THE INFLUENCE OF TOPOGRAPHY ON THE EMPLACEMENT DYNAMICS OF LAVA FLOWS ON MARS

The topography encountered by a lava flow during emplacement can exert a substantial control over the thermal, rheological, and morphological evolution of the flow. For example, lava encountering a break in slope or a sharp bend in its channel is likely to develop eddies within the flow, which result in disruption of the solid surface crust and enhanced radiative cooling of the hot flow core. This cooling will lead to an increase in flow viscosity, which may in turn lead to changes in flow morphology. We have developed a new modeling approach that accounts for such effects, and which can be applied to planetary flows to constrain key flow parameters such as flow rheology, effusion rate, and eruption duration. This model couples a treatment of the thermal dynamics of the flow before and after a topographic change with a fluid dynamic treatment of eddy formation as a result of the change in flow slope or direction. The decay of the eddies with distance determines the downstream extent to which the lava crust is disrupted and the resulting amount of flow cooling. This allows the change in viscosity, and hence the change in flow morphology, to be estimated. We have identified a range of flow types on Mars, representing a diversity of flow volumes, effusion rates, durations, and topographic settings, which are amenable to treatment with our approach. For example, in the Tharsis region we have identified several long (hundreds of km), meandering flows that were emplaced over low slopes. We observe that the total flow width commonly increases at pronounced channel bends. High resolution images suggest that this is the result of repeated overflows of the channel, which produce multiple generations of levees and/or limited-volume flow lobes adjacent to the main flow. We interpret this behavior in the context of our model as the result of cooling due to lava surface disruption induced by the channel bend, which leads to increased lava viscosity and a slowing of the lava within the channel downstream of the bend. This may then cause backing up of the lava upstream until it overflows the existing channel levees. Our preliminary analyses therefore show that channel bends and slope breaks do indeed induce recognizable morphological consequences in the flow, and we are currently working to develop quantitative inferences for flow eruption parameters.