Possible Hydroisostatic Influences on the Collective Geometry of Strandline Features Formed in Association with Ancient Martian Oceans

Overview: Some workers have proposed that large Martian basins contained extensive water bodies in the past, and have hypothesized that an ancient ocean extended over much or all of the northern lowland plains. With considerable depths over large areas, these water bodies would have represented massive surficial loads. The irregular form of some Martian basins, as well as spatial inhomogeneities in crustal and mantle properties , would have caused the magnitudes of vertical crustal displacements related to water loading to vary spatially. Similarly, the magnitudes of crustal displacements related to crustal unloading, associated with the gradual removal of water mass, would also have varied spatially. Terrestrial studies demonstrate that spatial variation in the displacement effects of oceanic loading and unloading can exceed 10% of the magnitude of vertical changes in water level (and when occurring in concert with other mass-transfer processes associated with e.g. glacial cycles, can exceed 200% of this magnitude). This suggests by analogy that hy-droisostatic processes alone may be sufficient to cause ancient Martian oceanic shorelines of common age to vary in elevation today by hundreds of meters. The modern horizontality of sets of possible strandline features should therefore not be considered a necessary precondition for their interpretation as having formed in association with a large water body; in some cases, the modern horizontality of such features may in fact act as evidence against their formation in this manner. Quantitative models of large water bodies on Mars are being constructed for use in the estimation of the effects of hydrological loading and unloading on the collective geometry of strandline features. Introduction: The past occurrence of the flow of surface water on Mars [e.g., 1-4] implies that there would likely have been, for some period of time, im-poundment of water in catchment basins to form small and possibly large surface water bodies [e.g., 5,6]. Large Martian basins in both the lowlands and highlands are proposed to have contained extensive water bodies in the past [7-10], and an ancient ocean is hypothesized to have extended over much or all of the northern lowland plains [6,11-15]. Terrestrial Analogs of Differential Crustal Loading by Large Water Bodies: Both solid and liquid water bodies at the surface of the Earth act to load the crust, and changes in their size and extent can cause significant crustal displacement. For example, during continental glaciation, the formation and expansion of an ice sheet causes elastic …