NOACHIAN-AGED LARGE IMPACT BASINS: A COMPARISON

Geological events in the Noachian Period of Mars (> 3.5 Ga) set the stage for subsequent evolution and are clues to the geodynamical processes operating then. We assess the characteristics and state of preservation of the two largest known impact basins, Utopia and Hellas. Utopia (D = ~3200 km; d = ~2.5 km), located in relatively thin crust in the northern lowlands, is highly degraded and is filled with thin layers of Amazonian Elysium flows and Upper Hesperian Vastitas Borealis Formation, which overlie ~1 km of Lower Hesperian volcanic ridged plains (Hr). Below these units, crater remnants suggest a Noachian-aged surface. No sharply defined remnants of ring structure, massifs, or basin floor structure remain; modification and fill appear to have been largely complete by the end of the Noachian. A large positive gravity anomaly is consistent with a filled basin. In contrast, Hellas (D = ~2300 km; d = ~78 km) lies in relatively thick crust in the southern highlands, is deep and morphologically fresh-appearing, is filled with Upper Hesperian sedimentary deposits overlying Lower Hesperian Hr, and is in near isostatic equilibrium. Utopia and Hellas show evidence for interior crustal thinning, attributable to impact excavation and mantle rebound, and annuli of thickened crust. Hypotheses for their different states of preservation include differences in age or thermal structure. An earlier Utopia may have formed in thinner lithosphere, and thus have undergone more initial rebound, lateral crustal flow, and floor shoaling, and/or been subject to increased impact and aqueous modification. Alternatively, if the two formed near the same time, thinner northern lowlands lithosphere could have contributed to basin shoaling. On the basis of available data, we favor the hypothesis that Utopia formed earlier than Hellas during a time of higher average heat flux and thinner lithosphere and during a period characterized by a more intense erosional environment. In this scenario, part of the subdued nature of Utopia (long wavelength) is due to initial relaxation, part (shorter wavelength) to intense erosion and degradation, and part to sedimentary and volcanic infilling. Important in distinguishing among these factors will be crater retention ages of the basins and their deposits revealed by MGS MOLA data