Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH)
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. <br&...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2018-01-01
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| Series: | Solid Earth |
| Online Access: | https://www.solid-earth.net/9/1/2018/se-9-1-2018.pdf |
| _version_ | 1818097256466743296 |
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| author | B. Laurich B. Laurich J. L. Urai C. Vollmer C. Nussbaum |
| author_facet | B. Laurich B. Laurich J. L. Urai C. Vollmer C. Nussbaum |
| author_sort | B. Laurich |
| collection | DOAJ |
| description | 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.
<br><br>
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 SiO<sub>2</sub> grains within
the gouge. Gouge has lower SEM-visible porosity and almost no calcite grains
compared to the undeformed OPA.
<br><br>
We present two hypotheses to explain the origin of gouge in the Main
Fault: (i) <q>authigenic generation</q>
consisting of fluid-mediated removal of calcite from the deforming OPA during
shearing and (ii) <q>clay smear</q> 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.
<br><br>
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. |
| first_indexed | 2024-12-10T23:17:38Z |
| format | Article |
| id | doaj.art-88eef87ab6bd4dc2a56a58e82e1c5720 |
| institution | Directory Open Access Journal |
| issn | 1869-9510 1869-9529 |
| language | English |
| last_indexed | 2024-12-10T23:17:38Z |
| publishDate | 2018-01-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Solid Earth |
| spelling | doaj.art-88eef87ab6bd4dc2a56a58e82e1c57202022-12-22T01:29:47ZengCopernicus PublicationsSolid Earth1869-95101869-95292018-01-01912410.5194/se-9-1-2018Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH)B. Laurich0B. Laurich1J. L. Urai2C. Vollmer3C. Nussbaum4Institute for Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Lochnerstrasse 4–20, 52056 Aachen, Germanynow at: Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hanover, GermanyInstitute for Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Lochnerstrasse 4–20, 52056 Aachen, GermanyInstitute for Mineralogy, University of Münster, Correnstraße 24, 48149 Münster, GermanySwiss Geological Survey, Federal Office of Topography Swisstopo, Seftigenstrasse 264, 3084 Wabern, SwitzerlandWe 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. <br><br> 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 SiO<sub>2</sub> grains within the gouge. Gouge has lower SEM-visible porosity and almost no calcite grains compared to the undeformed OPA. <br><br> We present two hypotheses to explain the origin of gouge in the Main Fault: (i) <q>authigenic generation</q> consisting of fluid-mediated removal of calcite from the deforming OPA during shearing and (ii) <q>clay smear</q> 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. <br><br> 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.https://www.solid-earth.net/9/1/2018/se-9-1-2018.pdf |
| spellingShingle | B. Laurich B. Laurich J. L. Urai C. Vollmer C. Nussbaum 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 |
| title | Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH) |
| title_full | Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH) |
| title_fullStr | Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH) |
| title_full_unstemmed | Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH) |
| title_short | Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH) |
| title_sort | deformation mechanisms and evolution of the microstructure of gouge in the main fault in opalinus clay in the mont terri rock laboratory ch |
| url | https://www.solid-earth.net/9/1/2018/se-9-1-2018.pdf |
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