Technischer Bericht NTB 14-02/III

Dossier III: Long-term geological evolution

In Dossier III on long-term geological evolution, the geological siting regions are characterised in terms of neotectonic movements and erosion over the next 105 and 106 years (time periods considered for the L/ILW and HLW repositories respectively). For the safety-based comparison, this Dossier provides the background information for the indicators 'conceptual models of long-term evolution (geodynamics and neotectonics; other processes)', 'seismicity', 'erosion in the time period under consideration', 'depth below terrain as relevant for rock decompaction', 'depth below top bedrock as relevant for glacial overdeepening' and 'depth below the local base level as relevant for formation of new ice-marginal drainage channels'.

In Stage 2 of the Sectoral Plan process, the work carried out on long-term geological evolution consisted mainly of literature studies and compilation of existing data. The key datasets for evaluating neotectonics and erosion were the high-resolution digital terrain model, the revised digital elevation model of the top bedrock surface and a newly compiled dataset on the distribution and elevation of gravel deposits. In order to detect recent crustal movements, geodetic data and data on seismicity and stress state were acquired and jointly analysed. Based on the reprocessed and densified 2D seismic dataset in Northern Switzerland, the knowledge of the location of tectonic structures in the basement and in the sedimentary cover was improved and the depth of the potential host rocks in the siting areas could be mapped with greater confidence (see Dossier II). This information is of key importance for identifying regional tectonic elements to be avoided and for evaluating the depth of the host rock with respect to future erosion.

For the area of Northern Switzerland, literature studies and detailed analyses of gravel terraces using high-resolution terrain models indicate that, since the Late Pliocene, neotectonic activity was minor. Owing to the very low movement rates, however, traces of neotectonic activity are expected to be difficult to identify. Thurs, ruling out neotectonic activity along known regional tectonic elements is currently not possible. Precision levelling data provide some indications of possible activity, for example in the region of the Folded Jura. Information on the recent stress field in the region implies that some pre-existing fault zones could potentially be reactivated in the future, most probably as strike-slip faults or normal faults. Whether deformation in the area of Northern Switzerland occurs mainly in the sedimentary cover, as was the case in the Early Miocene to Late Pliocene, or whether the basement is also affected, leading for example to a reactivation of the margins of the Permo-Carboniferous Trough of Northern Switzerland, cannot be fully clarified at present. Particularly when information from neighbouring regions is taken into consideration (e.g. the Upper Rhine Graben), and given the remaining uncertainties, both scenarios have to be considered. Because of these uncertainties, existing regional fault zones and other elements where future deformation would preferably occur should be avoided. In Stage 2 of the Sectoral Plan process, the margins of the Permo-Carboniferous Trough (that underwent post-Palaeozoic reactivation) and anticline structures are now considered as tectonic zones to be avoided in Northern Switzerland, along with the regional fault zones in overlying sedimentary cover. These zones, together with the remapped regional fault zones, are avoided, when defining the disposal perimeters to be evaluated in the safety-based comparison.

The Wellenberg siting region is generally seen as less favourable in terms of the conceptual understanding of the neotectonic situation compared to the siting regions in Northern Switzerland. Because of its tectonic situation between two alpine nappes, it shows higher tectonic overprinting than the siting regions in Northern Switzerland. The significantly higher recent uplift rates in this region and the higher earthquake activity mainly in the shallow crustal zone (< 5 – 15 km) are taken to be an expression of comparatively higher ongoing tectonic activity. In contrast to Northern Switzerland, systematic mapping of existing larger fault zones using reflection seismic methods is not possible in the Wellenberg region. Consequently, those zones, where future deformations would preferentially take place, cannot be avoided based on exploration from the surface.

When evaluating the siting regions in terms of future erosion, beside endogenetic processes (e.g. tectonic uplift), also exogenetic processes play an important role. The latter are strongly influenced by climatic conditions. When evaluating future erosion, a wide spectrum of climate evolutions is considered. Thereby, the glacial-interglacial cycles with repeated glacial advances into the external alpine foreland are assumed to be the persisting pattern in the future. The key basis for deriving erosion scenarios is therefore the development of the landscape in the past 2 million years, when the climate was characterized already by alternating cold and warm periods, initially in 40 ka and later in 100 ka cycles. Climate modelling for the next 105 years indicates that the next major ice age with glacial advances into the external Alpine foreland is to be expected in 60,000 years at the earliest and, depending on the assumptions made regarding anthropogenic CO2 emissions, may be significantly later.

For the long-term stability of a deep geological repository with respect to future erosion, the depth of the host rock in the siting region under consideration and particularly in the disposal perimeter within this region is of central importance. In general, the greater the overburden, the better protected a repository is against erosion-driven decompaction effects or exposure. In Stage 1 of the Sectoral Plan process, the overburden of the host rock with respect to terrain surface and with respect to bedrock surface was analysed and used for defining and evaluating preferred areas within the siting regions. This simplified approach assumed firstly a laterally constant erosion rate with the main features of the local topography remaining intact over time and secondly a preferential glacial erosion in existing valleys and bedrock overdeepenings. In Stage 2, the delimitation and evaluation of the disposal perimeters as part of the safety-based comparison, also takes into account that areas lying between main valleys could, under certain circumstances, be eroded more quickly than the valleys themselves. As a result of glaciation existing valleys can be closed and/or infilled with gravel in a relatively short period of time; subsequently, new bedrock valleys can be cut through bedrock ridges, forming so-called 'ice-marginal drainage channels'. To take into account the formation of such features in the future, the depth of the host rock under consideration is calculated not only relative to terrain and bedrock surface but now also relative to the local base level (defined by the altitude of existing fluvial bedrock valleys); this is taken into account for defining and evaluating the disposal perimeters as part of the safety-based comparison.

In the three HLW siting regions, the past development of the local base level (incision of the main rivers) was reconstructed over the past 2 million years using gravel deposits. The causes of fluvial incision in the past were considered to be both endogenetic (uplift) and exogenetic processes (e.g. increased runoff due to changes in climate or catchment area). The most dramatic and fastest lowering of the local base level in Northern Switzerland in the past 5 to 10 million years occurred due to the shift of regional watersheds, particularly associated with the diversion of the Aare-Danube into the Doubs-Rhone and finally the Rhine system. A large-scale analysis of the main drainage network in Northern Switzerland shows that the catchment area for the Aare-Rhine system will continue to grow compared to the Danube system, because of its lower elevation and that the potential for further lowering of the local base level by the displacement of the regional watersheds in Northern Switzerland in the course of the next 105 to 106 years is likely to be limited. Based on considerations of river network development in the past and the present-day network, various scenarios are formulated for the future evolution of the local base level; these differ between repository types (due to the different time periods under consideration) and partly also between siting regions.

Glacial overdeepening is an erosion process that can lower the top bedrock level significantly below the local base level, as is evidenced by several overdeepened valleys in Northern Switzerland. These occur mainly in the Molasse substrate. Glacial overdeepening in the Malm limestones exists only in few cases and reaches only a shallow depth. To become critical for a deep geological repository, a glacially overdeepened valley not only has to cut through the Molasse substrate, but, depending on the siting region, also through a limestone layer up to more than 200 m thick. Although glacial overdeepening occurs preferentially where the ice thickness and flow rates are high, i.e. in the area of existing valleys and overdeepened bedrock troughs, the potential formation of new overdeepenings has to be taken into account, particularly in flat terrain and over longer time periods with several glacial advances. Consequently, in phase 2 of the Sectoral Plan process, besides the deepening and widening of existing overdeepened valleys, the formation of completely new glacial overdeepenings reaching significantly below the lowered local base level at the end of the time of consideration is taken into account in the evaluation process of the potential siting regions.

Depending on the depth of the host rock and the repository type, the siting regions in Northern Switzerland and the disposal perimeters within these regions are affected differently by the erosion scenarios considered. In the L/ILW siting regions Zürich Nordost, Nördlich Lägern, Jura Ost and Jura-Südfuss, the disposal perimeters can be defined in terms of depth in such a way that the overburden of the host rock will remain large even after application of extreme erosion scenarios at the end of the time period under consideration. In the Südranden L/ILW siting region, the overburden relative to the local base level is comparatively small. Ice-marginal drainage channels were formed several times in the past in the vicinity and a filled valley is located within the siting region. For these reasons, the formation of a new ice-marginal drainage channel is assumed as a possibility for the time period under consideration for a L/ILW repository at the Südranden site. In the HLW siting regions Zürich Nordost and Nördlich Lägern, a considerable overburden also remains over the much longer time period under consideration (106 years) even when extreme erosion scenarios are applied. For the HLW region Jura Ost, the formation of an ice-marginal drainage channel is considered as an extreme erosion scenario, despite the high topographic relief and the lack of existing ice-marginal drainage channels west of the Lower Aare Valley, because of the longer time period compared to that considered for a L/ILW repository. For such a scenario, the overburden remaining at the end of the time period being considered is clearly smaller in this siting region.

In Stage 2 of the Sectoral Plan process, three new glacial erosion scenarios (additional glacial overdeepening by 50, 100 and 200 m) were developed for the Wellenberg siting region, in addition to scenarios from earlier investigations. It is assumed that, following a glacial overdeepening of the Engelberg valley, the deep-seated Altzellen slide mass that lies above the westernmost part of the repository zone will be reactivated at a deeper level and the upper slope areas will continue to slip until the equilibrium profile of the slopes has adjusted to the new level of the Engelberg valley. As the host rock in this siting region has the form of a thick, nearly vertically oriented layer caused by tectonic accumulation, the depth of the disposal zone can be selected relatively freely. The stricter requirements relating to erosion and decompaction (compared to earlier investigation phases; see Dossier VI) can therefore be fulfilled by selecting deeper disposal levels.