THE MOON HAS ITS FAULTS

Our Moon has a widespread array of tectonic landforms related to contraction, orbit, and lithospheric stresses without Earth-like plate tectonics. Early studies using Apollo photography revealed that the lunar surface hosts both extensional and contractional landforms (e.g., Schultz, 1976) beyond those seen by Earth-based telescopes. The main types of tectonic landforms identified on the Moon are wrinkle ridges, lobate scarps, and graben. Wrinkle ridges are morphologically complex landforms in the maria, interpreted as contractional landforms resulting largely from basin subsidence (Wilhelms, 1987; Watters & Johnson, 2010). Graben are long, narrow troughs formed by extensional stresses either from subsidence or locally induced flexure (Watters et al., 2012; French et al., 2015; Nahm et al., 2018). Lobate scarps are small-scale asymmetric thrust faults often located in the highlands and are the result of crustal compression mainly due to long-term interior cooling of the Moon (Lucchita, 1976; Binder & Gunga, 1986; Watters et al., 2010, 2015; Roggon et al., 2017).

In the last decade, the Lunar Reconnaissance Orbiter Camera (LROC) (Robinson et al., 2010) has returned an abundance of high-resolution imagery (down to 50 cm/pixel), allowing scientists to investigate lunar tectonic landforms in greater detail and extent (Watters et al., 2010; Banks et al., 2012; Williams et al., 2013; 2019). Extensive mapping, modeling, and multi-dataset investigations assist in understanding the global stress state of the lunar crust (Watters et al., 2012, 2015, 2019; Nahm et al., 2018; Thompson et al., 2018; Martin et al., 2019). LROC data has revealed the presence of high-albedo blocks on the crest of wrinkle ridges (French et al., 2014; Valantinas & Schultz, 2018), several or tens of meters wide craters being crosscut by ridges and scarps (Watters et al., 2010; Williams et al., 2019), and the appearance of small, 0.5-meter-deep graben near ridges and scarps (Watters et al., 2012; French et al., 2015). Ages determined via crater size-frequency distribution measurements (Senthil Kumar et al., 2016; Clark et al., 2017; van der Bogert et al., 2018) indicate that our Moon has been tectonically active in the last 1 Ga, and links to shallow moonquakes recorded by Apollo seismometers suggest current tectonic activity (Watters et al., 2019).