3D-architecture of a high-grade metamorphic gneiss terrain – How suitable are these rocks for of a deep geological repository site in Germany?

Most high-grade metamorphic gneiss units in Germany exhibit a strong late-Variscan thermal imprint recorded by the presence of migmatites and granitic intrusions.

A nuclear waste repository in this setting is challenging because the amalgamation of various gneisses and magmatic rocks must be predicted and assessed to a search depth of 1500 m. Understanding the 3D-architecture of such basement units relies on better exposed regions than found in Germany. In this contribution, we use selected field areas to show the difficulty to predict the variability of high-grade metamorphic rock types at depth.

The Argentera Massif in the western Alps represents an ideal field analogue as it displays a variety of para- and orthogneisses and their equivalent migmatites along continuous surface outcrops. Detailed field observations from this area help to constrain the structural and lithological anisotropy of a typical Moldanubian crustal segment.

Except for the occurrence of large amphibolite lenses, gneisses and migmatites exhibit textural gradients at various scales rather than distinct lithological boundaries. Sharp boundaries on the other hand result from cross-cutting shear-zones that developed under retrograde greenshist facies conditions and discordant intrusions.

Whereas the overall 3D-geometry and anisotropy of the gneiss terrane may be described with the help of 3D models it is generally not possible to predict the spatial distribution of lithotypes and dominant textures for a given rock volume.

This contribution indicates why foliated gneisses and migmatites are less suitable as host rocks for a high-level radioactive waste repository compared with late granitoid intrusions that are more homogeneous.