GEOGRAPHIC PROXIMITY OF LUNAR FLOOR-FRACTURED CRATERS AND SILICIC VOLCANIC LANDFORMS: COINCIDENCE OR NOT?

The Procellarum KREEP Terrane (PKT) is home to distinctive landforms, including floor-fractured craters (FFCs) and silicic volcanic edifices (SVEs). FFCs are impact craters that underwent subsequent modification by volcano-tectonic processes as evidenced by the presence of fractures and they sometimes host volcanic landforms such as mare basalt infill and pyroclastic deposits (Schultz, 1976; Jozwiak et al., 2012). The majority of FFCs (>60%) are located along the boundary of the PKT, as are the SVEs, with the exception of the Compton Belkovich Volcanic Complex (CBVC), which is located in the lunar farside [1,2]. We note that all lunar silicic landforms, including the outlier, CBVC, are located in close proximity (~50 km) to an FFC (e.g. Hansteen crater (HC) and Mons Hansteen (MH)). SVEs such as Gruithuisen domes were first proposed to be composed of silicic lithologies based on their strong UV absorption, leading them to be called “red spots” [3]. Christiansen Frequency (CF) values from the LRO DIVINER instrument confirmed them to be composed of ~70 wt% SiO₂ (Glotch et al., 2010). Two formation mechanisms have been proposed with regards to the formation of silicic magmas – basaltic underplating and silicate liquid immiscibility (SLI) [4]. Understanding the both the formation mechanism and potential geologic relationship between FFCs and SVEs are important in constraining the thermal and compositional evolution of the PKT.

Absolute Model Ages (AMAs) derived from Crater Size-Frequency Distribution (CSFD) measurements show that the formation of CBVC was contemporaneous with that of the basalt-flooded unit of the nearby FFC, Compton crater [5]. Therefore, we computed the AMAs of the basaltic floor unit (BFU) of HC (2.5 Ga), the ejecta blanket of HC (3.9 Ga), and MH (3.5 Ga) using LROC imagery. We conclude that while the formation of MH is not contemporaneous with HC, it is worth noting that the BFU is at least 500 Ma younger that the surrounding mare basalts, suggesting that source regions of S. Oceanus Procellarum were thermally active into the late Eratosthenian period (~2.5 Ga – 1 Ga).

[1] Schultz, P.H. (1976) The Moon, 15, 241-273; [2] Jozwiak, L.M. et al. (2012) JGR, 117, E11005; [3] Malin, M.C. (1974) EPSL, 21, 331-341; [4] Hagerty, J.J. et al. (2006) JGR, 111, E06002; [5] Shirley, K.A. et al. (2016), Icarus, 273, 214-223.