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https://doi.org/10.5194/se-2017-55
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
21 Jun 2017
Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Solid Earth (SE) and is expected to appear here in due course.
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 1Institute for Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Lochnerstrasse 4-20, D 52056 Aachen, Germany
2Institute for Mineralogy, University Münster, Correnstraße 24, D-48149 Münster, Germany
3Swiss Geological Survey, Federal Office of Topography Swisstopo, Seftigenstrasse 264, CH-3084 Wabern, Switzerland
anow at: Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Germany
Abstract. We studied gouge from an upper-crustal, low offset reverse fault in slightly overconsolidated claystone in the Mont Terri rock laboratory (CH). The laboratory is designed to evaluate the suitability of the Opalinus Clay formation (OPA) to host a repository for radioactive waste.

The macroscopically dark gouge displays a matrix-based, P-foliated microfabric bordered and truncated by μm-thin shear zones consisting of aligned clay grains, as shown by BIB-SEM and optical microscopy. TEM-SAED shows evidence for randomly oriented nm-sized clay particles in the gouge matrix, surrounding larger elongated phyllosilicates with a strict P-foliation. For the first time in 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 undeformed OPA.

We present two hypotheses to explain the origin of gouge in the Main Fault: (i) "authigenic generation": fluid-mediated removal of calcite from deforming OPA during shearing, (ii) and "clay smear": 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 which is weaker than OPA because of the lower fraction of hard grains. We infer that the long-term rheology of gouge is more strongly rate-dependent than suggested from laboratory experiments.


Citation: Laurich, B., Urai, J. L., Vollmer, C., and Nussbaum, C.: Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH), Solid Earth Discuss., https://doi.org/10.5194/se-2017-55, in review, 2017.
Ben Laurich et al.
Ben Laurich et al.

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Short summary
In Switzerland, the Opalinus Clay (OPA) formation is favored to host a repository for nuclear waste. Inevitable 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 defoms in a more viscous manner than undeformed OPA at laboratory conditions. Moreover, we speculate about the origin and evolution of the gouge layer.
In Switzerland, the Opalinus Clay (OPA) formation is favored to host a repository for nuclear...
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