Fabric and shear strain of a potential halite detachment below the Swiss Eastern Tabular Jura

The Swiss deep geological repository for radioactive waste is to be sited in the easternmost part of the Jura fold-and-thrust belt that forms the external allochthonous units in the Western Alps. Although it is commonly accepted that the Jura is a thin-skinned fold- and-thrust belt, which is detached from the underlying autochthonous units along Middle Triassic evaporites, this is not so clear for Nagra’s siting region. There, the amount of shortening accommodated in the so-called Siglistorf Anticline is only some 150-220m (Jordan et al., 2015, Nagra Arbeitsbericht NAB 14‐105). The location of the anticline above the northern boundary fault of a crustal-scale Paleozoic graben gave rise to a dedicated discussion whether the calculated shortening is in fact related to a thin-skinned detachment or related to thick-skinned deformation involving the underlying basement (e.g., Schöpfer et al., 2023, Terra Nova, and references therein).

To test the competing thin- and thick-skinned models, we analyzed cores from four wells drilled by Nagra through the evaporitic detachment for structures which can be used for quantifying shear strain and, hence, the thrust displacement. Cores of the drilled anhydrite and halite succession are oriented allowing to determine the true orientation of structures. Variably oriented stretching lineations along with sigma clasts, winged inclusions, shear bands and asymmetrical boudins prove polyphase kinematics with different transport directions arguing against a continuous high-strain detachment. Recorded shear directions are top-NNW, top-NNE and top-W. The preservation of primary sedimentary and early diagenetic fabrics, shapes of winged inclusions, angles between S and C planes in shear bands, elongations calculated from boudinaged layers and the abundance and sizes of survivor grains are used to estimate the finite shear strain for the different lithotypes, which are categorized in undeformed (tangent of the shear angle γ=0), low shear strain (γ<2), medium shear strain (γ<7), and high shear strain (γ<15). The total maximum displacement, calculated from the sum of the thickness-shear strain products for each strain category, are 41m, 66m, 79m and 123m, for the four investigated boreholes.

Balanced cross-sections from the area of interest based on the thin-skinned deformation model state a shortening of approximately 150 to 220m which is accommodated in the Siglistorf Anticline north of the investigated boreholes. In this thin-skinned model the whole respective displacement must be accommodated by the regional evaporitic detachment, which, however, is considering the estimated total displacements from the respective core intervals not fully supported. We propose that shortening accomodated in the Siglistorf Anticline is also related to shortening involving the strata below the regional décollement level. It is concluded that deformation of the easternmost part of the Jura fold-and-thrust belt is in fact a combination of thin-skinned and thick-skinned tectonics.