Abstract
Abstract. Highland crater Lalande (4.45° S, 8.63° W; D = 23.4 km) is located on the PKT area of the lunar near side, southeast of Mare Insularum. It is a complex crater in Copernican era and has three distinguishing features: high silicic anomaly, highest Th abundance and special landforms on its floor. There are some low-relief bulges on the left of crater floor with regular circle or ellipse shapes. They are ~ 250 to 680 m wide and ~ 30 to 91 m high with maximum flank slopes > 20°. There are two possible scenarios for the formation of these low-relief bulges which are impact melt products or young silicic volcanic eruptions. According to the absolute model ages of ejecta, melt ponds and hummocky floor, the ratio of diameter and depth, similar bugle features within other Copernican-aged craters and lack of volcanic source vents, we hypothesized that these low-relief bulges were most consistent with an origin of impact melts during the crater formation instead of small and young volcanic activities occurring on the crater floor. Based on Kaguya TC ortho-mosaic and DTM data produced by TC imagery in stereo, geological units and some linear features on the floor and wall of Lalande have been mapped. Eight geological units are organized by crater floor units: hummocky floor, central peak and low-relief bulges; and crater wall units: terraced walls, channeled and veneered walls, interior walls, mass wasting areas, blocky areas, and melt ponds. These geological units and linear features at Lalande provided us a chance to understand some details of the cratering process and elevation differences on the floor. We evaluated several possibilities to understand the potential causes for the observed elevation differences on the Lalande's floor. We proposed that late-stage wall collapse and subsidence due to melt cooling could be the possible causes of observed elevation differences on the floor.
Highlights
In the past lunar geological maps (e.g., Wilhelms and McCauley, 1971), all craters and their surroundings were mapped as an individual unit named as “crater material”
There is a chaotic landscape in crater floor area owing to its continuous evolution during and after the cratering process
The resulting geological maps produce the fundamental set of information to understand and interpret the geological history of impact craters
Summary
In the past lunar geological maps (e.g., Wilhelms and McCauley, 1971), all craters (with diameters larger than 10 km) and their surroundings were mapped as an individual unit named as “crater material”. Impact melts distributed widely on floor and wall of a crater. The morphological features and distributions of impact melts in and around craters have been broadly analyzed to understand the process of cratering, especially the relationship of melt movement and emplacement with various cratering parameters (e.g., Krüger et al, 2013; Plescia and Spudis, 2014; Srivastava et al, 2013). The geological settings of impact craters' floors are complex and various in the morphology and topography, they are not easy to infer. Geological mapping is a primary tool to interpret complicated terrains by sequentially grouping together materials with similar morphological or componential attributes (Nass et al, 2011; Zanetti, 2015). The resulting geological maps produce the fundamental set of information to understand and interpret the geological history of impact craters
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