Mudflow rheology under disequilibrium conditions: implications for the interpretation of Martian flow deposits

The presence of sedimentary volcanism on Mars has been proposed as an explanation for many characteristic features in areas such as Chryse, Acidalia, and Utopia Planitia. Orbital investigations and rovers have identified the presence of clay minerals including smectite, kaolinite, and chlorite on the Martian surface. However, the specific composition (lava vs. mud) of most flow deposits cannot be confidently described due to the scarcity of data. Interpreting the past behaviour of flow deposits on terrestrial or planetary bodies requires a comprehensive knowledge of the flow rheology. As such, constraining the composition of remotely observed flows relies on the use of rheological models. However, the rheological behaviour of sedimentary flows is not well constrained, especially under Martian conditions. The lower pressure, temperature, and gravity on Mars have been shown to produce different propagation conditions of sediment-water mixtures compared with those on Earth, highlighting the importance of investigating mudflow behaviour under Martian conditions through analogue experiments. Here, we choose a non-swelling kaolinite clay to firstly investigate the rheological behaviour of a clay-water suspension under different shear-rates and solid volume fractions. We analyse the relationship between yield stress, τ_y , and solid volume fraction, φ, to select realistic input values for modelling remote sedimentary flows on both Earth and Mars. We find the Herschel-Bulkley model provides the best fit to laboratory rheological data, but the Bingham model provides more utility with remotely sensed datasets. We then investigate the effects of simultaneous external cooling and internal frictional heating of our kaolinite clay-water mixtures, assessing the balance between the two processes. We find that the control of these disequilibrium conditions varies with both φ and the shear-rate, γ̇, (i.e., the flow velocity). For all values of φ, at high γ̇, we find that complete freezing/jamming is delayed compared with lower values of γ̇. We assess the morphology of inferred sedimentary flow deposits in Chryse Planitia by quantifying their flow length, local slope angle, flow thickness, and surface textures. Alongside our experimental data, these remotely sensed parameters serve as inputs for a non-Newtonian plug model designed to estimate realistic flow properties. This integrated approach allows us to better constrain the origin and composition of the Martian deposits.