Lunar Boulder Fields as Indicators of Recent Tectonic Activity

Abstract

Wrinkle ridges are the predominant tectonic structure on the nearside lunar maria. Although lunar wrinkle ridge formation began as early as ∼3.9–4.0 Ga, recent investigations have identified wrinkle ridges in the lunar maria that were tectonically active as recently as the Copernican period of lunar geologic history. Some of those geologically young wrinkle ridges were identified by the presence of dense fields of meter-scale boulders on their scarps and topographic crests. Other investigations have identified recently active lunar wrinkle ridges that lack the ubiquitous presence of boulder fields, thereby rendering the presence of boulder fields ambiguous in the search for ongoing tectonic activity on the Moon. Here we assess boulder populations associated with 1116 wrinkle ridge segments on the lunar maria that are inferred to be recently active (<1.5 Ga) based on their crisp morphologies and crosscutting relationships with small impact craters. We utilize data from the Lunar Reconnaissance Orbiter Mini-RF and Diviner Lunar Radiometer Experiment instruments to assess surface rock populations across these recently active structures. Our results indicate that, where present, meter-scale boulder fields are likely indicators of fault-slip-induced ground acceleration given the short lifespan of lunar surface boulders. However, elevated boulder populations are not observed on all recently active ridges analyzed here. This latter observation supports the notion that wrinkle ridge boulder fields are a nonunique indicator of recent tectonic activity. Furthermore, the spatial distribution of those boulder fields indicates that variable mare protolith properties may play a role in boulder field formation.