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.

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.

scholarly journals Microstructures and deformation mechanisms in Opalinus Clay: insights from scaly clay from the Main Fault in the Mont Terri Rock Laboratory (CH)

2016 ◽  
Author(s):  
Ben Laurich ◽  
Janos L. Urai ◽  
Christophe Nussbaum
Keyword(s):  
Ion Beam ◽  
Shear Zones ◽  
Deformation Mechanisms ◽  
Opalinus Clay ◽  
Reverse Fault ◽  
Pore Collapse ◽  
Power Law Distribution ◽  
Calcite Veins ◽  
Main Fault ◽  
Scaly Clay

Abstract. The Main Fault in the shaly facies of Opalinus Clay is a small reverse fault formed in slightly overconsolidated claystone at around one km depth. The fault zone is up to 6 m wide, with micron-thick shear zones, calcite and celestite veins, scaly clay and clay gouge. Scaly clay occurs in up to 1.5 m wide lenses, providing hand specimens for this study. After mapping of the macroscopic fabric at 10 cm–0.1 mm scale, Broad Ion Beam polishing combined with scanning electron microscopy (BIB-SEM) was used to study microscale deformation mechanisms. Results show a network of thin shear zones and microveins, separating angular to lensoid microlithons between 3 cm and 10 µm in diameter, with slickensided surfaces. Samples can be easily disintegrated into individual microlithons because of the very low tensile strength of the thin shear zones. Analyses of the size of microlithons show a power law distribution. We present a model to explain the progressive formation of a self-similar network of anastomosing thin shear zones during macroscopically continuous deformation in a fault relay. Localisation of strain in thin shear zones which are locally dilatant, and precipitation of calcite veins in dilatant shear fractures evolves into complex re-partitioning of shear, forming new shear zones at asperities while the microlithons remain much less deformed internally and the volume proportion of the µm-thick shear zones slowly increases. Grain scale deformation mechanisms are: microfracturing, boudinage and rotation of mica grains, pressure solution of carbonate fossils and pore collapse during ductile flow of the clay matrix. This provides a microphysical basis to relate the microstructures to macroscopic observations of strength and permeability of the Main Fault, and extrapolating fault properties in long term deformation.

scholarly journals Microstructures and deformation mechanisms in Opalinus Clay: insights from scaly clay from the Main Fault in the Mont Terri Rock Laboratory (CH)

Solid Earth ◽  
2017 ◽  
Vol 8 (1) ◽  
pp. 27-44 ◽  
Author(s):  
Ben Laurich ◽  
Janos L. Urai ◽  
Christophe Nussbaum
Keyword(s):  
Ion Beam ◽  
Shear Zones ◽  
Deformation Mechanisms ◽  
Opalinus Clay ◽  
Distribution Model ◽  
Reverse Fault ◽  
Deformation Localization ◽  
Calcite Veins ◽  
Main Fault ◽  
Scaly Clay

Abstract. The Main Fault in the shaly facies of Opalinus Clay is a small reverse fault formed in slightly overconsolidated claystone at around 1 km depth. The fault zone is up to 6 m wide, with micron-thick shear zones, calcite and celestite veins, scaly clay and clay gouge. Scaly clay occurs in up to 1.5 m wide lenses, providing hand specimens for this study. We mapped the scaly clay fabric at 1 m–10 nm scale, examining scaly clay for the first time using broad-ion beam polishing combined with scanning electron microscopy (BIB-SEM). Results show a network of thin shear zones and microveins, separating angular to lensoid microlithons between 10 cm and 10 µm in diameter, with slickensided surfaces. Our results show that microlithons are only weakly deformed and that strain is accumulated by fragmentation of microlithons by newly formed shear zones, by shearing in the micron-thick zones and by rearrangement of the microlithons.The scaly clay aggregates can be easily disintegrated into individual microlithons because of the very low tensile strength of the thin shear zones. Analyses of the microlithon size by sieving indicate a power-law distribution model with exponents just above 2. From this, we estimate that only 1 vol % of the scaly clay aggregate is in the shear zones.After a literature review of the hypotheses for scaly clay generation, we present a new model to explain the progressive formation of a self-similar network of anastomosing thin shear zones in a fault relay. The relay provides the necessary boundary conditions for macroscopically continuous deformation. Localization of strain in thin shear zones which are locally dilatant, and precipitation of calcite veins in dilatant shear fractures, evolve into complex microscale re-partitioning of shear, forming new shear zones while the microlithons remain much less deformed internally and the volume proportion of the µm-thick shear zones slowly increases. Grain-scale deformation mechanisms are microfracturing, boudinage and rotation of mica grains, pressure solution of carbonate fossils and pore collapse during ductile flow of the clay matrix. This study provides a microphysical basis to relate microstructures to macroscopic observations of strength and permeability of the Main Fault, and extrapolating fault properties in long-term deformation.

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