Importance of folding for the elastic anisotropy of foliated rocks in the NW-Tauern Window (Eastern Alps, Austria)

Most rocks show direction-dependent seismic wave velocities, which are largely controlled by the content and CPO (crystallographic preferred orientation) of elastically anisotropic minerals, especially phyllosilicates. In order to unravel the effect of complex structural features like folding, re-folding and the scale difference between seismic experiments and lab measurements, we investigate polyphasely deformed phyllosilicate-rich samples from the NW-Tauern Window. Based on anisotropy and modeled seismic velocity data we evaluate the effect of folding and hence upscaling to a larger scale.

The CPO of millimeter-thick foliated sample cylinders was measured using high energy X-ray diffraction at the German and European Synchrotron Facilities (DESY, ESRF). Folding structures were quantified from thin sections, drill cores and field data. CPOs were then synthetically folded and weighted based on observed and hypothetical geometries. Overall seismic velocities were computed using µXRF-based modal composition, single crystal stiffness tensors and folding-derived CPOs and considered to be representative up to kilometer-scales.

Folding and crenulation reduce the Vp-anisotropy by approximately 40% at interlimb angles of 60° and 45°, respectively. Polyphase folding around parallel fold axes shows only a slight further reduction. Re-folding at distinctly different fold axis orientations leads to an elastic anisotropy reduction up to 80%. In our samples, only wide interlimb angles of >120° occur that reduce the Vp anisotropy by <20%. Shear wave splitting results in a reduction of <30%. Hence, folding may have a significant effect on the elastic anisotropy of foliated rocks and can be crucial for any interpretation of seismic data.