SEARCHING FOR POSSIBLE REGIONAL VARIATIONS IN BASEMENT AGE IN THE LOWLANDS OF MARS USING QUASI-CIRCULAR DEPRESSIONS (QCDS)

Quasi-Circular Depressions (QCDs) revealed in MOLA elevation data often have no visible structural expression. Those that do are visible impact basins, and we have suggested those without visible structure are buried impact basins. If true, the combination of both visible and invisible QCDs provides the best estimate of the total crater retention age (CRA) for a given region. Recent comparison of total CRAs for the martian lowlands from two regional studies (western Arabia and Ismenius Lacus) of QCDs in the 20-200 km diameter range show cumulative frequency curves that plot along the same -2 power law trend as that for the global lowland population based on QCDs> 200 km diameter. N(100) and N(50) ages for these two regions are essentially the same as those extrapolated down to smaller diameters (using a -2 power law) from the global population. That is, for the two regions studied, the age of the lowlands basement is the same as the average age of the lowlands.

To explore whether this is true in general, we have begun a more systematic study of the total population (visible plus buried) CRAs for the martian lowlands. This has possible implications for both when and by what process(es) the lowlands may have formed. We compare cumulative frequency curves and selected crater retention ages for areas in western Acidalia, northern Utopia and Amazonis along with the regions previously studied. We find some local variations in the total population crater density for QCDs smaller than 200 km diameter. For example, eastern and western Amazonis have very different N(100) ages (~1.5 and 4.5) and N(50) ages (~5.2 and 19) compared to the region as a whole (which has N(100) ~ 3.2 and N(50) ~ 16), because eastern Amazonis is largely devoid of QCDs in this size range. This could be due to more complete burial of smaller features in the eastern portion of the area, which includes young smooth and largely crater-free units bordering on Olympus Mons. By contrast the western portion includes numerous partially buried craters revealed by abundant knobs showing through the plains. At larger diameters (> 200 km) QCDs and also circular thin areas in the crustal thickness model of Neumann et al. (which may be more deeply buried impact basins) are more uniformly distributed over the region.