Sandbox modeling has been used extensively over the last decades to study the structural evolution to model shallow geological processes. The materials used to date are however not well scaled for studying rocks with a high brittleness index, i.e. materials that are strong in comparison to the mean effective stress. Typically, materials like sand and clay do not develop brittle, dilatant structures observed in rocks like basalt and carbonate at shallow crustal depth. In this work a scaled analogue model is presented, using a fine-grained, cohesive powder (CaSO4 • ½ H2O). Deformation experiments document material properties that allow scaling with respect to the natural prototypes. The tensile strength of the powder is approximately 40 Pa, depending on the state of consolidation. Compression and shear tests show that the material parameters (Young’s modulus, cohesion, porosity) also depend on the state of consolidation/compaction, whereas the friction angle remains virtually constant. The behavior of these criteria allows analogue models of brittle rocks at scales between 1:1,500 and 1:600,000 depending on the properties of the prototypes carbonate or basalt. A model of a graben structure is presented using homogeneous or layered material sequences. In the layered sequences, sand and graphite/gypsum mixtures were used to decouple the layers forming a mechanical stratigraphy. Deformation is documented by time-lapse digital photography. These datasets are analyzed by Particle Imaging Velocimetry (PIV), which produces a high-resolution displacement field as a function of time and associated measures like strain or vorticity. The results show – among others – mode-I fractures, mode-II faults with dilatant jogs, fragmentation processes in asperities, vertical gravity-driven mass transport along the fault zones, and the effect of mechanical stratigraphy. The structures formed show good resemblance with natural prototypes. The PIV output can be used for a high-resolution analysis of displacement and strain over time. Using the PIV output, elastic strain was observed prior to brittle failure, and the evolution of the fault array over time can be studied
