Displacement-length scaling relations of nearside graben: Evidence of restricted normal faults on the Moon

Abstract

Evidence of significant extension on the Moon is associated with nearside mare. The load induced by the mare basalts resulted in lithospheric flexure and graben forming extension. We use high-resolution topographic and image data from the Lunar Reconnaissance Orbiter (LRO) and the SELenological and ENngineering Explorer (SELENE) to generate detailed fault displacement-length (D/L) scaling observations for graben bounding normal faults. Specifically, we focus on the Dmax/L relations of graben within three geologic settings: lunar highlands, mare, and mixed highlands-mare terrains. The population scaling factor ? for the measured lunar graben is 4.5 × 10-3 (n = 50), falling within the range for terrestrial normal faults. The scatter in the Dmax/L data for mare graben (? = 4.9 × 10–3,n = 25) is significantly higher than that for graben in highlands (? = 6.3 × 10-3, n = 16) and mixed terrains (? = 3.0 × 10-3, n = 9). The shape of a displacement profiles for faults is indicative of the mode of growth (i.e., restricted or unrestricted). Displacement plots show 52% of mare graben exhibit plateau-shaped profiles, indicating restricted fault growth, compared to 19% and 33% for highlands and mixed terrain graben, respectively. We attribute the large percentage of plateau-shaped profiles and the scatter in mare graben Dmax/L data to restricted fault growth controlled by the local thickness of the mare basalts. Finally, we consider whether observationally based estimates of the distribution of mare basalt thicknesses is consistent with the width-depth relation of the inferred restricted graben.