Analysis of tectonic structures and excavation induced fractures in the Opalinus Clay, Mont Terri underground rock laboratory (Switzerland)

Abstract

Excavated in the Opalinus Clay formation, the Mont Terri underground rock laboratory in the Jura Mountains of NW Switzerland is an important international test site for researching argillaceous formations, particularly in the context of deep geological disposal of radioactive waste. The rock laboratory is intersected by naturally formed tectonic structures, as well as artificial fractures primarily formed as a consequence of tunnel excavation and the associated stress redistribution. The description and characterisation of tectonic and artificial structures is, in many cases, of key importance for interpreting the results of the various in situ experiments conducted in the rock laboratory. Systematic small-scale mapping of the tunnel walls and floor, and adjacent niches, provides basic information about the geometry and the kinematics of the geological fractures intersecting the underground laboratory. A compilation of all tectonic structures identified is presented in this paper. The underground laboratory is located in the backlimb of the Mont Terri anticline, a NNW-vergent imbricate fault-bend fold, which is characterised by a pronounced along-strike asymmetry resulting from variously oriented inherited faults. The total shortening accommodated by this structure was estimated by mass (area) balancing to be approximately 2.1 km. The Mont Terri area is significantly affected by N- to NNE-striking normal faults of the Eo-Oligocene Rhine–Bresse transfer zone and by ENE-striking faults of Late Variscan age. Depending on their orientation with respect to the transport direction towards the NNW, these faults served as oblique and frontal ramps during the subsequent Jura thrusting in the Late Miocene. The various fault systems identified in the underground rock laboratory clearly correlate with the regional-scale structures. In addition to classical structural analysis, the anisotropy of magnetic susceptibility was measured to determine the magnetic fabric and strain imprint of the Opalinus Clay. Results indicate a well developed magnetic fabric with a magnetic foliation close to the bedding, and with two distinct magnetic lineations which are probably related to the Mont Terri anticline folding and layer-parallel shortening prior to the folding. Strain imprint is more pronounced in the overturned forelimb, which is consistent with the structural data.