52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2245.pdf

EVIDENCE FOR THE LUNAR PROCELLARUM BASIN: INSIGHTS FROM THE SPATIAL DISTRIBUTION OF FLOOR-FRACTURED AND CONCENTRIC CRATERS. S. Ravi and M.S. Robinson, School of Earth and Space Exploration, Arizona State University, Tempe, AZ – 85282, USA (Email: [email protected])

Introduction: Floor-fractured (FFCs) and Camera (LROC) Wide Angle Camera (WAC) GLD 100 DTM concentric craters (CCs) are impact craters that [13]. underwent modification by volcano-tectonic processes Both FFCs and CCs appear to be clustered within such as viscous relaxation or magmatic intrusion impact basins, with >60% of FFCs and CCs following basin-forming events (Fig. 1) [1-6]. concentrated along the western boundary of the proposed Procellarum basin [10] (Fig. 2). Whether the spatial distribution of FFCs and CCs are random or genetically related to impact basins is not well understood. If the latter is true, then we can not only use FFCs and CCs as proxies for identifying ancient basin Figure 1: Characteristics of floor-fractured and concentric rims, but we can also infer local and regional variations craters. (A) Vitello crater (D = 40km) exhibits radial/concentric fractures, (B) Gassendi crater (D = 100km) in the strength of the lunar lithosphere. Therefore, the exhibits mare infill, and (C) an unnamed concentric overarching goal of this study is to determine and crater (D = 5km) exhibits an uplifted concentric ridge. analyze the statistical significance of the spatial distribution of FFCs and CCs with respect to impact The Procellarum KREEP Terrane (PKT) is one of basins. three major crustal provinces on the [7]. Data Methods: We used the multi-distance spatial from the Lunar Prospector Gamma Ray Spectrometer clustering analysis (Ripley’s K-function or RKF) tool shows that the PKT is a geochemically distinct province (ArcGIS) to compute the spatial dependence of FFCs whose boundary is marked by the abundance of rare and CCs over a range of distances, particularly with earth elements (REEs; Fig. 2) [8]. There are two respect to concentric peak rings of impact basins (Fig. 3). competing hypotheses for the origin of the PKT. 1) The The RKF computes two correlation values: observed presence of the PKT and the crustal asymmetry is K value and expected K value. If observed K value > attributed to the formation of a “Gargantuan Basin” later expected K value, then the population is clustered. If known as the Procellarum basin early in lunar history at the observed K value < expected K value, then the ~ 4.1-4.2 Ga [9-10]. The Procellarum basin is bound by population is dispersed. to the north, Mare Vaporum to the east, The diff K value (observed K – expected K) Mare Cognitum to the and determines the significance of clustering. The larger the to the west (Fig. 3) [10]. 2) The presence of the PKT is diff K, the more significant the clustering of a given attributed as solely the result of the Imbrium impact population. We also computed the diff K values for the event [11-12]. In this model the proposed boundary of Procellarum basin rim that we outlined using FFCs and CCs and that of Wilhelms (1987) [10]. the Procellarum basin [10] is interpreted to be the outer

ring of the Imbrian basin [11-12].

Figure 3: Baseline dataset. Concentric rings represent basin Figure 2: Spatial distribution of FFCs and CCs. The PKT is rings and zones over which the randomness of crater outlined in red. Base map: Lunar Reconnaissance Orbiter 52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2245.pdf

populations was calculated. Base map: LROC WAC GLD100 statistical evidence for the Procellarum basin as the [13]. origin of the enigmatic PKT. The spatial distribution of 85% FFCs and CCs Results and Discussion: Based on the RKF provides evidence for the existence of the Procellarum analysis, we noted that the observed K value is > the basin (a pre-Nectarian basin), consistent with that of expected K value for FFC and CC populations previous studies [9-10] and the argument that FFCs and associated with all basins, statistically verifying our CCs occur in regions of subdued basin topography (i.e., visual observation that FFCs and CCs are clustered thin and weak crust), in particular, where the crust- within basins. Additionally, the diff K (i.e., the mantle interface was uplifted. The evidence for the significance of clustering) is highest for FFCs and CCs existence of the Procellarum basin also addresses associated with pre-Nectarian basins (i.e., the oldest previous studies that noted the lack of floor-fractured basins which exhibit subdued topographic rims and and concentric craters associated with the Imbrium weak gravity anomalies [14]) compared to relatively basin – since it is superposed within the Procellarum younger basins (Figs. 4-5). This correlation with pre- basin, at a time when the lunar lithosphere was likely Nectarian basins suggests viscous relaxation is a relatively rigid, as evidenced by the presence of the plausible formation mechanism for FFCs and CCs. Appenine mountains, which were uplifted during the Since FFCs and CCs share the same geospatial Imbrium impact event [9-10]. distribution, but only vary in size, we interpret that these Acknowledgments: This research is supported by two landforms are on a continuum and that they were the Lunar Reconnaissance Orbiter (LRO) project. likely formed by the similar processes. References: [1] Schultz P.H. (1976) The Moon, 15, 241-273. [2] Jozwiak L.M. et al. (2012) JGR, 117, E11005. [3] Jozwiak L.M. et al. (2017) Icarus, 283, 224-231. [4] Trang D et al. (2017) Icarus, 278, 62-78. [5] Ravi S. and Robinson M.S. (2019) LPSC L #2677. [6] Hall J.L. et al. (1981) JGR, 86, 9537-9552. [7] Jolliff B.L. et al. (2000). JGR, 105, 4197-4216. [8] Wieczorek et al. (2000). JGR, 105, 20417-20430. [9] Cadogan P.H. (1974) Nature, 250, 315-316. [10] Wilhelms D.E. (1987) USGS Prof. Paper 1348. [11] Schultz P.H. and Spudis P.D. (1985) LPI 16. [12] Spudis P.D. and Schultz P.H. (1985) LPI 16. [13] Scholten F et al. (2012) Figure 4: Spatial distribution of FFCs and CCs with respect JGR, 117, E00H17. [14] Mohit P.S. and Phillips R.J. to pre-Nectarian basins. Base map: LROC WAC GLD100 (2006) JGR, 111, E12001. [13].

Figure 5: Circumstantial evidence for the Procellarum basin. Base map: LROC WAC GLD100 [13].

Diff K calculations show more significant clustering associated with our Procellarum basin rim compared to that of Wilhelms (1987) [10], thereby providing