scholarly journals Geochemical signature of paleofluids in microstructures from Main Fault in the Opalinus Clay of the Mont Terri rock laboratory, Switzerland

2017 ◽  
Vol 110 (1) ◽  
pp. 105-128 ◽  
Author(s):  
Norbert Clauer ◽  
Isabelle Techer ◽  
Christophe Nussbaum ◽  
Ben Laurich
Keyword(s):  
Opalinus Clay ◽  
Geochemical Signature ◽  
Rock Laboratory ◽  
Main Fault ◽  
Mont Terri ◽  
Mont Terri Rock Laboratory

scholarly journals 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 ◽  
2018 ◽  
Vol 9 (1) ◽  
pp. 1-24 ◽  
Author(s):  
Ben Laurich ◽  
Janos L. Urai ◽  
Christian Vollmer ◽  
Christophe Nussbaum
Keyword(s):  
Electron Microscopy ◽  
Fault Zone ◽  
Shear Zones ◽  
Deformation Mechanisms ◽  
Opalinus Clay ◽  
Rock Laboratory ◽  
Main Fault ◽  
Clay Formation ◽  
Mont Terri ◽  
Mont Terri Rock Laboratory

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.

Tectonic structure of the “Main Fault” in the Opalinus Clay, Mont Terri rock laboratory (Switzerland)

2017 ◽  
Vol 110 (1) ◽  
pp. 67-84 ◽  
Author(s):  
David Jaeggi ◽  
Ben Laurich ◽  
Christophe Nussbaum ◽  
Kristof Schuster ◽  
Peter Connolly
Keyword(s):  
Opalinus Clay ◽  
Tectonic Structure ◽  
Rock Laboratory ◽  
Main Fault ◽  
Mont Terri ◽  
Mont Terri Rock Laboratory

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

2017 ◽  
Author(s):  
Ben Laurich ◽  
Janos L. Urai ◽  
Christian Vollmer ◽  
Christophe Nussbaum
Keyword(s):  
Shear Zones ◽  
Deformation Mechanisms ◽  
Opalinus Clay ◽  
Reverse Fault ◽  
Amorphous Sio2 ◽  
Rock Laboratory ◽  
Main Fault ◽  
Clay Formation ◽  
Mont Terri ◽  
Mont Terri Rock Laboratory

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.

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