The Jura Fold-Thrust Belt, Switzerland – a contractional deformation setting for nuclear waste disposal

The Jura, on the outer margin of the NW Alps, is classically interpreted as a “thin-skinned” fold-thrust belt, detached along Triassic evaporites. It includes the Opalinus Clay (Toarcian-Aalenian), the designated host formation for a radioactive waste repository in Switzerland. We develop a case study to illustrate how methods and approaches in cross-section balancing and restoration, based on seismic imagery, can be applied to assess risks and uncertainties for the integrity of the Opalinus Clay. We focus on the Nördlich Lägern area, announced in 2022 as the preferred site for nuclear waste disposal in Switzerland.  Between northern and southern fold-thrust structures, 3D seismic mapping defines a largely unfaulted domain within which there is no seismic indication of deformation in the Opalinus Clay. It is however seismically transparent. Bounded to the north and south by domains of strongly-faulted Mesozoic strata the low-strain zone has been translated tectonically and is underlain by a seismically resolved Permo-Carboniferous basin.

 

Here we consider multiple interpretations of key section lines with varying degrees of structural linkage and several conceptual models for the timing of the contractional deformation structures. By exploring multiple interpretations, we consider the key uncertainties in structural understanding and risks that might compromise the structural integrity of the waste repository site. Key questions include: were the thin-skinned thrust structures influenced by deep basement structures (that might reactivate in the future)? Has the Opalinus acted as an intraformational detachment that connects kinematically the northern and southern fold-thrust structures? Section balancing and restoration are used to assess whether strain in the two deformation domains can be matched on both sides of the Opalinus Clay. If not, then there is an increased risk of the Opalinus having acted as a detachment passing through the low strain domain, the candidate repository site. A combination of line-length and formation area balancing have been applied to the adjacent, upper and lower, units to the Opalinus Clay, evoking different interpretations of fault trajectories in the bounding fold-thrust domains. In all these interpretations, a balance in tectonic contraction can be achieved without involving distributed strain or requiring detachment within the Opalinus. On this basis, the structural integrity of the prospective repository site is not compromised; an inference consistent with borehole evidence. No significant layer-parallel fault zones have been identified in the four borehole penetrations in the Nördlich Lägern area. 

The work highlights the importance of structural interpretation and restoration techniques in assessing risks for energy and waste storage projects in contractional settings, even when searching for low strain zones.