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

Abstract

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.

The Sealing Mechanisms of a Fracture in Opalinus Clay as Revealed by in situ Synchrotron X-Ray Micro-Tomography

“…An eventual axial dilation of the bulk sample is not measurable given the "local area" setup used for the experiment. The deformation microstructures observed close to the fracture plane display affinities with those observed in the deformed zones at the Mont Terri site (Laurich et al, 2018).…”

Brittle Deformation of Carbonated Peridotite—Insights From Listvenites of the Samail Ophiolite (Oman Drilling Project Hole BT1B)

“…Clast sizes from 50–50,000 μm 2 in a prominent cataclasite at 31.4 m depth (Sample BT1B_17‐2_55‐60) follow a power law distribution with D values of ~1.9–2.0 (Figure 5). Such D values are typical for cataclasites and fault gouges that are characterized by extensive clast rotation and displacement (e.g., Billi, 2007; Keulen et al, 2008; Laurich et al, 2018). Cataclasites that overprint preexisting cataclasite contain fewer large clasts and a higher matrix fraction.…”

“…To measure the grain size distributions of clasts, the relative frequency of grain areas normalized to the image area and the bin width have been calculated as the number of grains N i in each bin i , using five logarithmic spaced bins per order of magnitude. Plotting the relative frequency against the grain area in log‐log plots yields the local linear slope D of a possible power law distribution (Keulen et al, 2008; Laurich et al, 2018).…”

Micro-cracking and incipient shear microstructures at low-strain deformation of Opalinus Clay: Insights from triaxial testing and broad-ion-beam scanning-electron-microscopy (BIB-SEM)

“…Subsequently, the surfaces of the sub-samples were polished using the BIB surface polishing procedure (Leica TIC3X) using a low incident Ar-ion beam angle (10.5 ° at 7kV for 45 min followed by 4.5° at 5 KV for 4 hours and for some selected sub-samples followed by 4.5° at 5kV for 18 hours, similarly to e.g. Schuck et al, 2020;Laurich et al, 2018;Oelker, 2019).…”

“…Currently, only few studies exist which investigate grain-scale processes using highresolution images. Laurich et al, 2014Laurich et al, , 2017Laurich et al, and 2018 microlithons. The formation of gauge is associated to both brittle deformation including micro-fracturing and cataclasis, pressure solution and ductile flow of the clay matrix forming relays which ultimately lead to the formation of scaly clay.…”