T. Frueh1, H.
Hiesinger1, C. H. van der Bogert1, J. D. Clark2, T. R. Watters3, and N.
Schmedemann1
1 Institut für Planetologie, Westfälische
Wilhelms-Universität Münster, Wilhelm-Klemm-Str.10, 48149 Münster,
Germany.
2 School of Earth and Space Exploration, Arizona State
University, 3603 Tempe, AZ, USA.
3 Center of Earth and Planetary Studies, National Air
and Space Museum, Smithsonian Institution, 37012 Washington, DC, USA.
Key Points:
- Early compressional tectonism in Tranquillitatis, in the form of wrinkle ridges, is presumably related to subsidence and basin loading.
- Later tectonism could reflect the evolution from a basin-localized to a global stress field and the continued growth of ancient faults.
- Recent wrinkle ridge and lobate scarp formation in Tranquillitatis occurred in the last 50 Ma and is influenced by a global stress field.
1 Introduction
The Moon’s surface hosts a variety of extensional and compressional tectonic features, which recorded the history of the acting regional and global stress systems. Compressional tectonism was initiated with the emplacement of the mare basalts and the shift of net global extension to net global contract at ~3.6 Ga, which led to the formation of the two major compressional tectonic landforms: lobate scarps and wrinkle ridges (Fagin et al., 1978; Lucchitta & Watkins, 1978; Solomon & Head, 1979; Wilhelms, 1987; Watters et al., 2009). Lobate scarps are the surface expressions of simple thrust faults and are the dominating tectonic landforms in the lunar highlands (Binder & Gunga, 1985; Watters et al., 2009, 2010). Lunar wrinkle ridges exclusively occur in the maria or basalt-covered regions and are a result of a complex interaction between thrust faulting and folding (Lucchitta, 1976; Wilhelms, 1987; Schultz, 2000; Watters et al., 2009). The compressional tectonism in the maria is thought to have originated from the superisostatic loading by dense mare basalts and the flexure of the lithosphere (Freed et al., 2001). This model has been established for the mascon (mass concentrations) maria, like Mare Imbrium or Mare Serenitatis. However, not all lunar maria are considered to be mascons because they lack the strong positive gravitational signal of mascon basins (Muller and Sjogren, 1968). The stress systems of those non-mascon basins are less well understood and still a matter of discussion.
Furthermore, the acting stress fields changed with time, and the age of tectonic landforms, therefore, contains important information on the stresses triggering their formation. Most of the deformation of the maria is thought to have occurred early in lunar history (e.g., Fagin et al., 1978; Ono et al., 2009; Watters et al., 2009; Yue et al., 2017). However, recent studies uncovered young tectonic features in the lunar highlands and maria, including young and recently active wrinkle ridges (e.g., Watters et al., 2010; Williams et al., 2019; Lu et al., 2019; Valantinas & Schultz, 2020; Nypaver & Thomson, 2022). The young landforms exhibit distinctive morphological features, like steep slopes, sharp edges, a crisp appearance, crosscutting relationships with craters, and the occurrence of small crisp graben in their close vicinity (Fig. 1). The trigger behind this recent tectonic activity is, also, still a matter of discussion.
Mare Tranquillitatis, which was the stage of the first human landing site as part of the Apollo 11 mission, is one of those non-mascon basins. Mare Tranquillitatis is an irregularly-shaped basin (Fig. 2), consisting of a deep and deeply basalt-filled western part and a shallow and shallow-filled eastern part (Dvorak & Phillips, 1979; De Hon, 1974, 2017; Konopliv et al., 2001; Zuber et al., 2013). The western part is associated with intensive deformation and circular, radial, and NS trending wrinkle ridge patterns, while the eastern part experienced less deformation and exhibits loose wrinkle ridge patterns. In addition to wrinkle ridges, lobate scarps, graben, and a large normal fault (called Rupes Cauchy) are present in the mare. A study by Yue et al. (2017), discovered an unusually young average age of ~2.4 Ga of large wrinkle ridges in Mare Tranquillitatis relative to wrinkle ridges in other maria. The reason behind this discrepancy remains unknown.
This study aims to contribute to the discussion on the age of tectonic landforms, stress systems of non-mascon maria, and the trigger behind recent tectonic activity. To achieve this goal, we created a tectonic map of Mare Tranquillitatis and studied the degradational state of compressional tectonic features to gain age information. By combining the tectonic analysis with the age of the tectonic features, we aim to uncover the evolution of the stress field acting in Mare Tranquillitatis.