Long-Wavelength Shoreline Deformation on Mars

A number of geologic and topographic features within the northern plains of Mars have been interpreted as shorelines formed by ancient oceans. Several recent studies have challenged this interpretation, arguing that the present topographic profiles do not appear to originate from surfaces of equal gravitational potential. Elevations along the ``shorelines'' are especially variable at long wavelengths (thousands of km), with amplitudes of hundreds of meters to kilometers. To test the hypothesis that the features in the northern plains are deformed shorelines, we compare the long-wavelength topography (solid surface position relative to the areoid) of the two most prominent shorelines (the Arabia and Deuteronilus contacts of Clifford and Parker, Icarus, 2001) with the deformation expected for: (1) flexural response of the lithosphere to surface loading from the growth of Tharsis and ocean (i.e., sea level) redistribution, (2) true polar wander (TPW), and (3) dynamic topography linked to internal convective flow. We find that TPW and dynamic topography are both capable of reconciling the longest-wavelength variation in topography (the former is a purely degree two signal). The predicted TPW path that best fits the shoreline record is a function of the effective elastic thickness of the Martian lithosphere and it is consistent with recent inferences based on paleomagnetic evidence. The inference is also compatible with our recent re-analysis of the rotational stability of Mars subject to Tharsis and internal loading, which we briefly summarize.