Topographic Variability: Implications for Lava Flow Modeling

Introduction: Glaze et al. [1] and Baloga et al. [2] have both recently indicated the importance of understanding the scale of a lava flow relative to the surrounding topography in extracting inferences about rheologic changes and the interpretation of the style of emplacement. The underlying scientific issue is whether flow emplacement is controlled by the largescale topographic inclination or whether the flow path and emplacement style are substantially influenced by small-scale topography and pre-existing lava flows. The as yet unanswered question is “What scale of underlying topography is significant for modifying emplacement behavior?” Baloga et al. [2] suggested that the flat topography found on the plains of Mars does not disrupt the emplacement of long thick flows sufficiently to expose the core. Thus there is virtually no cooling of the core and we do not see any evidence for a bulk viscosity increase over the length of the flow. Conversely, flows over “rougher” topography are shorter, exhibiting a greater degree of thickening and associated viscosity increase [1]. In addition to the science issues driving our desire to understand the scale of topographic variability, recent efforts have shown that reliable flow thickness profiles can be derived from MOLA Precision Experiment Data Records (PEDRs) [1]. For large flows (significantly thicker than surrounding topographic variation), we can use gridded MOLA data [2] to find flow thicknesses. However, as the flow thickness approaches the same scale as the surrounding topographic variations, the gridded MOLA data become increasingly unreliable. Intuitively, it seems that the influence of variable topography must be in some way measured relative to the thickness of the flow in question, i.e, a 100 m thick flow would be sensitive to topography at a much greater scale than a 10 m flow. Here, we present a method for quantifying topographic variability relative to lava flow thickness that uses MOLA PEDR profiles directly. The basic method is relatively straightforward, but very effective for characterizing topography. Methodology: Our approach is to analyze the residuals that result from standard statistical regression techniques applied to the MOLA PEDR profiles over several degrees in the region of a lava flow. Figure 1a shows seven MOLA PEDR profiles in the region north of Elysium Mons. The arrows in Figure 1a point to a 10 40 m thick lava flow oriented roughly perpendicular to the MOLA PEDR profiles. We can see that the thickness of this lava flow is at a similar scale to the ambient topography and that it is difficult to even identify the flow from the proffles alone [1].