USGS1978
The distribution of major geologic units of uniform relative age exposed in the far eastern maria (i.e., Mare Marginis, Mare Smythii, and Mare Crisium) and Mare Humorum was compiled in map form on the basis of a crater density mapping technique. Correlation of crater densities and radiometric ages of Apollo landing sites provide estimates of the absolute age for the units. Results indicate that the far eastern maria were emplaced over a period of approximately 1.25 b.y., from about 3.75 b.y. to 2.5 b.y. before present. Extensive young mare units (~ 2.5 ± 0.5 b.y.) occur in all three large far eastern maria. Major old units (~ 3.65 ± 0.05 b.y.) occur in central Mare Crisium and eastern Mare Marginis. Correlation of the age data and remote sensing data suggests that: (1) both young and old units have relatively high Mg/A1 ratios and are relatively rich in Fe and Ti, whereas intermediate age units have lower Mg/Al ratios and less Fe and Ti. A similar relationship previously noted for other maria suggests that this applied throughout the lunar maria; (2) older units generally have higher remnant magnetism than younger units - a relationship also previously noted for other lunar maria and attributed to a steady decline of a primodial lunar magnetic field, and (3) all age units in the eastern maria have relatively low natural radioactivity or abundances of Th, K, and U compared to the western maria (particularly in the Mare Imbrium area). The presence of young lava flows in both the eastern and western maria suggests that the relative abundance of these radioactive elements has played only a minor role in controlling the duration and location of mare volcanism. The crater-density relative age data presented here correlate in a linear fashion with crater-morphology relative age data. A combination of these data provides a nearly complete map of the distribution of units of uniform relative age of the lunar maria and shows that, as previously suggested, the older units (> 3.5 b.y. old) are generally located along the basin edges and the younger units (< 3.5 b.y. old) are generally in the basin centers. However, this map also shows that the vents for the flows of the young units must be located on the edges of the basins. These observations support tectonic models of basin subsidence caused by isostatic adjustment to the weight of the lava and to subsurface evacuation and subsequent collapse as the lavas were erupted.