Evidence for the sedimentary origin of imbrium sculpture and lunar basin radial texture

Abstract

The Imbrium sculpture texture, a distinctive ridged and furrowed pattern radial to the Imbrium basin and seen in other basins, has long been debated as to its origin (internal, formed by basin-related fractures; external, related to ejecta patterns). To test for the presence of deep radial fractures on the moon, the azimuth and length of linear rim segments of twenty-four post-Imbrium-basin craters were measured. Linear segments of crater rims parallel preexisting fracture patterns in terrestrial craters floored in an indurated substrate. Craters forming in a terrain containing pervasive fractures radial to Imbrium should show evidence of this tectonic influence by forming rim crest segments (terrace scarps) preferentially along these directions. No systematic relation of these segments with Imbrium radial structure was found. This suggests that the surface radial grooves may not extend to depth. The relatively young Orientale basin shows two types of radial structures: (1) pervasive subparallel ridges and furrows formed by a spectrum of sizes of secondary crater chains emanating from the main crater, and from flowage of material during secondary cratering; (2) parallel, generally radial ridges which appear to have formed on top of outward flows of debris. These types of radial textures (both depositional and erosional) appear unrelated to major faults or fractures. Therefore, these two lines of evidence suggest that much of the Imbrium-type sculpture surrounding major lunar basins is sedimentary, rather than tectonic, in origin.