The Structure and Evolution of Ancient Impact Basins on Mars

The number of Martian impact basins, once thought deficient owing to a low number of impactors, has been significantly increased through the identification of structural imprints that remain even after nearly complete erasure of basin massifs. Five ancient multiringed impact basins were selected for detailed study and exhibit a systematic pattern of rejuvenation of the multiring plan through endogenic modification. Concentric zones of unstable terrain are believed to reflect deep-seated, impact-related fractures that have localized regional igneous processes in a manner analogous with the basin-controlled istribution of basalts on the moon. Most Martian outflow channels were found to originate along the unstable ring zones. Hydrothermal melting and temporary confinement of channel source materials at sites analogous to lunar sinuous rille source regions are proposed to play an important role in the distribution of Martian outflow channels. Ancient basins also can be delineated by the concentric pattern of topographically and structurally controlled narrow valley networks that are particularly abundant along the outer rings where basin ejecta deposits occur. Remnant massifs and scarps commonly exhibit extensive furrowing by narrow valley networks, perhaps due to a different paleoclimate or sapping during a period of geothermal melting of trapped ices. The systematic patterns of valley networks, stable and unstable annuli, scarps, and massifs provide bases for interpreting impact structure in general and for recognizing multiringed structures elsewhere. The Chryse basin, in particular, is proposed to represent a major multiringed structure that controlled the formation and distribution of eastern Vailes Marineris and the chaotic terrains of Margaritifer Sinus in response to volcanism/tectonism related to Tharsis. We conclude that (1) Mars is probably not deficient in large-body impactors; (2) the multiring patterns of basins represent deep-seated fracture zones that can be reexposed through subsequent volcanism, hydrothermal activity, sapping, and differential erosion; (3) many Martian terrains and features (e.g., chaotic terrains and outflow channels) are controlled by ancient basins; and (4) the resurrection and desiruction of impact basins reveal an important third dimension (depth) for interpreting impact basin formation.