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Texture-related and experimental P-wave anisotropy of rocks from the TRANSALP seismic traverse
Authors: Klaus Ullemeyer and Jan H. Behrmann
The recently accomplished TRANSALP research program includes recording of a nearly 340 km long reflection seismic profile across the Eastern Alps. Cross-line observations indicate compressional wave (P-wave) velocity anisotropy of more than 10% at the northern border of the Tauern Window. Since the elastic behaviour of rocks largely depends on the mineral textures (Crystallographic Preferred Orientations), texture measurements were performed on a representative rock suite from the intersection of the seismic profile with the Tauern Window to determine the contribution of the mineral textures to bulk rock P-wave anisotropy. The texture-derived velocity distributions are compared to complete P-wave measurements on spherical samples at confining pressure of 200 MPa to determine the contribution of the crack fabric to bulk rock anisotropy as well.
A representative suite of deformed rocks from the TRANSALP reflection seismic traverse in the Eastern Alps was studied with respect to whole-rock texture and P-wave propagation velocities. Here 'texture' refers to crystallographic preferred orientations (CPOs), which were determined by TOF neutron diffraction at FLNP/JINR, Dubna, Russia. Mostly, the mineral textures of gneisses and schists are weak and, therefore, have only minor influence on P-wave anisotropy. Experimentally determined P-wave anisotropies at confining pressure of 200 MPa in the order of 5 to 15% may be explained by the microcrack fabric. In marbles (~100% calcite) and amphibolites (> 80% hornblende) the degree of anisotropy is between 5 and 10% and largely controlled by the mineral CPOs, whereas the contribution of microcracks to bulk rock anisotropy is much smaller compared to the gneisses. Summarizing, only more or less monophase rocks display well-pronounced CPOs leading to significant textural influence on elastic anisotropy.
Since in addition to missing textures density contrasts are small, very small reflection coefficients Rc are observed for lithological interfaces between various types of gneisses (Rc < 0.02). The maximum reflection coefficient of a marble-gneiss contact is about Rc = 0.06 and partly related to the calcite texture. In contrast to a gneiss-gneiss contact, such an interface produces a clear signal in a seismic experiment. The maximum reflection coefficient of an amphibolite-gneiss contact is Rc = 0.12 and largely caused by the density contrast between both units, with significant influence of the hornblende texture as well. Consequences regarding the origin of seismic reflections in the Alpine crust are that strong reflections are probably due to marble-gneiss and metabasite-gneiss contacts.