Eclogite as a seismic marker in subduction channels: Seismic velocities, anisotropy, and petrofabric of Cabo Ortegal eclogite tectonites (Spain)

High-pressure, sheath-like fold-nappes made of normal-mid ocean ridge basalt (N-MORB) eclogites, high-pressure volcanic-arc granulites, arc-root lithospheric mantle peridotites, and shallow crustal volcanosedimentary gneisses were amalgamated at deep realms of an Eohercynian orogenic channel and currently crop out in the Cabo Ortegal Complex. Compressional seismic-wave velocity measurements determined in eclogite at confining pressures of 600 MPa range from 8.4 to 7.3 km/s, whereas anisotropy varies between 0.7% and 3.2%. Shear-wave velocities range from 4.8 to 4.2 km/s, and the respective anisotropies vary between 0.1% and 3.4%. The Poisson's ratios are low (0.25-0.265), independent of the degree of retrogression or deformation. Seismic velocities and anisotropy are related to the fabric of omphacite, and calculated eclogite anisotropy is not symmetrical with respect to the tectonic fabrics. Calculated seismic-reflection coefficients induced by lithological, structural, and rock petrofabric variations of eclogites show that these can be identified as reflectors imprinting a particular, penetrative, and dipping fabric to discrete seismic-profile segments of paleosubduction settings.