1 Introduction
1.1 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; 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 (Golombek et al., 1991; Watters, 1988; Watters
et al., 2009). On the Moon, wrinkle ridges exclusively occur within the
mare basins (Lucchitta, 1976; Watters et al., 2009), to which they
typically appear radial and concentric (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 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).
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. Wrinkle
ridges in Mare Tranquillitatis, however, appear to be younger with an
average age of ~2.5 Ga (McGovern et al., 2022; Yue et
al., 2017).
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; Montési &
Zuber, 2003; Okubo & Schultz, 2003, 2004; Schultz, 2000; 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; Byrne et al., 2015;
Freed et al., 2001; 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). GRAIL Bouguer gravity gradient
data revealed a possible rectangular pattern of ancient deep rift
valleys that are proposed to influence the localization of some wrinkle
ridges (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 (Clark et al.,
2019; Lucchitta, 1976; Watters et al., 2009, 2010). 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 related to both landforms (e.g.,
Lu et al., 2019; Nypaver & Thomson, 2022; Valantinas & Schultz, 2020;
Watters et al., 2010; Williams et al., 2019). The evidence includes the
abundance of boulder fields (French et al., 2019; Valantinas & Schultz,
2020), a distinct crisp morphology, crosscutting of impact craters (Lu
et al., 2019; Nypaver & Thomson, 2022), ages <1 Ga determined
from CSFD methods (van der Bogert et al., 2018), and associated small
meter-scaled graben (Fig. 3; French et al., 2015; Watters et al., 2012).
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. 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 stresses generated by the South Pole-Aitken basin
(Schultz & Crawford, 2011) and by the existence of ancient deep-seated
intrusions (Andrews-Hanna et al., 2014). However, stresses related to
these ancient sources of activity may have largely relaxed long ago.
1.2 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.
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
(Bhatt et al., 2020; De Hon, 2017). The mare basalts of Mare
Tranquillitatis are of Imbrian age of 3.39 – 4.23 Ga (Hiesinger et al.,
2011; Hiesinger et al., 2000). 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 (Hiesinger
et al., 2000; Iqbal et al., 2019; Wilhelms et al., 1987). The western
part of Mare Tranquillitatis is slightly younger than the eastern part
(Hiesinger et al., 2011; Hiesinger et al., 2000). 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 (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 fault called Rupes Cauchy occur in eastern Mare Tranquillitatis
(Bhatt et al., 2020). Eastern Mare Tranquillitatis has a generally
thicker crust and is buried by a thinner cover of basalts (De Hon, 1974;
Rajmon & Spudis, 2004; Zuber et al., 2013). Many smaller volcanic domes
and cones are abundant in the eastern mare (Qiao et al., 2020; Spudis et
al., 2013). 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).
2 Data and Methods
In this study, a tectonic map and a tectonic feature map of Mare
Tranquillitatis and the adjacent highlands were created. Both maps were
done 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 ArcGIS. 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, unidentified features, large graben and troughs, and the large
normal fault of Rupes Cauchy. The polylines of wrinkle ridges were drawn
at the center of the anticline. Since the morphology of wrinkle ridges
is highly variable, visual 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 or shows a distinguishable asymmetric change in slope
and topography. For lobate scarps, 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 visual data 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. Attention was paid to their general
appearance, the number of crosscut and superimposed craters, and to
small associated graben. The boulder abundance was not used in the
classification.
3 Results
A total of 243 wrinkle ridges, 137 lobate scarps, and 148 unidentified
structures, with a total length of ~10,991 km, were
mapped in this study (Fig. 3). The length of individual segments ranges
from ~1 km to ~175 km, with a mean
length of ~21 km.
The differences in the appearance of the ridge segments allow
distinguishing four different classes. These classes are crisp,
degraded, advanced degraded, and heavily degraded (similar to Williams
et al., 2019). They differ from one another in their erosional state,
general structure, surface texture, crosscut relationships, and small
graben occurrence. However, transitions between the different
degradation classes are gradual. A total of 846 segments of
contractional tectonic features were mapped of which 658 segments were
classified (Fig. 4). Their appearances and occurrences are described in
the following.
A total of 49 segments with a cumulative length of ~451
km were classified as crisp. Consequently, they represent 5.1% of the
total mapped length. All of them occur scattered within the mare and are
often close to degraded features (Fig. 4). In general, they have a NE –
ENE orientation. Crisp features have sharp edges, and steep slopes on a
small scale (< 100 m; Fig. 5). They are generally relatively
small structures in terms of length and width and have a winding and
lobate appearance. They often braid and cross each other along strike.
The crisp wrinkle ridges often resemble a lobate scarp morphology, with
a simple asymmetrical profile and, in some cases, a seemingly missing
broad arch. Often smaller surface-breaking tectonic features occur in
their vicinity. Crisp features, generally, crosscut small craters (Fig.
5c; < 50 – 100 m diameter) and wrinkle ridges often appear to
be surface breaking when they crosscut craters. Clusters of small (width
< 50 m) crisp appearing graben and troughs are present on top
of and in the close vicinity of crisp features (Fig. 5). Generally, the
graben are located at the hanging wall and are oriented perpendicular
and parallel to the latter. Small boulder patches are visible
occasionally (Fig. 5b).
About 100 segments were classified as degraded. They have a total length
of ~780 km, which makes up 8.9% of the total mapped
length. On average, they generally show a NE orientation. The structures
are similar in size to the crisp segments, but the edges can be more
indistinct than crisp features. In general, they have a winding and
lobe-like morphology, and they often braid and cross each other. Only a
few small craters superimpose the segments. They typically crosscut
several craters along their length, which mostly have diameters of
larger than 100 m (Fig. 6). Small graben are generally not associated
with these structures. They occur throughout the mare and can be
spatially associated with crisp, advanced, and heavily degraded
features.
Advanced degraded features are the second most common class and are
dominantly comprised of wrinkle ridges. A total of 251 advanced degraded
wrinkle ridges have been mapped, resulting in a total length of
~2,762 km. This class represents 31.5% of the total
length. They are generally the most massive wrinkle ridges with respect
to width and topography (up to 10s of kilometers wide and hundreds of
kilometers high). Their rounded morphology mostly resembles the
traditional wrinkle ridge definition, with a, in some cases km scaled,
broad arch and an asymmetric superimposed steep crest (Fig. 7a). The
changes in the orientation of the wrinkle ridge asymmetry are either
gradual or abrupt. Smaller ridge segments of higher order occur in front
or back and on the top of these wrinkle ridges. Structures of higher
order can transition to first-order ridges along their strike. On slope
maps, advanced degraded wrinkle ridges show slopes up to >
30°. They have a larger number of superimposed craters than the
previously described morphological classes. However, the abundance of
superimposed craters is often lower than crater abundances in the
surrounding mare units. These wrinkle ridges can crosscut craters with
diameters of several hundred meters, but most segments do not crosscut
any craters. The surfaces often show a crisscross pattern that previous
studies described as an “elephant-hide” pattern (Gold, 1972).
Extensive boulder fields are associated with some advanced degraded
wrinkle ridges (Fig. 7a).
The most common class are the heavily degraded features, which also
dominantly consist of wrinkle ridges. 258 segments, with a total length
of ~3,140 km, were mapped. As a result, 35.8% of the
total length is represented by this class. While their overall structure
can be similar to advanced degraded wrinkle ridges, they generally have
an indistinctive and diffuse morphology with more rounded edges (Fig.
7b), and the classical wrinkle ridge structure is often only visible in
topographic data. They have many superimposed craters and generally only
crosscut craters with diameters of several hundred meters, but most
segments do not crosscut any craters. There are no associated small
graben present. Their surface texture can resemble a diffuse
elephant-hide structure. In general, advanced and heavily degraded
wrinkle ridges are similarly distributed. However, individual wrinkle
ridge assemblages are generally represented mainly by one of both
classes. In general, heavily degraded wrinkle ridges occur less commonly
together with crisp and degraded wrinkle ridges than advanced degraded
wrinkle ridges. Both classes represent the largest wrinkle ridge
structures in Mare Tranquillitatis in length, width, and height.
4 Discussion
The large size and strongly degraded morphology of advanced and heavily
degraded features suggest an older formation age relative to degraded
and crisp features. Advanced and heavily degraded features deform all
the mare units defined by Hiesinger et al. (2000), which have ages of
~3.4 to ~3.8 Ga. Consequently, they have
an upper age limit of at least 3.8 Ga. Since Rupes Cauchy and some large
graben are deformed by advanced and heavily degraded wrinkle ridges,
some of the wrinkle ridge formation occurred after 3.6 Ga (Lucchitta &
Watkins, 1978; Watters et al., 2009). They deform no craters to craters
of several hundred meters in diameter, which agrees with Nectarian,
Eratosthenian, and Imbrian formation ages (Trask, 1971). These proposed
ages agree with the results of previous studies (e.g., Fagin et al.,
1978; Ono et al., 2009; Watters et al., 2009; Yue et al., 2017). Yue et
al. (2017) found younger ages for large wrinkle ridges in Mare
Tranquillitatis (~2.4 Ga) relative to other lunar maria
(~3.3 Ga). With the focus on Mare Tranquillitatis and
the degradation-state approach of our classification, this age
discrepancy cannot be resolved. Relatively younger ages of advanced
degraded wrinkle ridges compared to heavily degraded ridges can only be
suggested and not conclusively proven. More precise dating methods are
needed to uncover the early tectonic evolution of the maria basins
(McGovern et al., 2022), however, CSFD measurements on wrinkle ridges
are challenging (Frueh et al., 2020).
The occurrence of the advanced and heavily degraded concentric and
radial wrinkle ridges in the western mare appears to have been localized
by a subsurface feature (Fig. 8; Freed et al., 2001; Schleicher et al.,
2019). These concentric and radial wrinkle ridges, as well as several
concentric large graben, can be attributed to the Lamont gravity
anomaly, which is argued to be a ghost crater (Dvorak & Phillips, 1979;
Scott, 1974) or a feature of volcanic origin (Zhang et al., 2018). Next
to the Lamont anomaly, western Mare Tranquillitatis is characterized by
a positive gravitational anomaly ranging from Mare Nectaris in the south
to Mare Serenitatis in the north (Fig. 8). Correlated with this positive
anomaly are the thickest basalts in Mare Tranquillitatis (Dvorak &
Phillips, 1979; De Hon, 1974, 2017; Konopliv et al., 2001; Zuber et al.,
2013). At the surface, this is expressed as an elongated depression.
Advanced and heavily degraded wrinkle ridges and large graben within
this depression occur parallel to the latter, which could also imply a
subsidence-related origin (Fig. 3). Mare Serenitatis most likely
influenced the northwestern Mare Tranquillitatis, resulting in a radial
wrinkle ridge and parallel graben to Mare Serenitatis (Fig. 3). Advanced
and heavily degraded wrinkle ridges in eastern Mare Tranquillitatis
occur generally close and parallel to mare boundaries, which is
consistent with an origin from basin loading and subsidence. The fewer
number and the less coherent patterns of features in the eastern mare
could be a result of the shallower basalts and, therefore, less basin
loading induced by subsidence. While basin loading and subsidence
influenced the regional stress field and the tectonic patterns in
Tranquillitatis, additional global stress fields contributing to wrinkle
ridge formation have been proposed. One possible influence on the global
stress field are deep transient stresses generated by the South
Pole-Aitken (SPA) basin (Schultz & Crawford, 2011), which predicts
antipodal failures on the lunar nearside due to extensions deep within
the Moon. Schultz and Crawford (2011) suggested reactivated deep-seated
faults localized the wrinkle ridges. Wrinkle ridge patterns of the lunar
nearside do spatially correlate with the predicted patterns (Schultz &
Crawford, 2011), however, it is not clear that SPA-related stresses
would not have largely relaxed before the period of wrinkle ridge
formation in Tranquillitatis. Another discussed potential model is the
fault adjustment correlated with deep-seated intrusions on the lunar
nearside (Andrews-Hanna et al., 2014). GRAIL Bouguer gravity gradient
data revealed a possible polygonal pattern of ancient deep intrusion
connecting most of the lunar maria and also Mare Tranquillitatis
(Andrews-Hanna et al., 2014). The western elongated positive
gravitational anomaly of Mare Tranquillitatis is proposed to originate
from these deep-seated intrusions. However, following the linear
unrestricted growth trends and the similar displacement values of
wrinkle ridges associated and not associated with these proposed
intrusions, there is no evidence that ridge faults were influenced by
buried structures associated with ancient rifts (Watters, 2022). In
addition, not all advanced and heavily degraded wrinkle ridges in Mare
Tranquillitatis are associated with the proposed intrusion and, thus, at
least those wrinkle ridges not associated with the intrusion presumably
formed by other stresses. In summary, the compressional stresses that
resulted in the formation of advanced and heavily degraded wrinkle
ridges originated primarily from load-induced subsidence with other
possible sources of regional or global stress that varied with time.
The sharp-edged morphology and the relatively small size of the crisp
and degraded wrinkle ridges and lobate scarps suggest a relatively young
formation age in contrast to degraded and heavily degraded segments.
Crisp features can crosscut craters with diameters of less than 50 - 100
m. Craters of these sizes are estimated to be of Copernican ages
(< 800 Ma; Wilhelms et al., 1987), because older craters of
this size would have been infilled and degraded since then (Trask,
1971). Thus, it is possible to establish a Copernican age, i.e., an
upper age limit of ~800 Ma for these landforms. Since
tectonic activity would result in seismic shaking and thus in enhanced
degradation of the small craters, the upper limit is presumably
overestimated (Williams et al., 2019). CSFD measurements also support
Copernican ages for lobate scarps with similar crisp morphologies (van
der Bogert et al., 2018; Clark et al., 2017). Accompanying crisp
features are small fresh graben and troughs. The existence of small
crisp graben situated near lobate scarps was first documented at the
back-limb of the Lee-Lincoln scarp, close to the Apollo 17 landing side
(Watters et al., 2010). Since then, more of these structures were found
in the vicinity of lobate scarps (French et al., 2015; Watters et al.,
2012) and wrinkle ridges (French et al., 2015; Williams et al., 2019).
Small graben observed in Mare Tranquillitatis are similar in their
dimensions to the graben described in the latter studies. They typically
have widths of less than 50 m and, in many cases, of even less than 10
m. Because of their similarity to sizes measured in other studies,
depths of ~17 m to ~1 m can be assumed
(Watters et al., 2012; Williams et al., 2019). Fill rates of shallow
depressions in lunar regolith are estimated to be 5 ± 3 cm/Ma (Arvidson
et al., 1975). Therefore, a ~1 m deep graben should be
filled entirely with regolith after ~12.5 to
~50 million years, which implies formation ages of less
than 50 Ma. Due to their association with lobate scarps, Watters et al.
(2012) suggested that these graben form by uplift and flexural bending
resulting from the movement at the underlying thrust fault. Thus, these
graben can be viewed as possible evidence for tectonic activity of crisp
features during the last 50 Ma (French et al., 2015; Watters et al.,
2012; Williams et al., 2019). Lu et al. (2019), used ejecta boulders of
craters crosscut by small wrinkle ridges in Mare Imbrium to calculate
the individual crater ages since boulder abundances decrease with
exposure time (Basilevsky et al., 2013; Lu et al., 2019). The derived
ages support wrinkle ridge formation during the last 10s of Ma (Lu et
al., 2019). The morphology of the young wrinkle ridges studied by Lu et
al. (2019) are indistinguishable from crisp wrinkle ridges in Mare
Tranquillitatis. In summary, different methods indicate the formation of
young wrinkle ridges and lobate scarps on the Moon during the last few
10 to 100 Ma. Thus, we propose tectonic activity for crisp wrinkle
ridges and lobate scarps in Mare Tranquillitatis at least during the
last 50 Ma, which further higlights the global recent wrinkle ridge
formation.
Based on our study we cannot conclusively estimate formation ages for
degraded features. Crater crosscutting relationships imply younger ages
for degraded wrinkle ridges and lobate scarps than advanced degraded
features. The main difference between degraded and crisp features, next
to a more rounded morphology of degraded features, is the apparent lack
of small graben. However, while small graben can be seen as possible
evidence for recent tectonic activity, it is unknown whether they
necessarily have to form during recent activity. Therefore, the lack of
crisp graben does not necessarily imply an older age. Furthermore,
because of the small size and faint appearance of these graben, as well
as the missing NAC coverage (incidence angles between 55° and 90°) of
some features, a wider distribution of undetected graben is possible. We
estimate that degraded wrinkle ridges and lobate scarps have a broad
range of formation ages in between crisp and advanced degraded features.
Crisp features occur scattered within Mare Tranquillitatis and do not
align with patterns predicted by basin loading and subsidence. Hence,
subsidence does not seem to be the major controlling factor of young
wrinkle ridge and lobate scarp formation. Additionally, they are not
correlated with positive gravitational anomalies within the mare.
However, as previously stated, Mare Tranquillitatis is of irregular
shape, which could influence subsidence-induced stress patterns, and the
role of the thickness of the elastic lithosphere in wrinkle ridge
formation is also a factor (Watters, 2022). Previous studies discussed
the prolonged cooling, triggered by the abundance of heat-producing
elements, of the Procellarum KREEP Terrane (PKT) to be a factor in
recent wrinkle ridge formation (Daket et al., 2016; Lu et al., 2019).
However, Mare Tranquillitatis is not associated with the PKT (Wieczorek
& Phillips, 2000). Therefore, this model does also not explain the
recent formation of wrinkle ridges and lobate scarps in Mare
Tranquillitatis. Late-stage global compressional stresses are consistent
with both an initially completely molten Moon and an initially hot
exterior and magma ocean (Binder & Gunga, 1985; Solomon & Head, 1979;
Watters et al., 2019; Williams et al., 2013). The interior cooling of
the Moon could result in compressional stresses of ≥ 2, but <
10 MPa (Watters et al., 2015, 2019). For shallow thrust faults to form,
an estimated ~2 - 7 MPa are sufficient (Watters et al.,
2019; Williams et al., 2013). Small-scale wrinkle ridges were likely
formed by shallow thrust faults (Lu et al., 2019; Watters, 2004). The
derived depths from Lu et al. (2019) for small wrinkle ridge thrust
faults are similar to suggested depths of shallow lobate scarps
(~1 km; Williams et al., 2013). Concluding, global
compression seems to be a likely candidate as the driving force behind
recent wrinkle ridge and lobate scarp formation on the Moon and in Mare
Tranquillitatis. Global lobate scarp patterns and the timing of detected
moonquakes highlighted the possible influence of tidal forces, such as
orbital recession, diurnal tidal stresses, and true polar wander onto
the lunar stress field (Matsuyama et al., 2021; Watters et al., 2019).
Models of an additional influence of SPA ejecta loading onto the global
stress field also showed good fitting results and are discussed as an
alternative or addition to the influence of tidal forces (Matsuyama et
al., 2021). N to NW orientated faults between ~20°E and
~40°E, and ~0°N to ~20°N
are predicted by a combination of recession stresses, diurnal tidal
stresses at apogee, and global contraction (Watters et al., 2015, 2019),
as well as by a combination of SPA loading, true polar wander, and
global contraction (Matsuyama et al., 2021). These predicted trends
approximately correspond with the NW to W orientation of crisp ridges
and scarps within Mare Tranquillitatis (Fig. 9), suggesting their
formation is consistent with those models. However, additional
influences by the regional geological setting in Mare Tranquillitatis,
such as by young volcanic activity (Braden et al., 2014; Qiao et al.,
2020), cannot be ruled out. The patterns of degraded wrinkle ridges
allign with both the patterns of advanced and heavily degraded features,
as well as with some crisp ridges and scarps (Fig. 4). Hence, degraded
ridges and scarps could reflect the evolution of the stressfield from
dominantly basin-localized to a dominately global stressfield, or they
represent the continued growth of ancient faults.
Previous studies discussed the possible activity of ancient wrinkle
ridges during the last Ma (French et al., 2019; Valantinas & Schultz,
2020). One possible evidence is the abundance of boulders at wrinkle
ridge crests (French et al., 2019; Valantinas et al., 2017; Valantinas
& Schultz, 2020; Watters et al., 2019). Valantinas and Schultz (2020)
suggested that layered mare basalts buckled and regolith drained into
small fractures during episodes of uplift, exposing the buckled material
below. Basilevsky et al. (2013) found that 50% of rock populations,
with fragment diameters larger than 2 m, are destroyed after 40 - 80 Ma
and 99% after 150 - 300 Ma. Following the boulder size of wrinkle ridge
boulder fields, Valantinas and Schultz (2020) proposed that wrinkle
ridges with high boulder abundance were active during the last tens of
millions of years. Boulder density increases with increasing slope. This
leads to the question whether boulders are simply associated with steep
slopes rather than ongoing wrinkle ridge activity since shallow seismic
shaking generated by impacts or tectonic activity unrelated to wrinkle
ridges could also result in the exposure of boulder fields (French et
al., 2019). For our classification, the abundance of boulders was not
used to determine the possible erosional state of a wrinkle ridge
segment. Crisp and degraded features do usually not appear in DiVINER
rock abundance maps and boulders are only visible occasionally in small
patches. Thus, no or merely a few boulders have been exposed during
their activity. Boulder-rich wrinkle ridges mapped by Valantinas and
Schultz (2020) tend to correlate with advanced degraded wrinkle ridges
rather than heavily degraded. However, it should be noted that the
boulder fields themselves could influence the morphological
classification since they typically appear brighter than the regolith
(Fig. 7a), possibly resulting in a greater contrast between the sunlit
and shadow side and, therefore, in a seemingly more defined appearance.
Five segments from Valantinas and Schultz (2020) can be associated with
degraded wrinkle ridges. These are located at the southeastern Lamont
ring and a single wrinkle ridge in southwestern Mare Tranquillitatis
(Fig. 10). All of these ridges occur together with advanced and heavily
degraded wrinkle ridges and are larger in relief than other degraded
features. They deform craters with ~100 m in diameter or
are accompanied by faint and small graben-like features (Fig. 10b). The
size of these degraded wrinkle ridges, their transitional morphology
between degraded and advanced degraded features, and their associated
patterns with advanced and heavily degraded wrinkle ridges suggest
possible ancient wrinkle ridges, which were later modified by more
recent activity. Our data do, however, not answer if this is the case
for all boulder-rich wrinkle ridges in Mare Tranquillitatis. Thus, we
can neither support nor reject that boulder fields along wrinkle ridges
are a sign of recent tectonism.
Boulder-rich wrinkle ridges on the lunar nearside are proposed to be
part of an active nearside tectonic system (ANTS). A possible origin for
this recent activity was assigned to the previously discussed deep
transient stresses generated by the South Pole-Aitken (SPA) basin
(Schultz & Crawford, 2011; Valantinas & Schultz, 2020) and a continued
fault adjustment correlated with deep-seated intrusions (Andrews-Hanna
et al., 2014; Valantinas & Schultz, 2020). Recorded deep moonquakes
might be evidence of the SPA-induced stresses (Valantinas & Schultz,
2020). However, as previously stated, the influence of these proposed
putative mare-filled ancient
rifts and intrusions on wrinkle ridge formation has been questioned
(Watters, 2022). Also, stresses attributed to SPA basin formation around
4.3 Ga are expected to have relaxed several Ga ago. Thus, the question
of young activity associated with ancient wrinkle ridges and the
implications of boulder fields remains unresolved.
5 Conclusions
In this study, compressional tectonic features were mapped in Mare
Tranquillitatis and classified into crisp, degraded, advanced degraded,
and heavily degraded, based on their morphology and erosional state.
This classification allows to suggest formation ages and possible
origins of these features:
- Crisp features show various signs of recent activity and presumably
have an age of tens of Ma (~50 Ma). Based on recent
studies and the shared orientation of crisp features, they likely
formed due to a combination of global contraction and an additional
influence of tidal forces and/or SPA loading.
- Degraded features, presumably, have a broad range of formation ages in
between crisp and advanced degraded features. They could reflect the
evolution of the stress field from dominantly basin-localized to a
dominantly global stress field, or they represent the continued growth
of ancient faults.
- Advanced and heavily degraded features presumably formed in the early
history of Mare Tranquillitatis, starting at ~3.8 Ga.
The distributions and orientations of these wrinkle ridges indicate
complex tectonic patterns and combined stresses. Ancient ridges in
western Mare Tranquillitatis have concentric, partly radial, and in
the case of Mare Tranquillitatis linear wrinkle ridge patterns
associated with basin loading and subsidence. There are scarce signs
of recent activity of some individual ancient wrinkle ridges within
the last 100 Ma.
Mare Tranquillitatis exhibits compressional tectonic features with a
variety of formation ages ranging from ancient to recent. The complex
and changing stress field behind wrinkle ridge formation is presumably a
result of a combination of different factors, which underlines the need
for new studies. Furthermore, our results highlight and strengthen the
case for a still tectonically active Moon within and outside of the
maria basins. To further uncover the active lunar tectonism, the future
installation of a geophysical network on the Moon is highly desirable
(Fuqua Haviland et al., 2022).