SEISMIC RESETTING OF THE CRATER RECORD AROUND LUNAR LOBATE-SCARP THRUST FAULTS

Previous studies estimated lunar lobate-scarp ages based on crater degradation rates (Binder and Gunga, 1985), the estimated lifetimes of the scarp’s fresh morphologies and associated small graben (Watters et al., 2010; 2012), or via buffered and/or traditional crater size-frequency distribution (CSFD) measurements (van der Bogert et al., 2012; Senthil Kumar et al., 2016; Clark et al. 2017). Using Lunar Reconnaissance Orbiter Narrow Angle Camera (NAC) images and NAC-derived/LOLA-SELENE digital terrain models, we compared the application of both CSFD approaches for determination of absolute model ages (AMAs) by re-examining five scarps previously dated by Binder and Gunga (1985).

In three cases, the traditional CSFDs produced ages (77, 79, 160, 98, 84 Ma) consistent with those from buffered counts (75, 61, 75, 56, 91 Ma), but the traditional counts have smaller error bars due to the larger number of craters measured. This indicates in these cases that fault activity is directly related to surface renewal adjacent to and even kilometers away from the scarps. We also observe that small craters (e.g., <20 m in diameter at Mandel’shtam-3) near the scarps exhibit fresh morphologies, whereas larger craters (typically >50 m in diameter at Mandel’shtam-3) exhibit highly degraded rims, hummocky textures, and are frequently funnel-shaped. Such features were attributed to seismic shaking effects (Schultz and Gault, 1975). The diameter ranges of the small fresh craters correspond to the fit ranges for young AMAs that define a resurfacing event indicative of scarp formation.

Thus, the study of CSFDs in the regions surrounding scarps can provide information about the extent and severity of scarp-related seismicity. Low to moderate ground motion due to scarp-generated shallow moonquakes was inferred based on the low runout efficiency of certain fallen boulders in Schrödinger basin (Senthil Kumar et al., 2016) – an interpretation consistent with our conclusion that scarp formation causes local resurfacing due to seismic shaking.

References: Binder and Gunga (1985) Icarus 63, 421; Clark et al. (2017) LPSC 48, 1001; Senthil Kumar et al. (2016) JGR 121, 147. Schultz and Gault (1975) PLPSC 6, 2845; van der Bogert et al. (2012) LPSC 43, 1847. Watters et al. (2010) Science 329, 936; Watters et al. (2012) Nature Geo., 10.1038/NGEO1387.