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Icarus 404 (2023) 115654
Available online 7 June 2023
0019-1035/© 2023 Elsevier Inc. All rights reserved.
Geological and topographical analysis of Anshar Sulcus, Ganymede:
Implications for grooved terrain formation
Mafalda Ianiri
a
,
b
,
*
, Giuseppe Mitri
a
,
b
, Davide Sulcanese
a
,
b
, Gianluca Chiarolanza
a
,
b
,
Camilla Cioria
a
,
b
a
International Research School of Planetary Sciences, Italy
b
Dipartimento di Ingegneria e Geologia, Universit`
a d’Annunzio, Italy
ARTICLE INFO
Keywords:
Ganymede
Tectonics
Satellites
Surfaces
Images processing
Geological precesses
ABSTRACT
The formation of grooved terrains on the icy surface of Ganymede is still debated and it could involve extensive
rifting or spreading, strike-slip tectonics, and minor cryovolcanic resurfacing. Anshar Sulcus is a terminal portion
of a groove network imaged at high resolution by the Galileo Solid State Imaging subsystem (SSI). To investigate
the origin and evolution of the grooved terrains, we performed a geomorphological and structural analysis of
Anshar Sulcus producing a geomorphological map of the study area in 1:500,000 scale, and we conducted a
topographical analysis producing a Digital Elevation Model (DEM) of this region. We found several indicators
that the formation of Anshar Sulcus’ grooved terrain is the result of two tectonic events, the rst being a right
lateral strike-slip type and the second a crustal spreading. We found that the right lateral strike-slip movement
that divided the dark terrain of this area into two distinct regions represents the precursor of the formation of the
sulcus, and that the crustal spreading subsequently occurred through transtensional movement of the regions. We
also found that the grooves were formed within the solid-state raised material by brittle fracturing and tilting of
the newly formed crust.
1. Introduction
The icy surface of Ganymede, the largest satellite of the Jupiter
system, presents two dominant terrains characterized by different al-
bedo: the dark and light terrains. The dark terrains are composed of
dark, albedo-heterogeneous material, modied by surcial processes,
such as sublimation, mass wasting, and sputtering (Prockter et al., 1998,
2000). The dark terrains are highly cratered with an estimated age older
than 4 Ga (Smith et al., 1979a,Smith et al., 1979b), preserving the relicts
of vast global scale sets of concentrically arranged structures, called
furrows (Pappalardo et al., 2004). The younger light terrains are more
extensive than the dark terrains, covering approximatively 64% of
Ganymede’s surface (Patterson et al., 2010). Light terrains are perva-
sively crossed by sets of sub-parallel, closely spaced ridges and troughs,
referred to as grooves (Patterson et al., 2010; Pappalardo et al., 2004).
Pappalardo et al. (2004) suggests up to four evolutionary stages leading
to the formation of the grooved terrains: 1) reactivation of dark terrain
tectonic structures; 2) deformation of preexisting terrain by extensional
tectonism; 3) possible cryovolcanic resurfacing; and 4) cross-cutting by
more recent lanes of grooved terrain. Extensive tectonism is believed to
be the main mechanism forming the surface morphology of Ganymede
(Collins et al., 1998, 2010; Pappalardo et al., 1998, 2004; Collins, 2006).
Furthermore, evidence of strike-slip tectonism has been reported in the
sulci areas (Pappalardo et al., 1998; Cameron et al., 2018).
The grooved terrain of Anshar Sulcus (centered at 168◦59′E; 11◦
54′N; Fig. 1) is located in the anti-Jovian hemisphere within the dark
terrain of Marius Regio, and evidence of fracture related both to
extensive and strike-slip tectonism (Prockter et al., 2000; Pappalardo
et al., 2004; Cameron et al., 2018) is present along its northern and
southern boundaries with the dark cratered unit. Several tectonic ob-
servations led to different hypotheses concerning the origin of the
grooved terrain in Anshar Sulcus region (Prockter et al., 2000; Pizzi
et al., 2017; Cameron et al., 2018). Prockter et al. (2000) suggested the
Anshar Sulcus’ grooved terrain was the result of a combination of a NNE-
SSW extension and a minor right-lateral shearing, which would have
produced a hanging wall rollover in the NE-SW direction against a
prominent normal fault. Such hypothesis is supported by Cameron et al.
(2018) analyses. Differently, Pizzi et al. (2017) proposed a spreading
* Corresponding author at: International Research School of Planetary Sciences, Viale Pindaro 42, Pescara 65127, Italy.
E-mail address: mafalda.ianiri@unich.it (M. Ianiri).
Contents lists available at ScienceDirect
Icarus
journal homepage: www.elsevier.com/locate/icarus
https://doi.org/10.1016/j.icarus.2023.115654
Received 25 August 2022; Received in revised form 26 May 2023; Accepted 1 June 2023
Icarus 404 (2023) 115654
2
center origin for Anshar Sulcus with possible transtension, with resur-
facing and formation of a novel crustal material.
To investigate the origin and evolution of the grooved terrains, we
performed a geomorphological and structural analysis of Anshar Sulcus.
In addition, we conducted a topographical analysis producing a Digital
Elevation Model (DEM) of this region. We selected Anshar Sulcus
because it is a terminal portion of a grooved system and it was imaged at
high resolution by the Galileo SSI. These two aspects helped us to
perform a palinspastic reconstruction of the area surrounding the sulcus
and, consequently, a reconstruction of the different stages of formation
of this sulcus.
2. Methods
We performed a geological analysis of the Anshar Sulcus area by
using the 2865_r and 2878_r Solid State Imaging (SSI) data onboard the
Galileo spacecraft, both images having a spatial resolution of 152 m/px.
To import these images to the Geographic Information System (GIS), we
calibrated, ltered and geo-referenced them through the Integrated
Software for Imagers and Spectrometers (ISIS4) (Houck and Denicola,
2000). We also conducted a topographical analysis producing a DEM of
Anshar Sulcus, obtained with the “shape-from-shading” tool (SfS) pro-
vided by the NASA Ames Stereo Pipeline tool suite (Beyer et al., 2018).
Since the brightness of each pixel is related to the solar incidence angle,
the shape-from-shading (photoclinometry) technique retrieves surface
relief from the light intensity of each pixel of the image analysed
(Alexandrov and Beyer, 2018). This technique generates a DEM with the
same spatial resolution of the source image, corresponding to ~152 m/
px for the selected images. However, since SfS requires a base topog-
raphy to start the process, and such topography is not available in
Anshar Sulcus, we used a at DEM (at DEM indeed as a raster having
0 elevation in each pixel), as suggested in Lesage et al. (2021).
Fig. 1. a) Ganymede Voyager - Galileo SSI Global Mosaic 1 km v1; the yellow polygon indicates the position of Anshar Sulcus on Ganymede’s surface; b) Anshar
Sulcus region (centered at 168
◦59′E; 11◦54′N) imaged by the Galileo SSI camera. In background the Ganymede Voyager – Galileo SSI Global Mosaic (Images credit:
USGS). (For interpretation of the references to color in this gure legend, the reader is referred to the web version of this article.)
M. Ianiri et al.
Icarus 404 (2023) 115654
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Moreover, long-wavelength topography artifacts might occur
because of the assumptions of this technique concerning the uniformity
of the surface’s photometric properties (Kirk et al., 2003). For this
reason, we have restricted the topographical analysis only to small
features (e.g., individual craters, troughs, and reliefs). Moreover, to
support the DEM height measurements, we also retrieved height values
from single shadow length measurements by using the QVIEW tool of
USGS ISIS4 (Gaddis et al., 1997). We produced a geomorphological map
of Anshar Sulcus in 1:500,000 scale, using the differences in tones,
textures and patterns of the Galileo and Voyager imagery data and the
DEM. The geomorphological map was created in the geographic infor-
mation system ArcMap by overlapping a vector layer composed by lines
and polygons to the Galileo images. Anshar Sulcus area hosts clear ev-
idence of displacement along the boundary between dark and grooved
terrain, which splits the dark terrain in two separate portions of terrain,
referred to as regions in this work. To better constrain the kinematics
affecting such regions, we re-joined the displaced features along the
regions’ boundaries by using both the Galileo images and the elevations
inferred by the DEM.
3. Results
3.1. Digital Elevation Model
We conducted a topographical analysis producing a DEM of Anshar
Sulcus area, that will be discussed in the next Sections. Fig. 2 presents
the produced DEM of Anshar Sulcus region.
3.2. Geomorphological map
Fig. 3 presents the produced geomorphological map of the Anshar
Sulcus area in 1:500,000 scale. In the geomorphological map we out-
lined both geomorphological units and linear features that are described
in subsections 3.2.1 and 3.2.2, respectively.
3.2.1. Geomorphological units
The dark cratered unit (dc) covers most of the study area (centered at
168◦59′E; 11◦54′N), and consists of a heavily cratered surface, char-
acterized by a low albedo in Voyager images. Collins et al. (2013)
classied the dark cratered material as a relatively low albedo material
with moderate to high crater densities, with brighter hummocks at local
scale. The dc unit of our map presents a moderately smooth appearance,
with a pervasive fracturing having variable dimensions and orientation.
Within the dc we observed several patches of hummocky material (h),
previously recognized by Prockter et al. (2000), consisting of an alter-
nation of few hundreds of meters-high mounds and valleys and resulting
in a heterogenous albedo. Some mounds are clustered along a linear
trend, forming elongated ridges.
The light grooved unit (lg) consists of a prominent lane cross-cutting
the dc and some craters, and dening a low cratered stripe, which sep-
arates the study area in two regions. Collins et al. (2013) classied the
light grooved material as a relatively high to moderate albedo material
dominated by closely packed groove structures. In our study area, lg
appears brighter than the dc in Voyager imagery and is characterized by
sets of sub-parallel linear grooves.
The last geomorphological unit is dened by impact craters having a
diameter larger than 4 km. The largest crater present in the study area is
in the southern part (Fig. 3) and it is characterized by a heavily fractured
oor and by an elongated shape in NE-SW direction with minor and
major axes of ~20 and ~ 24 km, respectively. We also observed several
smaller craters primarily within the dc and h units in both northern and
southern regions, but also within the lg.
3.2.2. Linear features
The geomorphological map of the study area also includes the linear
features, such as ridges and troughs (Prockter et al., 2000). We observed
sets of linear ridges in the hummocky terrain, having a maximum length
of ~25 km. The mean height of linear ridges and mounds is ~280 m as
derived from the DEM (Fig. 2). The dark cratered unit (dc) hosts several
Fig. 2. Digital Elevation Model (DEM) of Anshar Sulcus region. (For interpretation of the references to color in this gure, the reader is referred to the web version of
this article.)
M. Ianiri et al.
Icarus 404 (2023) 115654
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fractures that we divided into three main sets based on their spatial
distribution and dimensions (lengths, widths, and depths). Two main
sets of fractures are in the southern region and present a prevalent NW-
SE direction (letters “a” and “b” in Fig. 3).
Fractures forming the westernmost set (letter “a” and color blue in
Fig. 3) extend from ~3 km up to ~50 km in length, have a mean width of
~1.5 km, and a depth ranging from ~95 m up to ~650 m (depth
intended as vertical distance between local topographical maximum and
minimum of the elevation prole). In its central part, this set runs
through the largest crater, cutting its rims and deforming it. Moving
toward NW this fracture set laterally expands until it abruptly ends at
the grooved terrain and it continues his path in the northern region with
a right lateral displacement of ~15 km. To better constrain that the
fractures in the western part of the northern region belong to set “a”, we
traced topographic proles (Fig. 4) which coincide with each other,
conrming the previously observed morphological analogy. The east-
ernmost fracture set (letter “b” and color green in Fig. 3) appears to be
less pervasive than the previous one, with extensive fractures extending
from ~3 km up to ~30 km in length, with a mean width of ~0.7 km, and
a depth ranging from ~85 m up to ~450 m. Even in this case the
grooved terrain crosscuts these fractures; however, we do not observe
any continuation in the northern region. A right step-like series of ~1.7
km spaced fractures with en-echelon disposition (letter “c” and color
orange in Fig. 3), a NE-SW orientation and a mean length of ~9 km,
stretches in the NE part of the study area, ending its path in corre-
spondence with the southern grooved terrain. This path continues in the
southern region with a more linear disposition, having the same orien-
tation of the putative main strike-slip fault related the fracture set ‘c’
(Fig. 3).
The grooved terrain also shows a series of sub-parallel, closely
spaced structures. They consist of an alternance between grooves and
ridges having a mean depth of ~150 m (Fig. 5). This groove system can
be divided in two main sections, distinguished by a different orientation
and width. In particular, the rst section (166◦44′E 13◦16′N; 170◦14′E
11◦34′N) stretches from NW to SE, with a mean width of ~9 km. The
second section (170◦14′E 11◦34′N; 171◦24′E 11◦26′N) has a prevalent
W-E direction with a mean width of ~3 km. Topographical analyses of
the DEM revealed that, in the rst section, the elevation of the grooved
terrain increases toward its central part by a mean vertical relief of ~74
m (vertical relief calculated along the proles A-A’, B-B′and C-C′in
Fig. 5).
Fig. 3. Geomorphological map of Anshar Sulcus region. (For interpretation of the references to color in this gure legend, the reader is referred to the web version of
this article.)
M. Ianiri et al.
Icarus 404 (2023) 115654
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3.3. Terrain reconstruction
Fig. 6 shows the reconstructed apparent geometrical correspondence
between the southern and northern regions of some features (yellow-
dotted rectangle) by applying the same displacement of the northern
crater (green rectangle). The fracture set “a” and the associated northern
crater (Fig. 7) show a displacement of ~15 km. Even if the southern half
of the crater is not well-preserved because of an over-imposing cluster of
younger craters, we observed morphological and topographic similar-
ities of the fracture set “a” across the northern and southern parts of the
regions (Fig. 4). The half-crater, located in the terminal part of the
southern region associated with the set of fractures “b”, has been dis-
placed by ~13 km (Fig. 8). In this case, we found a discrepancy of 1 km
between the diameter of the northern half crater (8 km) and the
southern one (7 km). This is probably due to the extensive fracture set
“b” that could have affected and deformed the southern half of the
crater, resulting in an increase in the lateral distance between the two
crests of the southern rim. To quantify the possible inuence of the set of
fractures “b” in the southern half crater, which could have increased the
lateral distance between the diametrical crests of the rim, we measured
the mean diameter of both half craters and the mean width of the
fractures. To determine the size of the diameters of the two half craters,
we tracked radial topographic proles within the two half craters,
marking the crests corresponding to the crater rim. Then, we tracked
circular polygons passing through each marked crest point, nally
calculating the average measure of the resulting diameters. Table 1
presents DEM-derived measurements of length of fracture widths, and
the diameters of upper unfractured half crater and lower fractured half
crater in Figs. 8 and 9. We found that the diameter of the upper
unfractured half crater is (7070 ±200)m, whereas the diameter of the
lower fractured half crater is (8240 ±360)m. The sum of all fractures’
widths dissecting the lower half crater is (1150 ±260)m. We found that
the diameter of the upper half crater is consistent with the diameter of
the lower half crater deprived of the fractures’ widths. Additionally, we
compared topographic proles traced across the rims of the two half
craters and we found marked similarities in the general shape of the
resulting proles (Fig. 9).
Fig. 4. a) Digital Elevation Model of the area dissected by the fracture set “a” (upper panel). Topographic proles A-A’ (panel b) and B-B
′(panel c) from the set of
fractures “a”. (For interpretation of the references to color in this gure, the reader is referred to the web version of this article.)
M. Ianiri et al.
Icarus 404 (2023) 115654
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Fig. 5. Topographic proles of the grooves in Anshar Sulcus. The right panels show the elevation vs distance. The corresponding tracks are shown in the left panel.
(For interpretation of the references to color in this gure, the reader is referred to the web version of this article.)
Fig. 6. Apparent correspondence between the southern and northern regions of tted-back features (yellow-dashed rectangle in the upper panel, zoomed details in
panels a and b; the yellow signs in panel b highlight the apparent correspondence and continuity of linear features between the two boundaries of dark terrain
regions) by applying the same displacement of the set of fracture “a” (green rectangle in the upper panel). (For interpretation of the references to color in this gure
legend, the reader is referred to the web version of this article.)
M. Ianiri et al.
Icarus 404 (2023) 115654
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Fig. 7. The largest crater in the western portion of the northern region of dark terrain along the limit with the light terrain. Current position of the crater as seen from
image (a) and topographic (b) data (b); Reconstruction of the possible original position as seen from image (c) and topographic (d) data. (For interpretation of the
references to color in this gure, the reader is referred to the web version of this article.)
Fig. 8. Two half craters to the west, along the boundaries between the dark terrain and light terrain. Current positions of the half-craters as seen from image (a) and
topographic (b) data; Reconstruction of the possible original positions of the half-craters as seen from image (c) and topographic (d) data. (For interpretation of the
references to color in this gure, the reader is referred to the web version of this article.)
M. Ianiri et al.
Icarus 404 (2023) 115654
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3.4. Putative tectonic evolution
From the inferred reconstruction of the pre-deformed terrain pre-
sented in §3.2, we have modelled the putative tectonic evolution of
Anshar Sulcus area. Fig. 10 presents the inferred terrain reconstruction
using as piercing points two impact craters and the associated fracture
sets, whose original locations are highlighted in orange and green cir-
cles, and the displacements are shown in orange and green arrows. Panel
a presents the larger and older crater (orange circle) prior to the terrain
deformation in the Anshar Sulcus area. Remarkably, the margins of the
two regions forming the pre-deformed dark terrain are well-preserved
overall, suggesting the absence of degradation or damage processes of
the region margins.
Panel b presents the reconstruction of the terrains considering as
piercing point the location of the younger impact crater (green circle)
that was formed successively to the larger, older crater. The orange ar-
rows indicate the displacement of the rst larger crater at the time of the
younger crater’s formation. This evidence suggests that the fault has had
an initial component of a pure right-lateral strike-slip movement of the
northern region in a NW-SE direction. Panel c presents the reconstruc-
tion of the terrains up to the present time, involving an opening of the
two regions that formed the light grooved terrain, properly called
Anshar Sulcus. The presence of well-preserved structures along the
boundaries of the two dark terrain regions suggests that the formation of
Anshar Sulcus occurred as a spreading event and opening of the two
regions (Pizzi et al., 2017), leading to the formation of novel crustal
Table 1
Measurements of length of fracture widths, and the diameters of upper half crater and lower half crater shown in Figs. 8 and 9.‘N counts’ indicates the times we
measured the total width of the fractures within the lower half crater and the diameters of the upper and lower half craters. For the lower half crater, diameters of the
current, fractured crater and the unfractured crater - obtained by subtracting fracture widths, see text - are reported.
Fractures Upper half crater Lower half crater
N◦counts 8 6 6
With fractures Without fractures
Lenght (m) 1149 ±255 7070 ±200 8235 ±359 7086
Fig. 9. a) Tracks of the topographic proles of the two half craters to the west, located along the boundaries between the dark terrain and light terrain; Topographic
proles corresponding to the tracks C-C
′and D-D′. (For interpretation of the references to color in this gure, the reader is referred to the web version of this article.)
M. Ianiri et al.
Icarus 404 (2023) 115654
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Fig. 10. Reconstruction of the strike-slip displacement across Anshar Sulcus a) Older crater before displacement (orange circle). b) Formation of the second crater
(green circle), the orange arrows indicate the subsequent displacement of the rst crater at the moment of the impact that formed the second crater; c) Current
situation that shows different amount of displacement of the craters. (For interpretation of the references to color in this gure legend, the reader is referred to the
web version of this article.)
Fig. 11. Detail of two crater rims along the boundaries of the light terrain. Current position of the crater rims (red arrows) in image (a) and topographic (b) data.
Reconstruction of the possible original position of crater rims (red arrows) in image (c) and topographic (d) data. (For interpretation of the references to color in this
gure legend, the reader is referred to the web version of this article.)
M. Ianiri et al.
Icarus 404 (2023) 115654
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material likely due to an upwelling of fresh material. The formation of
the grooves that characterize the light terrain unit likely occurred in a
relatively short time scale, due to the instant brittle fracturing of the new
crust occurring simultaneously with the spreading event. The latter
could also have produced, tilting of newly formed crustal blocks.
The third phase of tectonic evolution of the Anshar Sulcus area also
involved a further displacement of the two impact craters highlighted in
orange and green arrows, evidenced by a further right lateral offset with
respect to the present location of the craters. Our reconstruction of the
terrains up to the present time has evidenced that during this third
phase, the lateral and perpendicular displacements of the craters did not
occur as a result of two different stages of tectonic evolution, repre-
sented by an initial pure strike-slip movement and a later spreading.
Instead, our terrain reconstruction has evidenced that the lateral offset
of the craters starts during the strike-slip stage (Fig. 10 a, b) and the
further lateral displacement occurred during the spreading stage (Fig. 10
c).
In addition, we present two uncertain observations that, if
conrmed, could better elucidate the tectonic evolution of Anshar Sul-
cus. We observed two crater rims along the boundary between the dark
cratered unit and grooved terrain, that could belong to the same crater.
Fig. 11 shows the current and reconstructed original position of two
crater rims located along the boundaries of the light terrain, as seen from
image and topographic data. However, the terrain reconstruction is
putative due to the uncertainty derived from the imperfectly recon-
structed shape and elevations of the crater (Fig. 11). The measured
displacement of this crater (~27 km) is the largest observed in the study
area, implying that the strike-slip movement could have occurred prior
to the displacement and prior the formation of the fracture set a.
The second uncertain observation is related to the presence of a
putative basin associated with a strike slip fault. Fig. 12 focuses on the
easternmost part of the study area, in correspondence with the en-
echelon strike-slip related set of fractures (“c” in Fig. 3) that charac-
terizes left lateral displacement. The realignment between the northern
and the southern region has evidenced a morphology that can be
geometrically interpreted as a pull-apart basin associated to the strike
slip fault with a fracture set that involves both the regions. This obser-
vation would imply that the set of fractures “c” formation occurred prior
to the strike-slip fault associated with the grooved terrain (“c” in Fig. 3).
4. Discussion and conclusions
The formation of Anshar Sulcus’ grooved terrain can be explained as
the result of rifting (Prockter et al., 2000; Cameron et al., 2018) or
spreading mechanisms (Pizzi et al., 2017), involving both the processes
of extensional tectonics. Using a structural analysis, Prockter et al.
(2000) proposed that the genesis of Anshar Sulcus grooved terrain
occurred by asymmetrical hanging wall rollover against a normal fault
with a prominent southern boundary, caused by extension in the
NNE–SSW direction with evidence of a small amount of right lateral
movement, resulting in a NE–SW transtension. The evidence of right-
lateral movement is given by the right-lateral offset (~10 km) of the
main set of fractures (letter “a” and color blue in Fig. 3) that crosscut the
main crater. The missing southern rim of such crater may have been
rifted and destroyed during Sulcus formation. Cameron et al. (2018)
recognized that prior to the rifting formation of Anshar Sulcus as
described by Prockter et al. (2000), the preexistent dark terrain was
subjected to a tectonic deformation by normal faulting, evidenced by the
presence of two sets of fractures in the direction NW–SE that formed in
response to extension in the NE–SW direction (letters “a” and “b” in
Fig. 3). Instead, using a structural analysis, Pizzi et al. (2017) proposed
that the formation of Anshar Sulcus’ grooved terrain occurred by
spreading, as they identied two preexistent impact craters within dark
terrain, crosscut and separated by the sulcus with same right lateral
displacement. Pizzi et al. (2017) found that Anshar Sulcus’ grooved
terrain formed through a separation of about 2.3 km of crust toward the
NE–SW direction, along with resurfacing of new material. The separa-
tion of the two regions by spreading was accompanied by about 500 m of
right lateral movement and a counter-clockwise rotation of the SE region
of about 2◦. The grooves within the sulcus may indicate a post break-up
brittle faulting of newly formed crust.
Using a structural analysis in combination with topographic ana-
lyses, we found that the formation of Anshar Sulcus grooved terrain
occurred by spreading and upwelling of novel material, conrming
overall the previous analysis of Pizzi et al. (2017). The DEM has shown
that the topographic elevations of the central regions inside the sulcus
are higher with respect to the sulcus boundaries and such scenario is not
typical of rifting processes. On Earth, in fact, rifting zones generally have
either a lower central topography or, in most cases, an asymmetry with a
prominent master fault bounding one side of the rift zone (Gibbs, 1984).
Interestingly we found that, differently from Pizzi et al. (2017), the
spreading event occurred only successively to an older right-lateral
strike-slip movement toward the NW-SE direction that separated the
two dark terrain regions, as evidenced by the different displacements of
two impact craters (Fig. 10). Therefore, we argue that the formation of
Anshar Sulcus by spreading had occurred in a crustal weakening due to
an older strike-slip fault. Furthermore, the absence of the fractures
(letter “b” and color green in Fig. 3) within the northern rim of the
smaller crater suggests that the formation of such fractures occurred
successively to the beginning of strike-slip event that separated the two
dark terrain regions, involving and deforming only the southern rim.
Fig. 13 presents the proposed tectonic reconstruction and Fig. 14 the
chrono-stratigraphy of Anshar Sulcus region. The older terrains that are
present in the Anshar Sulcus region are the dark cratered unit (dc) and
Fig. 12. Putative pull-apart basin associated to the strike slip fault.
M. Ianiri et al.
Icarus 404 (2023) 115654
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the hummocky material (h) (Figs. 14, 1◦phase). Fig. 13a presents the
reconstructed ancient dark terrain which formed prior to the strike-slip
event. Impact craters (cr) have characterized the surface of Ganymede
during its entire evolution (Fig. 14). The rst tectonic event involved in
the formation of the sulcus presents strike-slip kinematics indicators that
show a right lateral displacement of the northern region in the NW-SE
direction, prior to the formation of the Sulcus. In particular, geomet-
rical indicators and best ts of the two blocks lead us to assume a parallel
movement of both the regions toward NE direction, but at different rate
(R
N
>>R
S
), similarly to the case of the San Andreas fault (Atwater,
1970), resulting in a right lateral strike-slip fault as a precursor of the
formation of Anshar Sulcus (Fig. 13b). Previous morphological analysis
has already supposed a strike-slip movement as a precursor to the for-
mation of Anshar Sulcus (Prockter et al., 2000). The second phase of our
chronostratigraphic reconstruction (Fig. 14) is characterized by the
formation of the three sets of fractures and by the right strike-slip event
that produced the division of the ancient dark terrain of Marius Regio
into the two distinct regions. In particular, the sets “a” and “c” formed
rst, before the strike-slip event (Fig. 13a), while the set “b” has formed
after the start of strike-slip event (Fig. 13b). Indeed, the absence of the
set b in the northern region lead us to suppose a dispersion of the frac-
tures of set b within the crustal fracture previously formed by the strike-
slip fault, thus involving only the southern region of dark unit (dc). The
margins of the two regions are the conservative region boundary,
Fig. 13. Reconstruction of the tectonic evolution of Anshar Sulcus area. a) Reconstruction of the ancient dark terrain pre-deformation; b) Strike-slip right movement
of the northern dark region in the NW-SE direction (red arrows); c) Opening of the sulcus through transtension in the WSW-ENE direction (bold white arrows) with
upwelling of novel material. (For interpretation of the references to color in this gure legend, the reader is referred to the web version of this article.)
Fig. 14. Stratigraphic chart showing the evolutionary
temporal sequence of the Anshar Sulcus area; (dc)
dark cratered unit, (h) hummocky material, (cr)
crater, set of fracture a (set a), set of fracture b (set b),
set of fracture c (set c), beginning of strike-slip event
(black dashed line), (lg) light grooved unit. The 1
◦
phase presents formation by impact events of dc and h
surrounding Anshar Sulcus; the 2◦phase presents
fracturing by extensive and strike–slip tectonics,
including the right-lateral strike-slip movement pre-
cursor of Anshar sulcus; the 3◦phase presents for-
mation of lg through spreading center mode by
combination of extensive and strike-slip tectonics.
M. Ianiri et al.
Icarus 404 (2023) 115654
12
presenting the absence of resurfacing or degradation or damage pro-
cesses of the region margins. Due to the limits of long-wavelength
topography and uncertainties of the adopted technique used to pro-
duce the DEM of Anshar Sulcus region, the topographic analysis is not
able to show any evidence of possible vertical movements of the regions.
The second stage of tectonic evolution of Anshar Sulcus (Fig. 13c)
presents the start of the spreading event that caused the extension and
separation of the two regions previously formed as consequence of the
strike-slip event (Fig. 13b), corresponds to the formation of light
grooved unit (lg) (Figs. 14, 3◦phase), which is the youngest unit present
within the Anshar Sulcus area. The right lateral strike-slip tectonics
continued during the spreading stage, so the separation of the two re-
gions occurred with a transtensional movement toward a WSW- ENE
direction (Fig. 13c), as result of combination of the right-lateral
contribution of the strike-slip tectonics toward NW-SE direction and
the extensional contribution toward SSW-NNE direction of the
spreading. Remarkably, the conservative region boundary indicates that
the formation of a novel crustal material occurred by spreading together
with the upwelling of deeper crustal ice, differently from previous hy-
potheses that proposed crustal rifting (Prockter et al., 2000). In addition,
we observed a higher topography in the central part of the sulcus with
respect to the surrounding grooved terrains, which is consistent with
spreading centers at fast-intermediate rate (Macdonald, 1982).
Assuming isostatic compensation of a column of the upwelling of deeper
crustal ice, 1–3% enhancement in dark terrain crustal density, removal
of 1–3% of the dark terrains from the crater within the ejecta (Showman
et al., 2004; De Marchi et al., 2021), and considering a mean topography
height h= (118 ±56)m as derived from the DEM, we estimate that the
thickness of the dark terrain during the formation of the sulcus is H=
h
ρ
Δ
ρ
= (3.9±1.9)km. The formation of the grooves that characterize the
light unit occurred relatively quickly, due to the brittle faulting and
tilting of the new crust during the extension event that have formed the
light unit, explaining the absence of deformed or displaced craters inside
the light unit.
In conclusion, the formation of grooved terrains on the icy surface of
Ganymede is still debated and it could involve extensive rifting and
spreading, strike-slip tectonics and cryovolcanic resurfacing. Pappa-
lardo et al. (2004) have shown that the formation of the grooved terrains
could be correlated with the reactivation of tectonic structures present
in the dark terrain; in addition, Pappalardo et al. (2004) have shown that
the formation of the grooved terrains could foreshadow both tectonic
rifting and subsequent cryovolcanic resurfacing. The geological analysis
of Anshar Sulcus region has demonstrated that Anshar Sulcus is formed
from ancient dark terrain which surface is well preserved (Fig. 15, Stage
0); successively, a strike-slip movement divided the dark terrain into two
distinct regions (Fig. 15, Stage 1). The sulcus then formed in a trans-
tensional manner, maintaining well-preserved margins and the light
terrain is likely crustal material upwelling due to the crustal spreading.
The formation of the grooves likely occurred by brittle fracturing and
tilting caused by the spreading itself in the newly formed crust (Fig. 15,
Stage 2).
The identication of a strike-slip event preceding the formation of
the sulcus by spreading indicates that the sulcus was formed in a zone of
crustal weakening prior to the formation of the sulcus itself. Only after a
possible change of stresses, both in direction and in magnitude, did it
produce the sulcus by crustal opening. Such identication was possible
thanks to the peculiarity of Anshar Sulcus for which both high-resolution
images are available and the possibility of recognizing piercing points as
impact craters, which allowed the reconstruction of the crustal tectonics
and its evolution. Moreover, the production of a DEM of the area
allowed to conduct a topographic analysis which conrmed the
morphological and structural observations. Future studies will be able to
investigate whether the grooved terrain formation in two tectonic
stages, the rst of strike-slip type and the second of spreading type,
could be characteristic of the general formation of grooved terrains,
Fig. 15. 3D view of the formation stages of Anshar Sulcus. Stage 0: Pre-
deformation; Stage 1: Strike-slip tectonic deformation in NW-SE direction;
Stage 2: Transtension toward WSW-ENE direction with upwelling of
novel material.
M. Ianiri et al.
Icarus 404 (2023) 115654
13
thanks to the combination of images having spatial resolution up to 2.4
m / pixel, spectral, altimetry and radar sounder data of the JUICE
mission. In particular, the capability of the radar sounder RIME to detect
and differentiate the subsurface interfaces associated with different
geological components, structure and composition up to a depth of 9 km
(Sbalchiero et al., 2023), will provide a better support for the hypotheses
of formation of grooved terrain on Ganymede.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgements
G.M. acknowledges support from the Italian Space Agency (2023-6-
HH.0).
We would like to thank an anonymous reviewer for providing helpful
and constructive comments.
References
Alexandrov, O., Beyer, R.A., 2018. Multiview shape-from-shading for planetary images.
Earth and Space Sci. 5 (10), 652–666.
Atwater, T., 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of
western North America. Geol. Soc. Am. Bull. 81, 3513–3536.
Beyer, R.A., Alexandrov, O., McMichael, S., 2018. The Ames stereo pipeline: NASA’s
open source software for deriving and processing terrain data. Earth and Space Sci. 5
(9), 537–548.
Cameron, M.E., Smith-Konter, B.R., Burkhard, L., Collins, G.C., Seifert, F., Pappalardo, R.
T., 2018. Morphological mapping of Ganymede: investigating the role of strike-slip
tectonics in the evolution of terrain types. Icarus 315, 92–114.
Collins, G.C., 2006. Global expansion of Ganymede derived from strain measurements in
grooved terrain. Lunar Planet. Sci. XXXVII, 2077.
Collins, G.C., Head, J.W., Pappalardo, R.T., 1998. Formation of Ganymede grooved
terrain by sequential extensional episodes: implications of Galileo observations for
regional stratigraphy. Icarus 135, 345–359. https://doi.org/10.1006/
icar.1998.5978.
Collins, G.C., McKinnon, W.B., Moore, J.M., Nimmo, F., Pappalardo, R.T., Prockter, L.M.,
Schenk, P.M., 2010. Tectonics of the outer planet satellites. In: Watters, T.R.,
Schultz, R.A. (Eds.), Planetary Tectonics. Cambridge University Press, pp. 264–350.
Collins, G.C., Patterson, G.W., Head, J.W., Prockter, L., Pappalardo, R.T., Lucchitta, B.K.,
Kay, J.P., 2013. Global geologic map of Ganymede. US Department of the Interior, US
Geological Survey.
De Marchi, F., Di Achille, G., Mitri, G., Cappuccio, P., Di Stefano, I., Di Benedetto, M.,
Iess, L., 2021. Observability of Ganymede’s gravity anomalies related to surface
features by the 3GM experiment onboard ESA’s Jupiter ICy moons explorer (JUICE)
mission. Icarus 354, 114003. https://doi.org/10.1016/j.icarus.2020.114003.
Gaddis, L., Anderson, J., Becker, K., Becker, T., Cook, D., Edwards, K., Robinson, M.,
1997. An overview of the integrated software for imaging spectrometers (ISIS). In:
Lunar and Planetary Science Conference, 28, p. 387.
Gibbs, A.D., 1984. Structural evolution of extensional basin margins. J. Geol. Soc. 141
(4), 609–620.
Houck, J.C., Denicola, L.A., 2000. ISIS: an interactive spectral interpretation system for
high resolution X-ray spectroscopy. In: Astronomical Data Analysis Software and
Systems IX, 216, p. 591.
Kirk, R.L., Barrett, J.M., Soderblom, L.A., 2003. Photoclinometry made simple. In:
Proceedings of the ISPRS Commission IV Symposium, 34, p. 4.
Lesage, E., Schmidt, F., Andrieu, F., Massol, H., 2021. Constraints on effusive
cryovolcanic eruptions on Europa using topography obtained from Galileo images.
Icarus 361, 114373.
Macdonald, K.C., 1982. Mid-ocean ridges: ne scale tectonics, volcanic and
hydrothermal processes within the plate boundary zone. Annu. Rev. Earth Planet.
Sci. 10, 155–190.
Pappalardo, R.T., Head, J.W., Collins, G.C., Kirk, R.L., Neukum, G., Oberst, J., Giese, B.,
et al., 1998. Grooved terrain on Ganymede: rst results from Galileo high-resolution
imaging. Icarus 135 (1), 276–302.
Pappalardo, R.T., Collins, G.C., Head III, J.W., Helfenstein, P., McCord, T.B., Moore, J.M.,
et al., 2004. Geology of Ganymede. Jupiter: The Planet, Satellites and
Magnetosphere, pp. 363–396.
Patterson, G.W., Collins, G.C., Head, J.W., Pappalardo, R.T., Prockter, L.M., Lucchitta, B.
K., Kay, J.P., 2010. Global geological mapping of Ganymede. Icarus 207 (2),
845–867.
Pizzi, A., Di Domenica, A., Komatsu, G., Cofano, A., Mitri, G., Bruzzone, L., 2017.
Spreading vs. rifting as modes of extensional tectonics on the globally expanded
Ganymede. Icarus 288, 148–159.
Prockter, L.M., Head, J.W., Pappalardo, R.T., Senske, D.A., Neukum, G., Wagner, R.,
et al., 1998. Dark terrain on Ganymede: geological mapping and interpretation of
Galileo Regio at high resolution. Icarus 135 (1), 317–344.
Prockter, L.M., Figueredo, P.H., Pappalardo, R.T., Head III, J.W., Collins, G.C., 2000.
Geology and mapping of dark terrain on Ganymede and implications for grooved
terrain formation. J. Geophys. Res.: Planet. 105 (E9), 22519–22540.
Sbalchiero, E., Thakur, S., Cortellazzi, M., et al., 2023. Subsurface signatures of bright
terrain formation models on Ganymede by 3D radar sounder simulations. Icarus 394,
115351.
Showman, A.P., Mosqueira, I., Head, J.W., 2004. On the resurfacing of Ganymede by
liquid-water volcanism. Icarus 172, 625–640.
Smith, B.A., Soderblom, L.A., Beebe, R., Boyce, J., Briggs, G., Carr, M., et al., 1979a. The
Galilean satellites and Jupiter: voyager 2 imaging science results. Science 206
(4421), 927–950.
Smith, B.A., Soderblom, L.A., Johnson, T.V., Ingersoll, A.P., Collins, S.A., Shoemaker, E.
M., et al., 1979b. The Jupiter system through the eyes of voyager 1. Science 204
(4396), 951–972.
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