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\begin{document}

\title{Timing and Origin of Compressional Tectonism in Mare Tranquillitatis}



\author[1]{Thomas Frueh}%
\author[1]{Harald Hiesinger}%
\author[1,2]{Carolyn H. van der Bogert}%
\author[3]{Jaclyn D. Clark}%
\author[4]{Thomas R Watters}%
\author[1]{Nico Schmedemann}%
\affil[1]{Westfälische Wilhelms-Universität Münster}%
\affil[2]{Westfälische-Wilhelms-Universität Münster}%
\affil[3]{Arizona State University}%
\affil[4]{Smithsonian Institution}%


\vspace{-1em}



  
  \date{June 14, 2024}


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\selectlanguage{english}
\begin{abstract}
The lithosphere of the Moon has been deformed by tectonic processes for
at least 4 billion years, resulting in a variety of tectonic surface
features. Extensional large lunar graben formed during an early phase of
net thermal expansion before 3.6 Ga. With the emplacement of mare
basalts at \textasciitilde{}3.9 -- 4.0 Ga, faulting and folding of the
mare basalts initiated, and wrinkle ridges formed. Lunar wrinkle ridges
exclusively occur within the lunar maria and are thought to be the
result of superisostatic loading by dense mare basalts. Since 3.6 Ga,
the Moon is in a thermal state of net contraction, which led to the
global formation of small lobate thrust faults called lobate scarps.
Hence, lunar tectonism recorded changes in the global and regional
stress fields and is, therefore, an important archive for the thermal
evolution of the Moon. Here, we mapped tectonic features in the
non-mascon basin Mare Tranquillitatis and classified these features
according to their respective erosional states. This classification aims
to give new insights into the timing of lunar tectonism and the
associated stress fields. We found a wide time range of tectonic
activity, ranging from ancient to recent (3.8 Ga to \textless{} 50 Ma).
Early wrinkle ridge formation seems to be closely related to subsidence
and flexure. For the recent and ongoing growth of wrinkle ridges and
lobate scarps, global contraction with a combination of recession
stresses, diurnal tidal stresses, as well as with a combination of SPA
ejecta loading and true polar wander are likely.%
\end{abstract}%



\sloppy


\textbf{Plain Language Summary}

The lithosphere of the Moon has been deformed by tectonic processes for
at least 4 billion years, resulting in a variety of tectonic surface
features. Simple compressional asymmetric landforms are called lobate
scarps and complex compressional features, which form as a result of the
combination of faulting and folding, are known as wrinkle ridges. Lunar
wrinkle ridges only occur within the lunar maria. It has been argued
that their formation is linked to the subsidence of the dense mare
basalts, which would have happened in the early history of the Moon. We
mapped all of these features within one dark lunar region called Mare
Tranquillitatis and then studied their morphology on high-resolution
images. Based on their morphology, we found a wide time range of
tectonic activity, ranging from ancient to recent. Large wrinkle ridges
seem to be ancient and influenced by subsidence. Smaller wrinkle ridges
and lobate scarps show signs of recent activity. They likely formed
recently within the last hundred million years because of the Moon's
current state of global compression.

\par\null

T. Frueh\textsuperscript{1}, H. Hiesinger\textsuperscript{1}, C. H. van
der Bogert\textsuperscript{1}, J. D. Clark\textsuperscript{2}, T. R.
Watters\textsuperscript{3}, and N. Schmedemann\textsuperscript{1}

\textsuperscript{1} Institut f\selectlanguage{ngerman}ür Planetologie, Westfälische
Wilhelms-Universität Münster, Wilhelm-Klemm-Str.10, 48149 Münster,
Germany.

\textsuperscript{2} School of Earth and Space Exploration, Arizona State
University, 3603 Tempe, AZ, USA.

\textsuperscript{3} Center of Earth and Planetary Studies, National Air
and Space Museum, Smithsonian Institution, 37012 Washington, DC, USA.

Corresponding author: Thomas Frueh
(\href{mailto:thomas.frueh@uni-muenster.de}{\nolinkurl{thomas.frueh@uni-muenster.de}})

\par\null

\textbf{Key Points:}

\begin{itemize}
\tightlist
\item
  Early compressional tectonism in Tranquillitatis, in the form of
  wrinkle ridges, is presumably related to subsidence and basin loading.
\item
  Later tectonism could reflect the evolution from a basin-localized to
  a global stress field and the continued growth of ancient faults.
\item
  Recent wrinkle ridge and lobate scarp formation in Tranquillitatis
  occurred in the last 50 Ma and is influenced by a global stress field.
\end{itemize}

\par\null

\textbf{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 \textasciitilde{}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 \textasciitilde{}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.~

\par\null

\textbf{2 Background}

\emph{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
\textasciitilde{}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 \textasciitilde{}3.9 --
\textasciitilde{}4.0 Ga and generally ceased at \textasciitilde{}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
\textasciitilde{}1.2 Ga, crustal shortening associated with lunar maria
occurred at least as recently as \textasciitilde{}1.2 Ga (Watters et
al., 2009). A global survey of possible formation times found average
ages \textgreater{} 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 \textasciitilde{}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 \textless{} 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
\textless{}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.~

\par\null

\emph{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).

\par\null

\textbf{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 \textasciitilde{}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

\emph{Characteristics Used for the Classification of the Erosional
States of Wrinkle Ridges and Lobate Scarps}\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Class & Morphology & Crater crosscutting & Graben\tabularnewline
Crisp & Features with sharp and morphologically distinct edges and steep
slopes. & Can crosscut and deform craters with diameter ranges of
\textless{} 50 -- 100 m. & Small (width \textless{} 50 m) and crisp
clusters of graben are present.\tabularnewline
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.\tabularnewline
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.\tabularnewline
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.\tabularnewline
\bottomrule
\end{longtable}

\emph{Note}. Slopes and morphological descriptions are described
relative to each other.~

\textbf{4 Results}

A total of 242 wrinkle ridges, 137 lobate scarps, and 148 unidentified
structures, with a total length of \textasciitilde{}10,991 km, were
mapped in this study (Fig. 3). The length of individual segments ranges
from \textasciitilde{}1 km to \textasciitilde{}175 km, with a mean
length of \textasciitilde{}21 km. The mapped wrinkle ridges have a total
length of \textasciitilde{}7,852 km and range from \textasciitilde{}3.7
km to \textasciitilde{}175 km. The wrinkle ridge mean length is 32.8 km.
Lobate scarps have a total length of \textasciitilde{}946.4 km with a
minimum length of \textasciitilde{}1 km and a maximum length of
\textasciitilde{}58.5 km. The lobate scarp mean length is
\textasciitilde{}7 km.

The differences in the appearance of the ridge segments allow
distinguishing four different classes. These classes are crisp,
moderately degraded, advanced degraded, and heavily degraded. 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 \textasciitilde{}451
km and an average length of \textasciitilde{}9.2 km were classified as
crisp (Fig. 5). Consequently, they represent 5.1\% of the total mapped
length. Crisp features consist of wrinkle ridges and lobate scarps. All
of them occur scattered within Mare Tranquillitatis and are often close
to moderately degraded features (Fig. 4). In general, they have a NW --
WNW orientation. Crisp features have sharp edges, and steep slopes on a
small scale (\textless{} 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; \textless{} 50 -- 100 m diameter) and wrinkle ridges often appear to
be surface breaking when they crosscut craters. Clusters of small (width
\textless{} 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 moderately degraded (Fig. 6). They
have a total length of \textasciitilde{}780 km, which makes up 8.9\% of
the total mapped length. On average, they have a \textasciitilde{}7.8 km
length and generally show a NW orientation. Moderately degraded features
are comprised of lobate scarps and wrinkle ridges. 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 Mare Tranquillitatis and
can be spatially associated with crisp, advanced, and heavily degraded
features.

Advanced degraded features (Fig. 7) are the second most common class and
are dominantly comprised of wrinkle ridges. A total of 251 advanced
degraded wrinkle ridges with an average length of \textasciitilde{}11 km
have been mapped, resulting in a total length of \textasciitilde{}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 meters 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 \textgreater{} 30\selectlanguage{ngerman}°. 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 deform and 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 (Fig. 7b; Gold, 1972; Zharkova et al., 2020;
Bondarenko et al., 2022). Extensive boulder fields are associated with
some advanced degraded wrinkle ridges (Fig. 7).

The most common class are the heavily degraded features (Fig. 8), which
also dominantly consist of wrinkle ridges. 258 segments, with a total
length of \textasciitilde{}3,140 km and an average length of
\textasciitilde{}12.1 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. 8),
and the classical wrinkle ridge structure is often only visible in
topographic data. They have many superimposed craters and generally do
not crosscut any craters, but rarely can deform craters with diameters
of several hundred meters. There are no associated small graben present.
Their surface texture can resemble an 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 moderately
degraded wrinkle ridges than advanced degraded wrinkle ridges. Both
classes represent the largest wrinkle ridge structures in Mare
Tranquillitatis in length, width, and height.

\par\null

\textbf{5 Discussion}

The sharp-edged morphology and the relatively small size of the crisp
wrinkle ridges and lobate scarps suggest a relatively young formation
age in contrast to advanced degraded and heavily degraded features.
Crisp features can crosscut craters with diameters of less than 50 - 100
m. Craters of these sizes are estimated to be of Copernican ages
(\textless{} 800 Ma; Wilhelms et al., 1987), because older craters of
this size would have been infilled and degraded since then (Trask, 1971;
Basilevsky, 1976; Fassett \& Thomson, 2014). Thus, it is possible to
establish a Copernican age, i.e., an upper age limit of
\textasciitilde{}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) and possibly even wrinkle ridges (Valantinas et al.,
2018). Accompanying crisp features are small fresh graben and troughs
(Fig. 5). 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 \textasciitilde{}17 m to
\textasciitilde{}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
\textasciitilde{}1 m deep graben should be filled entirely with regolith
after \textasciitilde{}12.5 to \textasciitilde{}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;
Ghent et al., 2014; 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 highlights the global recent wrinkle ridge formation.

Based on our study we cannot conclusively estimate formation ages for
moderately degraded features. Crater crosscutting relationships imply
younger ages for moderately degraded wrinkle ridges and lobate scarps
than advanced degraded features. The main difference between moderately
degraded and crisp features, next to a more rounded morphology, 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 moderately 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 (Fig.
9, 10). 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 \selectlanguage{english}[?] 2, but \textless{}
10 MPa (Watters et al., 2015, 2019). For shallow thrust faults to form,
an estimated \textasciitilde{}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
(\textasciitilde{}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 \textasciitilde{}20degE and
\textasciitilde{}40degE, and \textasciitilde{}0degN to \textasciitilde{}20degN
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 W to NW orientation of crisp ridges
and scarps within Mare Tranquillitatis (Fig. 11), suggesting their
formation is consistent with those models. However, it should be
highlighted that the lithopsheric stressfield is a result from the
complex interaction and overlaying of multiple stresses, evolving with
time. Additional influences like, e.g., late stage mare basalt cooling
(Tian et al. 2021), stresses related to a possible movement of magma in
connection with young volcanic activity in Mare Tranquillitatis (Braden
et al., 2014; Qiao et al., 2020), or preexisting ancient faults in the
basement might have influenced the regional stress field. The patterns
of moderately 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, moderately degraded wrinkle ridges
and scarps could reflect the evolution of the stressfield from
dominantly basin-localized to a dominately global stressfield, and they
could represent the continued growth of ancient faults.

The large size and strongly degraded morphology of advanced and heavily
degraded features suggest an older formation age relative to moderately
degraded and crisp features. Advanced and heavily degraded features
deform all the mare units defined by Hiesinger et al. (2000), which have
ages of \textasciitilde{}3.4 to \textasciitilde{}3.8 Ga. Consequently,
they have an upper formation 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
can deform craters of several hundred meters in diameter but generally
deform no craters, which agrees with Nectarian, Eratosthenian, and
Imbrian formation ages (Trask, 1971). The deformation of craters with
diameter ranges of hundreds of meters implies on the basis of crater
degradation (Basilevsky, 1976, Fassett \& Thomson, 2014) that at least
some wrinkle ridges experienced ongoing activity throughout the
Nectarian, Eratosthenian, and possibly even the Copernican. Yue et al.
(2017) found a young average age of large wrinkle ridges in Mare
Tranquillitatis (\textasciitilde{}2.4 Ga) relative to other lunar maria
(\textasciitilde{}3.3 Ga). With the focus on Mare Tranquillitatis and
the degradation-state approach of our classification, this age
discrepancy between wrinkle ridges in Mare Tranquillitatis and other
maria 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 than our
morphological analysis are needed to uncover the early tectonic
evolution of the maria basins, however, standard CSFD measurements on
wrinkle ridges are challenging, because of steep slopes, small count
areas, and the often hummocky and heterogeneous terrain (Frueh et al.,
2020). Hence, buffered crater counting might be a more suitable option
to obtain formation ages of individual wrinkle ridges.

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. 9, 10; 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. 9, 10). 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 positive anomaly is expressed
as an elongated depression. Advanced and heavily degraded wrinkle ridges
and large graben within this depression occur parallel to the
gravitational anomaly and the topographic depression, which could also
imply a subsidence-related origin (Fig. 3; McGovern et al., 2022). Mare
Serenitatis most likely influenced the northwestern Mare
Tranquillitatis, resulting in a radial wrinkle ridge and parallel graben
to Mare Serenitatis (Fig. 3). Wrinkle ridges close to the eastern mare
boundary follow a NS trend similar to the general trend of the eastern
boundary itself, which is consistent with an origin from basin loading
and subsidence. The eastern mare boundary shows no clearly detectable
gravitational anomalies that have been associated with deep fractures of
the mare basement (Fig. 10; Andrews-Hanna et al., 2018). 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; Valantinas \& Schultz, 2020), 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. In this case, wrinkle ridges would be
the surface expression of these deep-seated intrusions (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 little evidence that most of the
ridge faults were influenced by buried structures associated with
ancient rifts (Watters, 2022). Having said that, we do observe wrinkle
ridges correlated with sharp increases in elevation, possible extensive
folding, and an elevation offset between both sites of the fold located
in this western part of the basin. Possible similar wrinkle ridges have
been associated with deeply rooted faults penetrating the base of mare
deposits (Byrne et al., 2015; Watters, 2022). The slopes of the trough
associated with the positive gravitational anomaly, however, complicates
the assessment of the actual extent of folding. Quantifying the
deformation in Mare Tranquillitatis will be part of a planned follow-up
study. In summary, the compressional stresses that resulted in the
formation of advanced and heavily degraded wrinkle ridges in Mare
Tranquillitatis originated primarily from load-induced subsidence with
other possible sources of regional or global stress, like SPA induced
stress and fault adjustment, which changed with time. Individual wrinkle
ridges in western Tranquillitatis could be correlated to the deep-rooted
faults.

It should be noted that the ancient age of advanced and heavily degraded
feature, which we we imply in this study, is the formation age and not
necessarily the age of the most recent activity along the fault.
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) and Ghent et al. (2014) 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
moderately 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. This could be either evidence against boulder fields as
a general sign of recent tectonic activity, related to the relation
between flow thickness and thrust fault size, or correlated with the
physical properties of the basalt flows, which result in different
predicted rock abundances (Elder et al., 2022). 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 moderately degraded wrinkle ridges. These are located
at the southeastern Lamont ring and a single wrinkle ridge in
southwestern Mare Tranquillitatis (Fig. 12). All of these ridges occur
together with advanced and heavily degraded wrinkle ridges and are
larger in relief than other moderately degraded features. They deform
craters with \textasciitilde{}100 m in diameter or are accompanied by
faint and small graben-like features (Fig. 12b). The size of those
moderately degraded wrinkle ridges, their transitional morphology
between moderately 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. Two large wrinkle ridges directly north of the
Lamont Anomaly could also contribute to this discussion (Fig. 3, 4, 9).
The eastern ridge (7.0degN, 22.7degE) shows a high boulder abundance, while
the western ridge (7.0degN, 22.1degE) only exhibits boulder fields in its
northern most part (Valantinas \& Schultz, 2020). Segments of the
eastern ridge are classified as advanced degraded and segments of the
western ridge, mainly, as heavily degraded. Additionally, the eastern
wrinkle ridge shows one small off-shoot segments, which is classified as
moderately degraded (Fig. 4). The off-shoot segment could highlight a
continuing growth of the eastern wrinkle ridge, which resulted in the
formation of boulder fields. In those cases, boulder fields might be a
sign of recent activity of the wrinkle ridges. However, since boulders
might also be exposed due to seismic shaking unrelated to wrinkle ridges
(French et al., 2019) and boulder fields can also be found on other
positive relief features than wrinkle ridges, we are not able to
conclude if all boulder rich wrinkle ridges in Mare Tranquillitatis are
boulder-enriched due to their recent tectonic activity. In addition,
crisp and moderately degraded features have not exposed extensive
boulder fields. Thus, we can neither clearly support nor reject boulder
fields along wrinkle ridges as a general 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, it remains unknown
if the current lunar stress field would allow for ongoing readjustment
of those ancient faults. Thus, the question of young activity associated
with ancient wrinkle ridges and the implications of boulder fields
remains unresolved. The reactivity or prolonged activity of ancient
wrinkle ridges, however, seems to be true for at least some individual
wrinkle ridges in Mare Tranquillitatis.

\par\null

\textbf{6 Conclusions}

In this study, compressional tectonic features were mapped in Mare
Tranquillitatis and classified into crisp, moderately 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:

\begin{itemize}
\tightlist
\item
  Crisp features show various signs of recent activity and presumably
  have an age of tens of Ma (\textasciitilde{}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 ejecta loading.
\item
  Moderately 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, and they
  represent the continued growth of ancient faults.
\item
  Advanced and heavily degraded features presumably formed in the early
  history of Mare Tranquillitatis, starting at \textasciitilde{}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
  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.
\end{itemize}

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).



\textbf{Acknowledgments}

The authors thank the LROC teams and LRO engineers for obtaining the
essential data. We also thank the SELENE (KAGUYA) TC team and the SELENE
Data Archive for providing the SELENE (KAGUYA) data. We acknowledge the
use of imagery from Lunar QuickMap
(\url{https://quickmap.lroc.asu.edu}), a collaboration between NASA,
Arizona State University \& Applied Coherent Technology Corp.



\textbf{Data Availability Statement}

Kaguya TC images can be obtained from the SELENE Data Archive
(\url{https://darts.isas.jaxa.jp/planet/pdap/selene/}). The TC morning
and evening image files can be downloaded from the
sln-1-tc-4-evening-map-v4.0/ and sln-1-tc-4-morning-map-v4.0/ folders in
the directory. LROC image data (Robinson, 2009) are available from the
Lunar Orbital Data Explorer, which is produced by the NASA Planetary
Data System Geosciences Node. The SLDEM2015 global map (Neumann, 2009)
is also available on the Planetary Data System, as well as the derived
GRAIL data (Kahan, 2013). Global GRAIL maps can be downloaded in the
rsdmap directory. The GRAIL bouguer gravity gradient map was accessed
from the supplementary material from Andrews-Hanna et al. (2018).
Additionally, LRO, Kaguya, and GRAIL data can be accessed with Quickmap
(\url{https://quickmap.lroc.asu.edu}). The shown rose diagram was
created with GeoRose 0.5.1 (Yong Technology Inc, 2014). ~

\par\null

\textbf{References}

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\par\null

\textbf{Figure Descriptions}

\textbf{Figure 1}. Schematic model of the signs of recent tectonic
activity of surface features. A small crisp wrinkle ridge segment in
Mare Tranquillitatis served as a template for the topographic profile.
The signs of recent tectonic activity apply, however, both for lobate
scarps and wrinkle ridges. These signs include crisp morphology,
deformed craters, cross-cut craters, small graben and troughs, lower
crater density, and boulder fields/patches. In this study, the boulder
abundance was not used to determine the degradational stage of a wrinkle
ridge or lobate scarp. Shallow moonquakes detected by the Apollo
missions have been previously correlated to the activity of lobate
scarps.

\textbf{Figure 2}. a) Location of Mare Tranquillitatis (white outline;
Nelson et al., 2014) near the lunar equator projected onto the global
merged WAC mosaic. b) Mare Tranquillitatis (black outline) projected
onto the LRO LOLA -- SELENE Kaguya DEM (Neumann, 2009; Barker et al.,
2016). A color-blindness-friendly version can be accessed in the
supplementary materials (Fig. S1). c) The black outline of Mare
Tranquillitatis projected onto the GRAIL Free Air Gravity map (Kahan,
2013; harmonic degree and order of 660). The black lines sketch the
proposed quasi-rectangular pattern of ancient intrusion (after
Andrews-Hanna et al., 2012).

\textbf{Figure 3}. Tectonic map of Mare Tranquillitatis projected on the
merged LRO LOLA -- SELENE Kaguya DEM (Barker et al., 2016). A
color-blindness-friendly version can be accessed in the supplementary
materials (Fig. S2). Parts of the lobate scarp cluster in the northern
mare cross the highland boundary and continue into Mare Serenitatis near
the Taurus-Littrow valley. 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).

\textbf{Figure 4}. Tectonic feature map with all degradational
classified segments colorized according to their respective class and
projected onto the WAC global mosaic (Robinson et al., 2012). This map
includes wrinkle ridge, lobate scarp, and unidentified features.
Tectonic features in the western part are mostly comprised of advanced
and heavily degraded features. Crisp and moderately degraded features
occur scattered in clusters throughout the mare.

\textbf{Figure 5}. NAC images of crisp features. White arrows show
representative graben. a) Wrinkle ridge north of Ross Crater with a
crisp morphology and small graben (M1184668142RE; 11.82\selectlanguage{ngerman}°N, 24.27°E).~ b)
Image of the same wrinkle ridge further west. Visible are several sets
of small graben and a small boulder patch (black arrow; M1184668142RE;
11.90°N, 24.17°E). c) Small and faint lobate scarp in the vicinity of
Taurus-Littrow valley. The image shows some faint graben-like features
and deformed craters with \textasciitilde{}100 to \textasciitilde{}50 m
in diameter (black arrows; M1154023134RE; 19.11°N, 29.93°E). d) Set of
graben in close vicinity of a crisp lobate scarp cluster near
Taurus-Littrow (M1157549836RE; 18.52°N, 30.55°E).

\textbf{Figure 6}. NAC images of moderately degraded features with
relatively sharp contacts (white arrows) in Mare Tranquillitatis. a) A
moderately degraded wrinkle ridge in the eastern mare deforming and
cross-cutting several craters (black arrows; M1245756057LE/RE; 12.29°N,
39.82°E) and b) a small moderately degraded lobate scarp in the
northwestern mare which also deforms a \textasciitilde{}100 m diameter
crater (black arrows; M1279976340LE; 14.60°N, 20.04°E).

\textbf{Figure 7}. Kaguya Terrain Camera images of a representative
advanced degraded wrinkle ridges. The advanced degraded wrinkle ridge
(7.54°N, 22.75°E) has a well-developed wrinkle ridge morphology
consisting of a broad arch and a superimposed ridge (white arrows). In
addition, it exhibits several dominant boulder fields, which are visible
as bright spots along the ridge (black arrows). b) Close up NAC image
(M1234102538LE) of the ridge shown in 7a, featuring boulder fields
(black arrow) and the ``elephant-hide'' texture (white arrows).

\textbf{Figure 8}. Kaguya Terrain Camera images of representative
heavily degraded wrinkle ridges (white arrows). Both wrinkle ridges (a),
1.31°N, 22.56°E; b), 8.10°N, 22.20°E) have gentle slopes and less
well-developed wrinkle ridge morphologies than seen in figure 7. The
\textasciitilde{}1 km sized crater in the center of image a) resembles a
rare case, which shows the possible deformation of a crater by a heavily
degraded ridge. Survival times of \textasciitilde{}1 km sized craters
are still estimated to be several billion years (Fassett \& Thomson,
2014).

\textbf{Figure 9}. Bouguer anomaly map of Tranquillitatis superposed on
the WAC global mosaic. The map has the same spatial extent as the map in
Fig. 4, and shows the tectonic feature map of Fig. 4. The outline of
Mare Tranquillitatis is shown as a fine white line. Yellowish colors
indicate positive gravitational anomalies, which implies a thin crust
and mantle upwelling, as well as a thick abundance of basalt. Mascon
basins like Mare Serenitatis in the northwestern part of the map are
represented in yellow colors, whereas non-mascon basins like Mare
Tranquillitatis appear in more heterogenous and mainly blue and green
colors. The western part of Mare Tranquillitatis has more pronounced
positive gravitational anomalies than the eastern part. Concentric
wrinkle ridges occur at the positive Lamont anomaly in southwestern
Tranquillitatis. Crisp and moderately degraded features are not
correlated with gravitational anomalies.

\textbf{Figure 10}. Grail bouguer gravity gradients map of Mare
Tranquillitatis (supplement material from Andrews-Hanna et al., 2018) in
units of Eötvös (1 E = 10--9 s--2) and our tectonic map of Fig. 3.
Gravity gradient maps are used to identify buried deep-seated
structures, like large igneous intrusions and ring-faults in impact
basins (e.g., Andrews-Hanna et al., 2013, 2014, 2018; Valantinas \&
Schultz, 2020). Eastern Tranquillitatis does not exhibit clearly
detectable anomalies known from deep faults.

\textbf{Figure 11}. Rose diagram of the orientations of crisp features
within Mare Tranquillitatis, including lobate scarps and wrinkle ridges.
Crips features share a western to northwestern orientation.

\textbf{Figure 12}. Evidence for recent activity by ancient wrinkle
ridges in Mare Tranquillitatis. (a) Shows the topographic map of the
region southeast of the Lamont anomaly. The stars mark the locations of
(b) and (c). (b) Shows NAC image (M1108125194LE; 3.43°N, 23.97°E)
showing a part of a concentric wrinkle ridge at the southeastern Lamont
anomaly. It crosscuts craters with \textasciitilde{}100 m in diameter
(white arrow) and exhibits several boulder fields (black arrows). (c)
NAC image (M162134363LE) of faint graben-like features on the hanging
wall of a wrinkle ridge (0.45°S, 26.47°E). A color-blindness-friendly
version can be accessed in the supplementary materials (Fig. S3).

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