Lunar Boulder Fields as Indicators of Recent Tectonic Activity

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, several recent investigations have identified wrinkle ridges as young as ~101–103 Ma on the lunar maria. Those young wrinkle ridges were identified by the presence of dense fields of meter-scale boulders on their bounding scarps and topographic crests. Other investigations have separately identified young lunar wrinkle ridges that appear to lack the ubiquitous presence of boulder fields – thereby rendering the presence of boulder fields ambiguous in the search for recent or ongoing tectonic activity on the Moon. Here we assess boulder populations associated with 1,116 winkle ridge segments on the lunar maria that are inferred to be recently active (<1.5 Ga) based on their crisp morphologies and cross cutting relationships with small impact craters. We utilize derived data products from the Lunar Reconnaissance Orbiter (LRO) Mini-RF and Diviner Lunar Radiometer Experiment instruments to assess surface rock populations across these recently active structures and we compare their rock populations with mean ridge slope. Our results indicate that, where present, meter scale boulder fields are likely indicators of fault slip-induced ground acceleration given the short lifespan of boulders on the lunar surface. However, elevated boulder populations are not observed on all recently active ridges analyzed here. This latter observation concurs with the notion that wrinkle ridge boulder fields are an inexhaustive indicator of recent tectonic activity, and multiple geomorphic attributes should be considered in the search for ongoing lunar tectonic activity.