2 Background
2.1 Lunar Tectonics
The tectonic history of the Moon began with a period of net thermal expansion, which is argued to have shifted to net contraction around 3.6 Ga (Lucchitta & Watkins, 1978). Since then, global cooling induced a dominantly contractional global stress field (Solomon & Head, 1979; Wilhelms, 1987; Watters et al., 2009). This shift in the thermal state of the Moon is preserved in its tectonic landforms. Large scale crustal extension and, thus, the formation of large lunar graben ended at ~3.6 Ga (Lucchitta & Watkins, 1978; Watters et al., 2009). Following the shift, compressional features, i.e., lobate scarps, became the dominant globally forming tectonic landforms. The emplacement of the mare basalts started at ~3.9 – ~4.0 Ga and generally ceased at ~1.2 Ga (Hiesinger et al., 2011). With the main period of basalt emplacement at about 3.6 – 3.8 Ga (Hiesinger et al., 2011), the formation of wrinkle ridges began (Fagin et al., 1978; Watters, 1988; Watters et al., 2009).
Wrinkle ridges are common contractional tectonic features on the Moon, Mercury, Mars, and Venus (Plescia & Golombek, 1986; Watters, 1988; Golombek et al., 1991; Watters et al., 2009). On the Moon, wrinkle ridges exclusively occur within the mare basins (Lucchitta, 1976; Wilhelms, 1987; Watters et al., 2009), to which they typically appear radial and concentric (Whitaker, 1981; Watters et al., 2009). They typically show an asymmetric profile and consist of a broad arch and a superimposed irregular ridge (Plescia & Golombek, 1986; Strom, 1972; Watters, 1988), but their morphology is highly variable (Plescia & Golombek, 1986; Watters, 1988). Wrinkle ridges reach up to 300 km in length and 20 km in width (Sharpton & Head, 1988). Often one flank of the wrinkle ridge, the vergent side, has a steeper slope than the other, but this asymmetry can reverse along the wrinkle ridge. The superposed ridge usually is located near the steeper flank of the arch (Plescia & Golombek, 1986; Watters, 1988). However, both structures can occur independently from one another (Watters et al., 2009). Wrinkle ridge segments often occur in en-echelon arrangements (Watters et al., 2009). Smaller secondary or tertiary ridges occur near or on top of larger primary ridges (Watters, 1988; Watters et al., 2009). The surface texture of wrinkle ridges often resembles a crisscross “elephant-hide” structure (Gold, 1972). Elephant-hide structure can be found on slopes everywhere on the Moon and is thought to form due to regolith creep and seismic shaking (Zharkova et al., 2020; Bondarenko et al., 2022).
Since wrinkle ridges deform even young mare basalts with an age of ~1.2 Ga, crustal shortening associated with lunar maria occurred at least as recently as ~1.2 Ga (Watters et al., 2009). A global survey of possible formation times found average ages > 3.0 Ga for large wrinkle ridge structures (Yue et al., 2017). Wrinkle ridges in Mare Tranquillitatis, however, appear to be younger with an average age of ~2.4 Ga (Yue et al., 2017; McGovern et al., 2022).
While the exact kinematics of wrinkle ridge formation are still debated, the formation is generally explained by a combination of thrusting and folding (Schultz, 2000; Watters et al., 2009). Hence, wrinkle ridges can be interpreted as anticlinal structures above a non-surface breaking blind thrust fault (Schultz, 2000; Watters et al., 2009). For these processes to occur, a layered stratigraphy of the mare basalts is necessary (Schultz, 2000). The fault geometry may be planar or listric, there may be a single or multiple faults, and the depth of faulting may be shallow or deep (i.e., thick- or thin-skinned deformation; Schultz, 2000; Montési & Zuber, 2003; Okubo & Schultz, 2003, 2004; Watters, 2004, 2022). Wrinkle ridge formation is thought to be the result of superisostatic loading by dense mare basalts inducing subsidence and flexure of the lithosphere (i.e., mascon tectonics; Freed et al., 2001; Byrne et al., 2015; Schleicher et al., 2019). This led to compressional stresses in the basin center and extensional stresses at the basin margins, and, consequently, in basin concentric and radial wrinkle ridges (Freed et al., 2001). It is also suggested that global cooling instead of subsidence was the dominant cause of wrinkle ridge formation after 3.55 Ga onwards (Ono et al., 2009; Watters et al., 2009). Another proposed influence on the global stress field and wrinkle formation is deep transient stresses generated by the South Pole-Aitken (SPA) basin (Schultz & Crawford, 2011). This model predicts antipodal failures on the lunar nearside due to extensions deep within the Moon, which would have reactivated deep-seated faults. Wrinkle ridge patterns of the lunar nearside do spatially correlate with wrinkle ridge patterns predicted by this model (Schultz & Crawford, 2011; Valantinas & Schultz, 2020). GRAIL Bouguer gravity gradient data revealed a possible quasi-rectangular pattern of ancient deep rift valleys that are proposed to influence the localization of some wrinkle ridges (Fig.2; Andrews-Hanna et al., 2014). Wrinkle ridge formation might, therefore, be a result of an interplay of various factors on the regional and global stress fields, which will be discussed later.
Lobate scarps are linear to curvilinear small-scaled compressional structures, which mainly occur in the lunar highlands. They are asymmetric with a steeply sloping scarp face and gently sloping back scarp. The scarp face's direction often reverses along the strike (Binder & Gunga, 1985; Watters et al., 2009, 2010). In contrast to wrinkle ridges, they are thought to result from shallow surface-breaking thrust faults (Watters et al., 2009). In some cases, wrinkle ridges transform into lobate scarps at mare highland boundaries (Lucchitta, 1976; Watters et al., 2009, 2010; Clark et al., 2019). Lobate scarps are thought to be among the youngest tectonic features on the Moon (e.g., Binder & Gunga, 1985; van der Bogert et al., 2018; Watters et al., 2009, 2010, 2019). Binder and Gunga (1985) suggested that highland scarps are younger than 1 Ga. Crater size-frequency distribution (CSFD) measurements of lobate scarps support late Copernican ages (van der Bogert et al., 2018). From infilling rates of small-scale back-scarp graben, the age of the lobate scarps is likely < 50 Ma (Watters et al., 2012).
Recent studies revealed fresh activity of wrinkle ridges and lobate scarps (e.g., Watters et al., 2010; Williams et al., 2019; Lu et al., 2019; Valantinas & Schultz, 2020; Nypaver & Thomson, 2022). The evidence includes for both landforms (Fig. 1), the abundance of boulder fields and patches (French et al., 2019; Watters et al., 2019; Valantinas & Schultz, 2020), a distinct crisp morphology (e.g., Watters et al., 2010; Williams et al., 2019), crosscutting of impact craters (Watters et al., 2010; Lu et al., 2019; Nypaver & Thomson, 2022), ages <1 Ga determined from CSFD methods (van der Bogert et al., 2018; Valantinas et al., 2018; Lu et al., 2019), shallow moonquakes (Watters et al., 2019), boulder falls (Kumar et al., 2016), and associated small meter-scaled graben (Fig. 3; Watters et al., 2012; French et al., 2015; Valantinas & Schultz, 2020). The correlation between boulder falls and seismic activity, however, has been questioned lately (Bickel et al., 2021; Ikeda et al., 2022), highlighting the ongoing and early state of the study of recent tectonic activity. Late-stage global contraction is consistent with both an initially molten Moon (Binder & Gunga, 1985; Watters et al., 2019) and a near-surface magma ocean (Solomon, 1986; Solomon & Head, 1979; Watters et al., 2019), however, the magnitude of the late-stage stresses predicted in the totally molten Moon model is inconsistent with the population of small lobate thrust fault scarps (Watters et al., 2012, 2015). Global contraction would result in scarps with random orientations (Watters et al., 2015, 2019). However, since scarp orientations are non-randomly distributed, Watters et al. (2015, 2019) proposed a significant contribution of tidal stresses in the current stress state on the Moon. These stresses might also be an important influence on recent wrinkle ridge formation and activity (Williams et al., 2019). A model including South Pole-Aitken ejecta loading, true polar wander, and global contraction is also able to reproduce the observed scarp distribution (Matsuyama et al., 2021). Valantinas and Schultz (2020) proposed that active wrinkle ridges are part of an active nearside tectonic system (ANTS), resulting from the fault adjustment of ancient deep-seated intrusions, which were reactivated by the SPA forming impact. Deep moonquakes could be possible signs of those readjustments (Valantinas & Schultz, 2020). However, stresses related to these ancient sources of activity may have largely relaxed long ago and further models are needed to quantify their influence on today’s global stress field.
2.2 Mare Tranquillitatis
Mare Tranquillitatis is centered at 8.35°N, 30.83°E, and extends over approximately 875 km in diameter (Fig. 2). In the northwest, it borders Mare Serenitatis and Mare Fecunditatis in the southeast. Mare Tranquillitatis is irregularly shaped and dividable into two regions. The eastern part has a higher topographic elevation of up to –350 m (Fig 2). The western region has a lower elevation of below –2,000 km. The somewhat irregular shape of Tranquillitatis does not resemble the typical circular mare basin shape (e.g., Mare Imbrium, Mare Serenitatis, or Mare Crisium).
Mare Tranquillitatis is a non-mascon basin of pre-Nectarian age (Wilhelms et al., 1987). The mare fills at least one multi-ring basin (De Hon, 1974; Spudis, 1993), but a second overlapping basin is possible (De Hon, 2017; Bhatt et al., 2020). The mare basalts of Mare Tranquillitatis are of Imbrian age of 3.39 – 4.23 Ga (Hiesinger et al., 2000; Hiesinger et al., 2011). Most of the basalts show a CSFD age of 3.6 – 3.7 Ga (Hiesinger et al., 2000). These ages agree with the radiometric age of 3.67 Ga of the returned Apollo 11 samples (Wilhelms et al., 1987; Hiesinger et al., 2000; Iqbal et al., 2019). The western part of Mare Tranquillitatis is slightly younger than the eastern part (Hiesinger et al., 2000; Hiesinger et al., 2011). Crustal thickness varies from west to east as well. With a thickness between 10 and 30 km, the crust is thinnest in the west. This agrees with the free air data, which indicate a positive gravity anomaly in the western region (Fig. 2; Zuber et al., 2013). This gravitational anomaly suggests a trough-like structure connecting Mare Tranquillitatis with Mare Nectaris in the south and Mare Serenitatis in the north (De Hon, 1974). Recent publications suggest that this trough is part of a system of deep-seated intrusions that form a rectangular pattern on the near side of the Moon (Andrews-Hanna et al., 2014; Valantinas & Schultz, 2020). The deepest basalt-filled regions of the trough in Mare Tranquillitatis are the Lamont region and a structure near Torricelli crater (Dvorak & Phillips, 1979; De Hon, 1974, 2017; Konopliv et al., 2001; Zuber et al., 2013). The Lamont region represents a circular positive free air anomaly in the southwest of Tranquillitatis and is superficially recognizable as a circular ring of wrinkle ridges and an overall topographic low (Dvorak & Phillips, 1979; Scott, 1974). It has been interpreted to be either a buried impact crater or ghost crater (Dvorak & Phillips, 1979; Scott, 1974) or a feature of volcanic origin (Zhang et al., 2018). Several large graben occur throughout the mare, but most of them in the western region of Mare Tranquillitatis. The large graben Rima Cauchy and a parallel normal fault called Rupes Cauchy occur in eastern Mare Tranquillitatis (Bhatt et al., 2020). Many smaller volcanic domes and cones are abundant in the eastern mare (Spudis et al., 2013; Qiao et al., 2020). Spudis et al. (2013) proposed two large shield volcano-like structures in eastern Mare Tranquillitatis as an explanation for the abundance of volcanic features. Mare Tranquillitatis has the largest abundance of irregular mare patches, which were interpreted to be evidence of volcanism within the past 100 Ma (Braden et al., 2014; Qiao et al., 2020).
3 Data and Methods
In this study, a tectonic map and a tectonic feature map of Mare Tranquillitatis and the adjacent highlands were created using ESRI's ArcGIS version 10.5.1 and ArcGIS Pro. Wrinkle ridges and lobate scarps typically consist of a variable number of individual segments. In the tectonic map, e.g., a wrinkle ridge consisting of several individual segments is represented by one continuous polyline. This map was used for the tectonic analysis. For the feature map, we mapped the individual segments for morphological analysis, because individual segments might have varying formation ages. Both maps were created on Kaguya TC images (pixel scale of ~10 m; Ohtake et al., 2008) at a scale of 1:80,000. To achieve complete coverage of Mare Tranquillitatis, 84 TC tiles of both west and east illumination maps were integrated into the ArcGIS environment. Topographic information was gathered from the merged LRO LOLA – SELENE Kaguya DEM (Barker et al., 2016). Hillshade maps with different azimuth and height combinations, as well as slope maps were derived from this DEM.
For the tectonic map, features were classified as wrinkle ridges, lobate scarps, and unidentified. Unidentified features are linear positive topographic features with a possible but unproven tectonic origin (other possible origins are, e.g., dikes, lava flows, surface expressions of buried structures, or ejecta remnants). Additionally, we mapped extensional features, i.e., graben and the normal fault segments of Rupes Cauchy for complete coverage of the tectonic setting of Mare Tranquillitatis and for the following tectonic analysis. The polylines of wrinkle ridges were drawn at the center of the anticline. Since the morphology of wrinkle ridges is highly variable, Kaguya TC images, topographical data, slope maps, and hillshade maps were used to identify wrinkle ridge structures. A wrinkle ridge was mapped if it exhibits the classical morphological characteristics (as described in section 2.1) or shows a distinguishable asymmetric change in slope and topography. For lobate scarps and normal faults, the polylines were drawn at the scarp face base and for graben, the polylines were drawn at the graben center.
For the feature map, we focused on Kaguya TC images to identify individual features of wrinkle ridges and lobate scarps. Polylines were drawn on top of each wrinkle ridge crest. Every polyline represents a continuous wrinkle ridge crest segment. A new polyline was drawn if the orientation of the wrinkle ridge changes or if the crest segment is interrupted. Since mapping took place on the 1:80,000 scale, smaller structures are mostly represented by a single polyline. If no crest could be visibly identified, the edge of the steeper side was used for mapping. Lobate scarps features were mapped at the scarp face base. The morphology of each of these mapped features was then examined on NAC images in Quickmap and ArcGIS, with incidence angles of between 55° and 90°. Each wrinkle ridge segment was classified according to their respective appearances and erosional states into the classes crisp, moderately degraded, advanced degraded, and heavily degraded (similar to Williams et al., 2019). Attention was paid to their general appearance, the number of crosscut and superimposed craters, and to small associated graben (Table 1). The boulder abundance was not used in the classification, because we want to compare our results to previously published boulder abundance maps (French et al., 2019; Valantinas & Schultz, 2020).
Table 1
Characteristics Used for the Classification of the Erosional States of Wrinkle Ridges and Lobate Scarps
Class | Morphology | Crater crosscutting | Graben |
Crisp | Features with sharp and morphologically distinct edges and steep slopes. | Can crosscut and deform craters with diameter ranges of < 50 – 100 m. | Small (width < 50 m) and crisp clusters of graben are present. |
Moderately Degraded | Features with slightly rounded edges and steep to moderate slopes. | Can crosscut and deform craters with, generally, ≥ 100 m in diameter. | Generally, not associated with small graben. Rarely, diffusive troughs can be associated with features of this class. |
Advanced Degraded | Features with moderate to gentle slopes and well-rounded edges. | Rarely deform and crosscut craters with diameters of several kilometers. | No small graben associated with those features. |
Heavily Degraded | Features with gentle slopes and often indistinctive morphologies, not following the standard wrinkle morphology described in section 2.1. | Generally, do not crosscut superimposed craters. | No small graben associated with those features. |
Note. Slopes and morphological descriptions are described relative to each other.