Forecasting the tectonic evolution of the Swiss Jura fold-and thrust belt: Structural analyses of oriented cores from deep exploration boreholes for the Swiss radioactive waste disposal

During the siting process the Swiss radioactive waste management organisation Nagra completed nine drillings through the Jura fold-thrust-belt and its evaporitic detachment, mostly down to the underlying basement. A total of around 5.5 km of oriented cores were recovered providing a unique basis for structural analyses. We use this data to review and amend the tectonic history of the wider region of the easternmost Jura which also comprises the site of the future deep geological repository for nuclear waste.

Based on the structural core analysis we characterise the kinematics of four main deformations: (1) a NNW-SSE-extension, which is unequivocally dated to the Jurassic and related to the rifting of the European margin during the opening of the Penninic ocean, (2) a NNW-SSE-extension by the Paleogene-Neogene flexure of the Molasse foreland basin, (3) a local NE-SW extension correlated to the formation of nearby Paleogene-Neogene graben systems, and (4) Neogene (21 Ma) to Quaternary heteroaxial SSE-NNW- and SSW-NNE-directed shortening. Repeated changes of shortening directions probably relate to strain partitioning in a zone of oblique convergence. Convergence at oblique angles with respect to pre-existing basement faults is partitioned into mild sinistral-transpressive shear parallel to the prevailing basement faults and orthogonal shortening perpendicular to these faults.

The outlined tectonics of the last about 21 Ma allows a qualitative extrapolation into the tectonic future of the planned repository considering the past processes as key information for possible future evolutions. Past tectonic history provides sound evidence for pre-dominant concentration of deformation along pre-exisiting faults with aforementioned possible changes in shortening directions. Future tectonic scenarios that can be expected include switches of the deformation regime between orthogonal shortening and sinistral transpression, and changing stress/strain coupling and decoupling between the allochthonous units and the units below the basal evaporitic detachment. However, it should be noted that the site selection process has identified a region that has seen very little deformation in the past. At the site larger faults have been carefully delineated with 3D-seismics and by avoiding these zones the repository is expected to remain intact even in the case of re-activations of regional faults.