Distinct Element Modelling of Landslides in Mechanical Multilayers on Mars

Mass wasting events such as landslides are an important component of the processes that have shaped the surface of Mars. Landslides are interpreted to have been active during much of the geologic history of Mars including the very recent past. The main scarp and displaced materials of landslides can tell us much about the mechanical nature of the surface and shallow subsurface of Mars. We use vertical two-dimensional distinct element models parallel with the slide direction to examine the effects of mechanical layering upon the morphology of slip surfaces and scarps that form as a result of slope failure on Mars. Bulk layer mechanical properties incorporated into the models and scaled to values likely be present on Mars include density, tensile strength, Young's modulus, Poisson's ratio, internal friction angle, cohesive strength, and unconfined compressive strength. Here we model horizontal layers with thickness range of 100 m to 500 m for a total thickness of 2500 m. Initial geometry is a 5 km long rectangle under conditions of Mars gravity where the top surface and one lateral boundary are free surfaces, and the horizontal base and opposing lateral boundary are rigid surfaces with friction coefficient of 0.5. Each layer represents one of five rock strengths, with strongest (strong basalt) to weakest (unconsolidated deposits) unconfined compressive strengths of 83, 44, 25, 8, and 2 MPa, respectively. Our models show that an initial slip surface forms some distance from the lateral free surface and subsequently migrates away from the free surface in discrete increments with concomitant decreasing slope of successive failure surfaces. Relative and absolute layer strength, thickness, and order control the morphology of the failure surfaces, the location and shape of the initial failure surface, and the kinematics of displaced material. In general, the size of coherent blocks and tendency towards sliding and spreading of displaced blocks increases with layer strength. In most cases, kinematic styles such as toppling, sliding, spreading, flow, and fall occur in some combination during development of a final kinematically stable displacement surface, although not all styles occur simultaneously nor persist throughout the modelled landslide event. Landslides are commonly observed in the Valles Marineris region on Mars, including portions of Coprates Chasma. Our models show strong similarities to landslide features observed on the north wall of Coprates Chasma, including features that are likely influenced by mechanical layering in the canyon walls. These similarities include slip surface shapes and apparent remnant layer blocks in the displaced materials. Our preliminary results demonstrate the power of distinct element modelling as an appreciable step toward a new understanding of the stratigraphy and mechanical nature of the upper crust of Mars.