LUNAR BOULDER FIELDS AS INDICATORS OF RECENT TECTONIC ACTIVITY

Recent structural deformation of the lunar crust is evident from the presence of small, morphologically crisp lobate scarps in the lunar highlands and wrinkle ridges in the lunar maria. Past investigations have cited the presence of meter-scale boulders on the slopes of lunar wrinkle ridges as evidence for recent fault slip associated with those ridges given the short (<300 Ma) lifetime of rocks at the lunar surface (French et al., 2019; Valantinas and Schultz, 2020). However, some recently active wrinkle ridges — inferred from cross-cut craters — are observed to lack the presence of meter-scale boulder fields all together (Nypaver and Thomson, 2022). Hence, the relationship between recent tectonic activity and meter-scale boulder fields on the Moon remains unclear. In the work presented here, we test the hypothesis that lunar boulder fields indicate recent tectonic activity by characterizing the surface rock populations associated with 1,122 recently active lunar wrinkle ridges using Rock Abundance (RA) and Circular Polarization Ratio (CPR) data from the Lunar Reconnaissance Orbiter Diviner and Mini-RF instruments, respectively. We find that 54.2% of recently active wrinkle ridges measured here exhibit RA medians greater than 0.005 and 22.2% of ridges exhibit CPR medians greater than 0.5. Those wrinkle ridges with elevated RA and CPR values are predominantly located in Mare Humorum, Frigoris, Serenitatis, and Procellarum. Furthermore, weak correlations exist between wrinkle ridge RA, mean ridge slope, and background surface age — indicating that more steeply sloping wrinkle ridges on younger terrains are more likely to exhibit enhanced boulder populations. When present on slopes of wrinkle ridges, meter-scale boulder fields may signify the occurrence of recent slip along the associated fault structure given the relatively short lifetimes of meter-scale boulders at the lunar surface. However, our results suggest that recent tectonic activity does not always lead to the presence of boulder fields. Hence, defining tectonic features as recently active based on the presence of boulder fields alone may lead to a spatially incomplete assessment of recent lunar crustal deformation.