Small-scale lunar graben: Distribution, dimensions, and formation processes

Abstract

The Lunar Reconnaissance Orbiter Camera (LROC) is the first instrument to provide widespread coverage with a range of incidence angles at the resolution required to detect small-scale landforms. A sample (n=238) of globally distributed, small-scale graben average 26m wide and 179m long. When dividing the population into those located within mare and highland regions, we observe that graben located within mare tend to be narrower, shorter, and more irregularly spaced than those in highland terrane. For graben associated with contractional landforms, those in mare are smaller in width and length than those in highlands; the same is true for graben independent of contractional landforms. Assuming a simple geometry, widths of mare graben associated with scarps or ridges are used to estimate the minimum depth range to a mechanical discontinuity (e.g., base of the regolith) resulting in values of ∼4–48m. These values are similar to the ranges estimated for regolith thickness from previous workers using Apollo 14 seismic data (3.9–8.5m), crater counting techniques (8–33m), crater morphology techniques (2.5–9m), and crater blockiness (8–31m). Widths of highland graben yield minimum depths of faulting of 209–296m. While this range agrees well with models for regolith production (an older surface will have thicker regolith), this estimate likely does not represent the thickness of a mechanical unit due to the fragmented nature of the highland crust (it does not provide a defining boundary between bedrock and regolith). Spacing of mare graben not associated with contractional landforms is used to estimate maximum local mare unit thickness for two graben groups: 190m for Posidonius and 296m for Vitello. Maximum graben ages range from late Eratosthenian to early Copernican based on superposed and crosscut crater ages with a group of graben deforming ejecta from Copernicus crater. Data presented here provide further evidence of a globally distributed, young, small-scale graben population that has formed as a result of localized extension either from flexural bending or dilation due to contractional faulting or volcanic uplift, indicating a significant level of recent geologic activity.