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Arbeitsbericht NAB 12-41

Characterisation of Cenozoic brittle deformation of potential geological siting regions for radioactive waste repositories in Northern Switzerland based on structural geological analysis of field outcrops

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This report presents the results of an extensive analysis of brittle deformation structures (fractures) across a large area of Northern Switzerland including five potential siting regions for the disposal of radioactive waste (Jura-Südfuss, Jura Ost, Nördlich Lägern, Zürich Nordost and Südranden). Over 80 field outcrops were mapped in detail resulting in more than 2300 fracture measurements and representing the most extensive dataset of this kind available for the region to date. The gathered data, mostly stemming from outcrops in Upper Jurassic limestones, enabled an enhanced structural geological characterisation of the five geological siting regions in terms of fracture orientation and tectonic regime.

The most common brittle fractures observable in the field across the entire study area are bedding orthogonal joints whose general high frequency varies only slightly depending on the rock type. The most commonly observed joint orientations are NE-SW, NW-SE and N-S. The timing of their formation is only weakly defined. At the latest they formed in conjunction with Early Cenozoic tectonic events (e.g. during uplift associated with the initialisation of the Alpine forebuldge or due to extension related to the formation of the European rift system). The bedding orthogonal joints hardly ever show significant offsets of more than a few centimetres and as such do not represent tectonic faults in the strict sense. However, kinematic indicators are very commonly found on the planes of this kind of fractures allowing for their kinematic analysis for paleostress reconstruction. Relative overprint and cross-cutting criteria of different fracture types allowed establishing a regional chronology of Cenozoic brittle deformation.

Most kinematic indicators found on fracture planes suggest strike-slip faulting. In the region of the Jura Fold-and-Thrust Belt bedding orthogonal joints showing strike-slip kinematics were frequently found to be rotated with the folded sedimentary stack. This indicates that strike-slip faulting initiated pre- or syn-kinematically with the main contractional deformation phase. At some locations strike-slip faulting also appears to postdate folding. Based on the kinematic analysis of strike-slip faults it was possible to constrain a robust paleostress tensor across the entire region indicating NNW-SSE shortening. Occurrence of reverse faults was mostly limited to high strain areas affected by the tectonics of the Jura Fold-and-Thrust Belt. In most cases these reverse faults were found to be kinematically compatible with the strike-slip faults and yielded roughly the same paleostress orientation (e.g. NNW-SSE shortening). Some indications for earlier extensional deformation in the region are given by the occurrence of extensional flexures and normal faults in the Tabular Jura that predate strike-slip faults associated with Neogene Alpine shortening. However, kinematic data related to these extensional events remained scarce and no reliable regional paleostress tensor could be established except for the area of Schaffhausen.

In general, the results of this investigation indicate a comparatively simple Cenozoic deformation history of the region. For example, extension related to the nearby Rhine Graben appears to only have mildly influenced the area. On the other hand, the field observations reported herein also raise some new questions regarding the regional tectonic setting. In particular these include the definition of the Jura Fold-and-Thrust Belt front which seems to reach farther to the north than expected (e.g. beyond north of the Rhine River at some locations) and the complex extension kinematics in the border zone of the Hegau-Bodensee Graben (in the area of Schaffhausen).

The appendix of this report (available from the Nagra info centre on request) includes an outcrop catalogue for the area of investigation. Naturally it has no claim for completeness but, together with the mapping protocols for each analysed outcrop that allow a comprehensive re-evaluation of the field observations and their kinematic interpretation, it is considered a valuable data base for the planning and execution of future investigation steps (e.g. quantitative fracture investigation, sampling campaigns etc.).

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