Slope Deformation Associated with Recent Tectonism and the Lasting Effect of Local Subsurface Geometry in the Taurus-Littrow Valley, Apollo 17 Landing Site.

The Taurus-Littrow Valley, location of the Apollo 17 landing site, hosts recent, late-Copernican geomorphological landforms and tectonic structures, namely the Light Mantle avalanche deposit and the Lee-Lincoln lobate scarp. The Light Mantle deposit represents a unique case of a hypermobile avalanche on the Moon (El-Baz 1972; Schmitt et al. 2017). The Lee-Lincoln lobate scarp is the surface expression of a recent thrust fault (Watters et al. 2010), which is considered to be the source of strong seismic shaking throughout Taurus-Littrow Valley (van der Bogert et al. 2012, 2019), and potentially still active (Watters et al. 2019).

The Light Mantle represents the only extraterrestrial landslide for which an absolute age is provided (70-110 Ma), thanks to the Apollo 17 returned samples (e.g., Schmitt et al. 2017). Therefore, the Light Mantle deposit can be used as a geomorphological marker and time constraint for surface changes that occurred since its emplacement. By applying the principle of superposition, surface changes superposed on the Light Mantle deposit, and on the slope from which it was generated (the NE-facing slope of the South Massif), must post-date the landslide event. For example, small scale grabens (10-20 m wide) associated with the Lee-Lincoln lobate scarp are found superposed on the Light Mantle unit (Watters et al. 2010). These troughs likely formed less than 50 Ma and are thought to be generated by the flexural bending of the hanging wall (Watters et al. 2010, 2012). Similarly, boulder tracks, whose survival time is estimated to range up to 35 Ma (e.g., Arvidson et al. 1976; Kumar et al. 2019), are found on the NE-facing slope of the South Massif, therefore evidence that boulder falls have occurred after the Light Mantle landslide event.

Here, we extend the body of evidence of surface changes that have affected the South Massif since the emplacement of the Light Mantle deposit. We map boulder tracks, areas of disturbed regolith, linear slope structures, and other structures associated with the summit of the South Massif. We identified features (i.e., slope structures oblique to contours, the Nansen Moat and the trough at the NE-base of the Sout Massif) directly related to back-thrust faults associated with the Lee-Lincoln thrust fault, which are re-activating the buried fault that bounds Taurus-Littrow Valley; we identified other features (i.e., crestal graben-like structures, slope structures parallel to contours) that derived from gravitational adjustment following basal slope support removal due to back-thrust faulting. Moreover, the overlapping relationships between the boulder tracks and regolith disturbance suggests that continuous slope deformation has been affecting the NE-facing slope. We attribute the efficiency of the process to repeated ground-shaking perturbation, which maintains the slope in a perpetually unstable state.

We conclude that the NE-facing slope of the South Massif has been recently and continuously affected by slope deformation processes. We suggest that the efficiency of these processes is the product of lasting, and perhaps ongoing, effects of activity of the Lee-Lincoln thrust fault, coupled with the influence of the subsurface geometry of the valley inherited from the impact basin formation.