Williams, Nathan - Fault Dislocation Modeling of Tectonic Landforms in Mare Frigoris

Abstract: 
Previous work suggested that large-scale nearside basin-localized extensional tectonism on the Moon ended ~3.6 billion years ago and mare basin-related contractional deformation ended ~1.2 billion years ago [Lucchitta and Watkins, 1978; Solomon and Head, 1979, 1980; Hiesinger et al., 2003]. Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) [Robinson et al., 2010] high resolution (50-200 cm/pixel) images enable the detailed study of lunar tectonic landforms and further insight into the evolution of stresses. Populations of wrinkle ridges, lobate scarps, and graben are now observed at scales much smaller than previously recognized, and their morphology and stratigraphic relationships imply a complex deformational history [Watters et al., 2010, 2012]. Mare Frigoris (~45°N-60°N, 40°W-40°E) is one such area with abundant tectonic landforms now revealed by LROC [Williams et al., 2014].

The most common tectonic landforms in mare basins are sinuous wrinkle ridges that have up to hundreds of meters of relief and are interpreted as folded basalt layers overlying thrust faults [Plescia and Golombek, 1986; Golombek et al., 1991; Schultz, 2000; Watters, 2004; Watters and Johnson, 2010]. They often consist of a narrow, asymmetric ridge atop a broad arch and sometimes occur radial to or concentric with the centers of some mare basins. Wrinkle ridges with these patterns have previously been associated with mascons – dense concentrations of mass identified by positive gravity anomalies. The thick basaltic lava thought responsible for lunar mascons causes flexure and subsidence to form wrinkle ridges [Solomon and Head, 1979, 1980]. However, Mare Frigoris is not associated with a mascon [Zuber et al., 2013], yet wrinkle ridges deform the mare basalts there [Whitford-Stark, 1990; Williams et al., 2014]. The origin of compressional stresses in non-mascon environments remains an outstanding question.

A key step to better understanding the occurrence of wrinkle ridges in non-mascon basins is characterizing the behavior of the underlying faults. We expand upon methods used in Williams et al. [2013] and apply fault dislocation modeling to estimate geometries and displacements for selected wrinkle ridge faults in Mare Frigoris. Digital terrain models (DTMs) derived from LROC NAC stereo pairs [Tran et al., 2010] are used to constrain fault models. Using the system of analytical equations for deformation of a half-space defined by Okada [1985, 1992], we apply a genetic algorithm to invert ridge relief for fault parameters including dip angles, displacements, and depths of faulting along fault segments. Preliminary results for a portion of an S-shaped wrinkle ridge in western Mare Frigoris include maximum depths of faulting within the upper ~1-2 km, displacements of up to 200 m, and shallow (<40°) dip angles. These preliminary modeled values are comparable to estimates for other lunar and martian wrinkle ridges [e.g. Plescia and Golombek, 1986; Golombek et al., 1991; Schultz, 2000; Watters, 2004; Watters and Johnson, 2010], and suggest this faulting is likely confined to within the mare fill and not rooted deeply in anorthositic crust.