Articles | Volume 9, issue 1
Solid Earth, 9, 1–24, 2018
Solid Earth, 9, 1–24, 2018

Research article 09 Jan 2018

Research article | 09 Jan 2018

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

Ben Laurich1,a, Janos L. Urai1, Christian Vollmer2, and Christophe Nussbaum3 Ben Laurich et al.
  • 1Institute for Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Lochnerstrasse 4–20, 52056 Aachen, Germany
  • 2Institute for Mineralogy, University of Münster, Correnstraße 24, 48149 Münster, Germany
  • 3Swiss Geological Survey, Federal Office of Topography Swisstopo, Seftigenstrasse 264, 3084 Wabern, Switzerland
  • anow at: Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hanover, Germany

Abstract. We studied gouge from an upper-crustal, low-offset reverse fault in slightly overconsolidated claystone in the Mont Terri rock laboratory (Switzerland). The laboratory is designed to evaluate the suitability of the Opalinus Clay formation (OPA) to host a repository for radioactive waste.

The gouge occurs in thin bands and lenses in the fault zone; it is darker in color and less fissile than the surrounding rock. It shows a matrix-based, P-foliated microfabric bordered and truncated by micrometer-thin shear zones consisting of aligned clay grains, as shown with broad-ion-beam scanning electron microscopy (BIB-SEM) and optical microscopy. Selected area electron diffraction based on transmission electron microscopy (TEM) shows evidence for randomly oriented nanometer-sized clay particles in the gouge matrix, surrounding larger elongated phyllosilicates with a strict P foliation. For the first time for the OPA, we report the occurrence of amorphous SiO2 grains within the gouge. Gouge has lower SEM-visible porosity and almost no calcite grains compared to the undeformed OPA.

We present two hypotheses to explain the origin of gouge in the Main Fault: (i) authigenic generation consisting of fluid-mediated removal of calcite from the deforming OPA during shearing and (ii) clay smear consisting of mechanical smearing of calcite-poor (yet to be identified) source layers into the fault zone. Based on our data we prefer the first or a combination of both, but more work is needed to resolve this.

Microstructures indicate a range of deformation mechanisms including solution–precipitation processes and a gouge that is weaker than the OPA because of the lower fraction of hard grains. For gouge, we infer a more rate-dependent frictional rheology than suggested from laboratory experiments on the undeformed OPA.

Short summary
In Switzerland, the Opalinus Clay (OPA) formation is favored to host a repository for nuclear waste. Thus, we must know its deformation behavior. In this study, we focused on the microstructure of gouge, a thin (< 2 cm), drastically strained clay layer at the so-called Main Fault in the Mont Terri rock laboratory. We suggest that in situ gouge deforms in a more viscous manner than undeformed OPA in laboratory conditions. Moreover, we speculate about the origin and evolution of the gouge layer.