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

Over the last 18 years we have extensively studied an intra-Opalinus Clay fault zone that crops out within the Mont Terri rock laboratory in NW-Switzerland. We performed micro- and macrostructural characterization on four outcrops of this so-called “Main Fault”, which crosscuts the entire Mont Terri rock laboratory. Combining detailed structural mapping, analysis down to the nanometer-scale, and geophysical investigations leads to a better understanding of fault zones within clay-dominated lithologies. The multi-scale, multi-technique approach that we applied in this study on four individual outcrops is critical for describing such a complex system. In these four outcrops, we differentiate five macroscopic structural elements of the Main Fault and have studied their occurrence and spatial distribution. In general, scaly clay, including S–C bands (S = “schistosité” = cleavage, C = “cisaillement” = shear parallel to shear zone boundaries) and microfolds, occurs in isolated, sharply bound lenses and in larger zones at the top and bottom of the Main Fault. A cm-thin, continuous layer of gouge runs along the upper boundary of the fault zone. The non-scaly part shows rather low strain and consists of rhombohedral blocks of undeformed rock (horses), bound by slickensides. The µm-thin shear zones are considered to be elementary building blocks for the structural elements of the Main Fault. Direct comparison of the four studied outcrops to each other highlights the significant lateral variability of the Main Fault. In addition to a reduction in thickness from west to east, size and distribution of structural elements are highly variable. Correlation of these structural elements between closely spaced outcrops is not possible. Fortunately, the upper and lower boundary of the Main Fault, as well as thicker sheets of scaly clay, can be recognized using seismic attributes and thus yield indirect information about size and internal structure of fault zones in clay.