What model material to use? A Review on rock analogs for structural geology and tectonics

Analog, or physical, models have been used to investigate problems in structural geology and tectonics for over two centuries. Together with field observations and theoretical considerations they form the basis for significant advances in our understanding of structural and tectonic evolution in space and time. A fundamental requirement for the design of a successful scaled experiment is the appropriate choice of model material. Scaling relationships between the rock analog and nature have to be established and the interactions between the rock analog and the boundary conditions, provided by the experimental apparatus, investigated. With an increased understanding of the rheological properties of Earth's crust and mantle, model materials with corresponding rheology have to be developed and/or established. Over the past two hundred years, a wide variety of model materials have been used in deformation experiments. While some materials, such as sand and clay, are still used today, many new materials have been tested and used in experiments in more recent times. Furthermore, new quantification methodologies, such as particle image velocimetry, require model materials with specific properties. While the specific conditions are unique to each experiment we present here a review of commonly used analog modeling materials in experiments that capture structural and tectonics processes. This review focuses on 1) scaling approaches; where we discuss alternatives to the classic dynamic scaling in structural geology and tectonics experiments. 2) rock analogs used to model deformation within the upper crust characterized by localized failure, 3) materials used to model the lower crust and mantle characterized by distributed deformation, and 4) different approaches to model the semi-brittle middle crust. We discuss all the model materials in light of the different scaling approaches. Furthermore, we provide a best-practice section intending on helping researchers new to the field of analog modeling to choose the appropriate rock analog.