Modeling mobile shales under contraction: Critical analyses of new analog simulations of shale tectonics and comparison with salt-bearing systems

Abstract

Weak substrates, such as salt and mobile shales, exert a strong control on deformation styles in all structural settings, especially those undergoing contraction. Despite both materials being very weak, they are mechanically very different. Salt is weak and will flow in a ductile fashion under most geologic conditions, whereas shales only become mobile after reaching a critical state. Many sandbox-style physical or analog modeling studies have typically used a salt analog, viscous silicone polymer, as a proxy for mobile shales. However, to more accurately model mobile shale behavior, the model material needs to exhibit yield strength. One such material is Carbopol, which is made up of microgel grains that are elasto-plastic, separated by a viscous interstitial fluid. The abundance of the grains depends on the concentration of the mixture. Our results show that Carbopol does behave much differently than the traditional salt analog during contraction. Polydimethylsiloxanes typically undergoes bulk deformation and inflation under contraction, whereas Carbopol forms discrete, intense shear zones and contains zones of little to no strain where its yield strength has not been exceeded. Below the shale analog, brittle layers typically form imbricate thrust stacks, jacking up the overburden, with shear zones propagating out from thrust tips along and through the shale proxy. Strain analyses reveal complex switching of activity within the Carbopol and overlying sediments. Models reveal that even a very thin layer of Carbopol can act as a highly efficient detachment, and form more geologically realistic shortening structures, especially where these detachments are vertically stacked and horizontally offset. We believe that Carbopol is a powerful mobile-shale analog and opens new modeling directions because, as far as we are aware, this material has never been incorporated into a traditional sandbox model. Future work will seek to incorporate this material into more complex and 3D sandbox-style models.