On the relative and absolute ages of seven lunar front face basins: I. From Viscosity Arguments

The seven basins, Orientale, Imbrium, Crisium, Nectaris, Humorum, and an unnamed basin between Werner and the Altai Ring show rims whose absolute and relative heights are correlated with the sharpness and crispness of the features. On the assumption that the decline in average outer rim height, not scarp height, measures the age of the basin and also that the decline represents a hot creep of rocks of very high viscosity, absolute ages were derived. Basins were found to increase in age in the sequence listed above, with a range from about 3.82 to 4.30 × 10 9 years. The average or effective viscosity of the surface layers down to whatever level was involved in the creep was calculated as increasing from 9.46 × 10 24 poises at about 4.30 × 10 9 years to 1.86 × 10 30 poises at present. It should be clear at the outset what the assumptions and associated observations are and why they are necessary to a solution. They will be listed in this abstract and expanded upon in the text. 1. (1)The original rim height of each basin was a function only of basin diameter. 2. (2)The original rim height was given by Pike's (1983) relation for fresh craters extrapolated to basin diameters. 3. (3)The present rim height is that of the most prominent ring structure. 4. (4)The smaller rim height of all seven basins, relative to the height predicted by (2) is due largely to creep in the lunar rocks down to some undetermined level. Other forces may contribute to the sinking of the rims, but these are considered to be of lesser importance and are discussed in the text. 5. (5)The relative ages of the seven basins are as given in Table I. This sequence differs slightly from that of Wilhelms (1984), for example, but it is that found in Baldwin (1974, 1987) and is consistent with the results of this paper. 6. (6)The age of formation of the Imbrium basin (3.85 × 10 9 years) inferred from lunar sample studies (particularly Apollo 15) is correct. 7. (7)The age of formation of the Serenitatis basin (3.87 +/− .04) × 10 9 years, inferred from petrologic and geochemical studies of Apollo 17 boulders is incorrect. This is not an assumption, but is a result of the analysis of this paper. 8. (8)The rheology of the Moon may be described, for the purposes of this paper, by an effective viscosity valid throughout the layers involved in the creep. 9. (9)This effective viscosity is used as a tool to determine basin ages and is not important in itself. It does appear to vary in the same range as terre