Tectonic evolution around the Mont Terri rock laboratory, northwestern Swiss Jura: constraints from kinematic forward modelling

Abstract

We propose a geometrically, kinematically, and mechanically viable thin-skinned kinematic forward model for a cross section intersecting the Mont Terri rock laboratory in the frontal-most part of the Jura fold-and-thrust belt, Switzerland. In addition to the available tunnel, borehole, and surface data, initial boundary conditions are crucial constraints for the forward modelling scenarios, especially the inherited topography of the basement and any pre-compressional offset within the Mesozoic sediments. Our kinematic analysis suggests an early-stage formation of the Mont Terri anticline located above ENE-trending, Late Paleozoic extensional faults, followed by back-stepping of the deformation developing the Clos du Doubs and Caquerelle anticlines further south. In this model, the thrust sequence was dictated by the inherited basement faults, which acted as nuclei for the ramps, detached along the basal décollement within the Triassic evaporites. The mechanical viability of both the thrust angles and thrust sequence was demonstrated by applying the limit analysis theory. Despite numerous subsurface geological data, extrapolation of structures to depth remains largely under-constrained. We have tested an alternative model for the same cross section, involving an upper detachment at the top of the Staffelegg Formation that leads to duplication of the sub-Opalinus Clay formations, prior to detachment and thrusting on the Triassic evaporites. This model is geometrically and kinematically viable, but raises mechanical questions. A total displacement of 2.9 and 14.2 km are inferred for the classical and the alternative scenarios, respectively. In the latter, forward modelling implies that material was transported 10.8 km along the upper detachment. It is not yet clear where this shortening might have been accommodated. Despite the differences in structural style, both models show that pre-existing basement structures might have interfered in time and space. Both styles may have played a role, with lateral variation dictated by basement inherited structures.

Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH)

“…We found amorphous, well rounded SiO 2 in the gouge. The SEM-visible porosity of gouge is strongly reduced compared to the undeformed OPA; however, an increased porosity for gouge from pores smaller than the SEM resolution limit (see Orellana et al, 2016) is possible.…”

“…The undeformed OPA has 18 % porosity as the best estimate (Nussbaum and Bossart, 2008;Houben et al, 2014). In recent work, high porosities for the OPA gouge (> 20 %) were found using mercury injection porosimetry and He pycnometry in Orellana et al (2016), dominated by pores < 14 nm. However, it is unclear to what extent the applied methods are comparable to the BIB-SEM findings: are artificial openings excluded and are poro-elastic effects considered (see Sigal, 2009;Klaver et al, 2012;Hemes et al, 2013;Houben et al, 2014) …”

Seismotectonic analysis around the Mont Terri rock laboratory (Switzerland): a pilot study

“…Thus, based on our analysis it was possible to determine three synthetic focal mechanisms from the microseismic events near the Mont Terri rock laboratory. These are linked with the major thrust faults interpreted in the Mont Terri ramp-related anticline, especially with the basal ramp and the Main Fault known from the tunnel system (Nussbaum et al 2017). …”

“…A high number of b Fig. 1 Nussbaum et al 2017) with backsection hemisphere projection of focal mechanisms of microseismic events in the vicinity of the Mont Terri rock laboratory corresponding to the sub-groups in Fig. 4 recording stations and a spatial distribution with a good radial coverage will considerably improve constraining the focal mechanism solution using the unsupervised grid search approach.…”

“…The highest correlation factor between the naturally generated waveform and the synthetic waveforms represents the maximum likelihood estimate of the possible focal mechanism solution. The locations of the earthquakes from the microseismic catalog are further evaluated for their depth in relation to the geological context (see Freivogel and Huggenberger 2003;Nussbaum et al 2017). Subsequently, we determine in which major geological unit (sedimentary cover or basement) each earthquake is located.…”