Abstract - Mechanical heterogeneity of a sedimentary sequence exerts a primary control on the geometry of fault zones and the
proportion of offset accommodated by folding. The Wildensbuch Fault Zone in the Swiss Molasse Basin, with a maximum
throw of 40 m, intersects a Mesozoic section containing a thick (120 m) clay-dominated unit (Opalinus Clay) over- and
underlain by more competent limestone units. Interpretation of a 3D seismic reflection survey indicates that the fault zone
formed by upward propagation of an east–west-trending basement structure, through the Mesozoic section, in response to NE–
SW Miocene extension. This configuration formed an array of left-stepping normal fault segments above and below the
Opalinus Clay. In cross-section a broad monoclinal fold is observed in the Opalinus Clay. Folding, however, is not ubiquitous
and occurs in the Opalinus Clay where fault segments above and below are oblique to one another; where they are parallel the
fault passes through the Opalinus Clay with little folding. These observations demonstrate that, even in strongly heterogeneous
sequences, here a four-fold difference in both Young’s modulus and cohesion between layers, the occurrence of folding may
depend on the local relationship between fault geometry and applied stress field rather than rheological properties alone.
Journal of the Geological Society, 177, 493-508, doi: https://doi.org/10.1144/jgs2019-052, 2020.