Contribution of crystallographic preferred orientation to seismic anisotropy across a surface analog of the continental Moho at Cabo Ortegal, Spain

Abstract

The lithostratigraphic sequence of the Upper Allochthon of the Cabo Ortegal complex in northwestern Spain provides an excel lent analog for the direct study of petrophysical properties of the continental Moho transition. The various lithologies present were sampled for velocity measurements on minicores and determination of seismic veloc ities through microstructural analyses, with emphasis on the crustal part of the sequence. Here, we present data from electron backscatter diffraction (EBSD) to determine the crystallographic preferred orientation of all major rock-forming minerals. The orientation data are then combined with the elastic properties, densities, and modal fractions of the mineral phases to calculate P-wave and S-wave velocities. Calculated velocities coincide very well with direct measurements made on minicores of the same samples in the high-pressure laboratory. The threedimensional velocity patterns show the signifi cant contribution of the crystallographic preferred orientation of certain constituent minerals on the bulk properties of the rock. The bulk anisotropic signal is dominated by highly anisotropic phases that are susceptible to both shape and crystal preferred orientation. Factors of particular importance are small modal fractions of micas in gneisses, and amphiboles and clinopyroxenes in eclogites and high-pressure granulites. In the lower-crustal rocks, the direction of maximum P-wave velocities, although contained in the foliation plane, does not coincide with the orientation of the mineral and stretching lineation and therefore cannot be used as an indicator of the main strain direction in rocks. The maximum seismic birefringence is often contained within the foliation plane but very rarely coincides with the lineation. Our data set also illustrates the strong effect of the breakdown reactions of clinopyroxene and the appearance of plagioclase on the petrophysical properties of mafi c lower-crustal rocks. While it does not enhance anisotropy, it does produce a drop in velocity and as a consequence enhances the refl ectivity of the contact between lower-crustal rocks and ultra mafi cs from the mantle.