Investigating the Seismic Record Around Small Mare Ridges and Lobate Scarps

The formation of large wrinkle ridges in the lunar maria is estimated to have initiated soon after basalt emplacement or synchronous with mare basalt emplacement and cooling. Several studies suggest most large wrinkle ridges formed between 3.5 and 3.1 Ga, and some as recently as 2.4 Ga. Although the timing of early lunar wrinkle ridge formation is well-constrained, we have only recently started to explore the timing of formation and slip-event(s) at small mare ridges using crater counting methods. A large number (2,697) of small mare ridges (SMRs) have been identified across the lunar surface, creating the most complete map to date. Based on their fresh morphologies, cross-cutting relationships with decameter-scale impact craters, and the presence of nearby small-scale graben, SMRs are presumed to have been recently formed and reactivated (< 100 Ma) via stress fields corresponding to global contraction, orbital recession, and solid body tides similar to lobate scarps in the lunar highlands.

To further constrain SMR formation and the timing of coseismic events, we have implemented age determination methods previously used for the lobate thrust fault scarps in the highlands. These methods allow us to investigate the geologic history of SMRs where the cratering record adjacent to the tectonic landform is reset due to seismic shaking related to coseismic slip events on ridge faults. Here, we explore the variations in the crater size-frequency distributions (CSFDs) and derive absolute model ages (AMAs) for a selection of SMRs on the near- and far side of the Moon. The age variations of SMRs and lobate scarps across the lunar surface have the potential to improve our understanding of the evolution of stresses and the degree to which the Moon is currently tectonically active.

Methods:  We used high-resolution (2–5 m/px) Narrow Angle Camera (NAC) images from the Lunar Reconnaissance Orbiter Camera (LROC) with high solar incidence angles (55–80°) for our CSFD measurements. Individual NAC images were calibrated and georeferenced in the Integrated System for Imagers and Spectrometers: Version 3 (ISIS3). The CSFD measurements were conducted in QGIS using OpenCraterTools and then exported to Craterstats for plotting and fitting. We used the techniques outlined by [16] and the production and chronology functions of [17], which are valid for lunar craters >10m and <100 km in diameter, to determine AMAs.

Figure 1: Global distribution of 34 lunar scarps ages (circles) and 7 small mare ridge ages (stars, [5] and this study) on an equidistant cylindrical projection of the LROC WAC global mosaic. The color scale goes from yellow to purple, where yellow represents the youngest ages between 0 – 50 Ma, and purple represents ages over 250 Ma.

Clementine Color-Ratio images were used to locate secondary impact crater chains to avoid skewing derived ages. Count areas were placed on relatively flat surfaces with less than a 10-degree slope to minimize mass-wasting effects that could result in younger apparent ages . The sizes of the count areas for SMRs range from 0.5 - 8 km² and vary based on the size of the tectonic landform and the availability of homogeneous, flat areas. Due to their small size, we estimate that the areas produce errors up to ~20-30% per derived AMA. Past studies of small count areas (0.5 - 2 km²) have found that count area size does not significantly affect the precision of AMAs. Therefore, we assume that the AMAs vary by a few to 10s of million years. A rectangular shape was often used for our count areas, but occasionally, a more irregular shape is required to omit steep slopes, uneven terrain, and limited optimal image coverage for CSFD measurements. The crater diameter fit range has been discussed in detail in previous works.

Results and Discussion:  Results from traditional CSFD methods for seven SMRs reveal an age range of ~50 - 310 Ma with an average age of 124 Ma (Fig. 1). This range of ages for SMRs is similar to ages derived for the lobate thrust fault scarps (~24 – 400 Ma) in the highlands [Fig.1]. The temporal similarities between SMRs and lobate scarps suggest that the crustal stresses forming the lobate thrust fault scarps (e.g., late-stage global contraction and tidal forces) are also being expressed by the SMRs.

As seen in other studies using traditional CSFD methods at wrinkle ridges, their application to SMRs is challenging and requires great care to make sure that derived ages are robust and representative. For all CSFD measurements, we explored the crater population in cumulative, differential, and relative forms to get the best fit. The cumulative resurfacing correction was applied to the subset of craters used to derive AMAs. Occasionally, an age cannot be determined because the entire crater population is parallel to the equilibrium line as defined by [30], meaning that the count area is in a crater equilibrium state (i.e., new craters form at the same rate that old craters are erased).

Figure 2: The maximum and minimum fit crater diameters affected by seismic events related to ridge activity at lobate scarps (shades of red, [11]) and seven SMRs (shades of blue).

Many of the ages that we fit use a minimum fit near 10 m (Fig.2) because this is the limit allowed by the lunar production function [16]. At present, the 10 m diameter is a boundary condition for our analysis until the production function has been extended to smaller craters. The maximum crater diameter reset by the scarp activity minus the minimum crater diameter defines the ∆Crater diameter (Fig.3). For the seven SMRs, the diameter range is 10 m to 35 m. Compared to the ∆Crater diameter for lobate thrust fault scarps (blue, Fig.3), SMRs (red, Fig.3) have a reduced affected size range and a shallower upward trend toward older ages. The differences in target properties between lobate scarps and SMRs could be one factor controlling the distinct ∆Crater diameter slopes.

Figure 3: Crater diameter range affected by slip motion at lobate scarps (red) and SMRs (blue).