the three crustal layers below the Scottish Caledonides are most likely to be, in order of increasing depth: (1) mixed metamorphic rocks in amphibolite or lower facies, plus granites, (2) mixed pyroxene-granulite facies rocks, of overall intermediate composition and (3) mafic garnet granulites. Poisson's ratios of isotropic aggregates of mafic amphibolites and serpentinized peridotites in the Lewisian are high (0.28-0.31) because of the presence of hydrous minerals. Only a highly preferred orientation of minerals in such rocks in the lower crust would lead to observed values of Poisson's ratio of about 0.25.