Probing Mare Basalt Thicknesses Using Nearside Lunar Graben

Abstract:

The nearside mare are closely associated with concentric and radial graben frequently attributed to mascon tectonics. Previous work suggests that in the cases of mare not associated with mascons, the loading of the basalt on a thinner or weaker lithosphere may be sufficient to produce graben comparable to mascon mare. To assess whether there are sufficient loads to induce lithospheric flexure we must first understand the spatial distribution and variability of mare basalt thickness. Thus, an accurate assessment of the thickness is critical to constraining stresses from loading by mare basalts on the lunar lithosphere.

The thicknesses of mare basalt on the nearside has been the focus of myriad studies, employing various techniques including the use of ghost craters to estimate the minimum basalt fill. We aim to better understand the controls on mare localized graben formation by using graben as a means of inferring the minimum depth of the brittle basalt sequence in which they formed. In the case of a simple graben made up of two antithetic normal faults, the graben width is related to the depth of convergence of the faults and is often controlled by the depth to a mechanical discontinuity. For graben in mare basalts, this convergence depth may be to an interbed in lava flow sequence or more likely the contact between the basalt sequence and the highlands substrate. Thus, assuming a simple graben made of two 60° dipping faults, the depth where the normal faults intersect should represent a local minimum estimate of the thickness of the mare basalts.

We find graben widths in mare range from ~400 m to ~5 km with a mean of ~2 km. This corresponds to a mean convergence depth of ~1.7 km. If our assumption that fault convergence depths represent a minimum mare basalt depth, 1.7 km is significantly larger than previous estimates based on depth-diameter relations of buried impact craters.