JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. E6, 5063, doi:10.1029/2002JE001994, 2003
Syrtis Major and Isidis Basin contact: Morphological and topographic
characteristics of Syrtis Major lava flows and material of the Vastitas
Borealis Formation
Mikhail A. Ivanov
Vernadsky Institute for Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia
Department of Geological Sciences, Brown University, Providence, Rhode Island, USA
James W. Head III
Department of Geological Sciences, Brown University, Providence, Rhode Island, USA
Received 18 October 2002; revised 10 March 2003; accepted 14 April 2003; published 28 June 2003.
[1] The floor of Isidis Basin is covered by materials of the Vastitas Borealis Formation
(VBF) that appear to be emplaced essentially as a single unit. Along its western
boundary, Isidis Basin is in contact with volcanic flows from Syrtis Major Planum. The
contact between the Isidis unit and volcanic flows from Syrtis Major is sharp to
gradational and in places is characterized by a high (500 m) scarp or by a network of
faults that separate pieces of lava plains off the main plateau of Syrtis. Clusters of knobs
and mesas, sometimes arranged in flow-like features, are also typical features of the
transition zone. Several important characteristics of the transition from Syrtis Major to
Isidis Basin are documented. (1) The small-scale surface texture seen in MOC images
appears to be the same for both the Syrtis lava plateau and the knobs and mesas that
characterize the transition. (2) There is strong evidence for the breakup of the coherent
surface of Syrtis Major where it is in contact with materials in Isidis Basin. (3) The
plateau breakup (the knobby terrain) occurs basinward after the major break of slope of
Syrtis Major where it enters the Isidis Basin. (4) There is no evidence for plateau
breakup anywhere up on the slopes of Syrtis Major Planum. (5) The lavas of Syrtis
remain morphologically intact where they are in contact with other units, such as the
Noachian cratered terrain or where lava flows are stacked within Syrtis Major itself.
These characteristic features of the transition zone from Syrtis to Isidis are readily
explained if the zone of plateau breakup consists of relatively young lava flows that have
been superimposed onto the surface of a volatile-rich substratum, such as the interior
unit of Isidis Basin (the Vastitas Borealis Formation). Thus simple superposition of
volcanic materials on top of volatile-bearing sediments can explain the key geological
and topographic aspects of the transition zone from Syrtis Major to Isidis Basin. On the
basis of our findings, we outline the following scenario for the evolution of this region.
In the Early Hesperian, volcanic plains are emplaced in Syrtis Major (the lower part of
the Syrtis Major Formation), and wrinkle ridges deform their surfaces soon thereafter.
Concurrently, volcanic plains are emplaced on the floor of the Isidis Basin, and wrinkle
ridges deform their surfaces soon thereafter. The apparent simultaneity of these units
may mean that Syrtis Major was the source of many of the flows in the Isidis Basin. In
the early part of the Upper Hesperian, subsequent to the formation of most of the
wrinkle ridges, the Vastitas Borealis Formation was emplaced in the Isidis Basin and
elsewhere in the northern lowlands. Following the emplacement of the Vastitas Borealis
Formation, the upper part of the Syrtis Major Formation was emplaced, erupting from
the eastern margins of Syrtis Major Planum and flowing down into the westernmost part
of the Isidis Basin on top of the recently emplaced Vastitas Borealis Formation.
Modification of the superposed lavas by degradation and evolution of the VBF formed
the scarps and unusual morphology of the marginal areas. We found no compelling
evidence for massive or sudden erosion from Syrtis Major to produce the plains
currently on the surface of the floor of the Isidis Basin (the Vastitas Borealis
Copyright 2003 by the American Geophysical Union.
0148-0227/03/2002JE001994$09.00
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IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Formation).
INDEX TERMS: 6225 Planetology: Solar System Objects: Mars; 5480 Planetology: Solid
Surface Planets: Volcanism (8450); 5470 Planetology: Solid Surface Planets: Surface materials and
properties; 5464 Planetology: Solid Surface Planets: Remote sensing; 5415 Planetology: Solid Surface
Planets: Erosion and weathering; KEYWORDS: Mars, Isidis Basin, Vastitis Borealis Formation
Citation: Ivanov, M. A., and J. W. Head III, Syrtis Major and Isidis Basin contact: Morphological and topographic characteristics of
Syrtis Major lava flows and material of the Vastitas Borealis Formation, J. Geophys. Res., 108(E6), 5063, doi:10.1029/2002JE001994,
2003.
1. Introduction
[2] The largest ancient impact structures on Mars may
have potentially served as first-order sinks for volatiles.
Among these, the Isidis Basin [Wilhelms, 1973; Schultz
and Frey, 1990] could be a unique site because (1) it is a
large basin at the very end of the southern highlands with a
highly degraded and lowered north-eastern rim and (2) it is
in direct contact with the extensive volcanic plains of Syrtis
Major. Specific features at the contact between lava flows
from Syrtis and materials covering the floor of Isidis may
illustrate key aspects of the nature of materials in Isidis Basin
and the timing of emplacement of both Syrtis Major and
Isidis Basin material units. The history of the Isidis Basin is
of sufficient interest since it has been designated the landing
site for the Beagle 2 lander [Bridges et al., 2003].
[3] According to the geological mapping of the eastern
equatorial region of Mars by Greeley and Guest [1987], five
principal units are represented in the area of the transition
from Syrtis Major Planum to Isidis Basin. The oldest is the
unit HNu (plateau and high-plains undivided material) of
Noachian/Hesperian age that occurs as a relatively small
area directly at the boundary between Syrtis Major and
Isidis Basin. The surface of the unit appears as a tight and
chaotic collection of knobs, hills, and mesas of different size
and orientation. The next unit, Hs (Syrtis Major Formation,
Late Hesperian), covers the surface of Syrtis Planum and
forms vast, morphologically smooth plains with individual
lava flows occasionally seen in places [Schaber, 1982;
Greeley and Guest, 1987]. Three units make up the Isidis
lowland. The oldest is the ridged member of the Vastitas
Borealis Formation, unit Hvr, Late Hesperian in age, that
occupies the central portion of the basin and is surrounded
by an annulus of two Amazonian units, Aps (smooth plains
material) and Apk (knobby plains material).
[4] The new topography and image data provided by the
Mars Global Surveyor (MGS) spacecraft have led to
reevaluation of the stratigraphic scheme for the northern
lowlands and Isidis Basin as well [Tanaka and MacKinnon,
1999; Tanaka and Kolb, 2001; Tanaka et al., 2002a]. The
new stratigraphy includes eight units, and the four oldest
ones appear to be most important in the understanding of
the late Noachian - late Hesperian evolution of the northern
plains of Mars. The units (from the older to younger) are as
follows [Tanaka et al., 2002a]: (1) The knobby plateau unit
that occurs at the transition from the southern cratered
highlands to the northern lowlands. The unit is interpreted
to be formed due to massive erosion of the highland
materials; (2) The boundary plains material, which is
topographically lower than the previous unit and is interpreted to be due to the erosion and redeposition of highland
materials; (3) The channeled plains materials that occur
within the floor of the large circum-Chryse outflow chan-
nels. This unit is absent in the area of our study; (4) The
Vastitas Borealis Formation, which is the most widespread
unit in the northern lowlands and within Isidis Basin as
well. In the earlier stratigraphic schemes, the Vastitas
Borealis Formation had been divided into grooved, ridged,
mottled, and knobby members [Scott and Tanaka, 1986;
Greeley and Guest, 1987; Tanaka and Scott, 1987]. The
ridged member dominates in the central portion of Isidis
Basin [Greeley and Guest, 1987; Grizzaffi and Schultz,
1989; Kargel et al., 1995].
[5] The new stratigraphic scheme is closely related to a
specific mechanism for the formation of the lowland units.
This is the sedimentary filling of the northern plains due to
massive erosion of southern highlands [Tanaka, 1997;
Tanaka and Kolb, 2001; Tanaka et al., 2000, 2001a,
2002a, 2002b]. One of the specific elements of this mechanism is the catastrophic erosion of material from rims of
large impact basins where large areas of the Hesperian
ridged plains occur, such as Hesperia and Malea Plana at
Hellas basin, Syrtis Major Planum at Isidis Basin [Tanaka et
al., 2001b, 2002b], and within Chryse Planitia [Dohm et al.,
2001].
[6] Tanaka et al. [2000] describe the nature of the interior
unit presently covering the Isidis Basin (predominantly the
Vastitas Borealis Formation) and its relation to the wrinkleridged volcanic plains materials of Syrtis Major Planum.
They describe the character of the interior plains unit and
conclude that the ‘‘unit grades into knobby plains materials
in western Isidis Planitia, which appears to have formed by
the degradation of the wrinkle-ridged, Hesperian plateau
rocks of Syrtis Major Planum and the underlying and
adjacent Noachian cratered materials of Arabia Terra.’’
They describe a scenario in which ‘‘catastrophic breakup
of Syrtis Major Planum rocks produced a series of mass
flows that become deposited within Isidis Basin as essentially one unit’’ (the present Vastitas Borealis Formation).
They further speculate that ‘‘magmatic activity at Syrtis
Major Planum may have instigated hydrothermal activity
along the Isidis margin’’ to produce the catastrophic event.
[7] Tanaka et al. [2002b] outline a more detailed scenario
for the Hellas Basin rim in which they propose a three stage
model (see their Figure 3): 1) sills intrude shallow friable
rocks of the basin rim and mobilize volatile rich sediment;
2) surface rocks are fluidized and flow into the adjacent
basin, removing massifs from the basin margin, and producing volatile-rich sedimentary deposits across the basin
floor; 3) subsequent eruptions cover eroded basin rim
surface and basin inner slopes with lava plains, but leave
deposits in the basin interior floor exposed and largely
undisturbed. Tanaka et al. [2002b] also point out that this
model can be applied to the Syrtis Major Planum-Isidis
Basin origin [e.g., Tanaka et al., 2000].
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
[8] In our study, we analyzed in detail the transition zone
from Syrtis Major Planum to Isidis Basin in order to address
the following questions: (1) What is the general sequence of
events in the transition zone; (2) How does this sequence
correlate with the proposed origin of the Vastitas Borealis
Formation [Greeley and Guest, 1987; Grizzaffi and Schultz,
1989; Scott et al., 1995; Tanaka and Kolb, 2001; Tanaka et
al., 2000, 2001a, 2002b]; (3) What is the possible nature of
the Vastitas Borealis Formation; (4) What is the place and
role of the Hesperian ridged plains that make up the
majority of Syrtis Major?
[9] We analyzed the contact between Syrtis Major and
Isidis Basin using the new MGS data such as the MOLA
topography (gridded into the 1/64 degree topographic map
and individual profiles as well) and MOC images with the
resolution several meters per pixel. The geological context
of the area of our study was provided by the Viking Orbiter
(VO) images of medium (180– 200 m/px) and high (25 – 28
m/px) resolution that cover significant and contiguous areas
at the contact.
[10] In the beginning of the paper we describe the general
characteristics of the transition zone that may have played
the key role in interpretation of the morphology and
geologic history of the area under study. Then we describe
and analyze two areas of detailed study using the complete
data set available for this area. In the discussion section of
the paper the above questions are addressed with the
information collected during the study and a summary of
our work is given as conclusions.
2. Major Features of the Transition Zone
[11] The area of our study (Figure 1) is between 5 –19N
and 277– 287W, in the western portion of Isidis Basin
where it is in contact with the lava plateau of Syrtis Major.
The floor of the basin (Figure 1) is approximately outlined
by the 3500 m contour line below which the floor is very
flat with the variations in elevations less than 500 m over an
area about 800 800 km across. The floor is gently sloped
toward the southwestern sector of the basin. The whole
interior portion of the basin is characterized by a large
positive gravitational anomaly up to 400– 500 mGal [Smith
et al., 1999; Zuber et al., 2000].
[12] The floor of Isidis Basin is covered by material
which appears either as morphologically smooth to hillocky
or as hosting numerous low and narrow curvilinear and
arcuate ridges [Grizzaffi and Schultz, 1989]. The ridged
member of the Vastitas Borealis Formation [Greeley and
Guest, 1987; Tanaka et al., 2002a] characterizes the central
portion of the basin and it is the reference locality of the
so-called ‘‘thumbprint terrain’’ [Lockwood et al., 1992] that
also occurs elsewhere within the northern plains [Kargel et
al., 1995]. The ridges are morphologically prominent and,
as a rule, consist of dense chains of small mounds, many of
which have a central pit. The ridges are about 10– 40 km
long and 0.5– 1 km wide and the individual mounds are
about 400 m wide on the average [Grizzaffi and Schultz,
1989]. There is a variety of interpretation of the ridges and
mounds ranging from a volcanic origin [Frey and Jarosewich, 1982; Hodges and Moore, 1994], to ice-related landforms such as pingos and moraines [Rossbacher and
Judson, 1981; Lucchitta, 1981; Grizzaffi and Schultz,
17 - 3
1989; Kargel et al., 1995]. The thumbprint terrain, which
is the most important feature of the Isidis interior, does not
occur, however, within the transition zone from Syrtis
Major to Isidis Basin where the knobby terrain is the most
common feature.
[13] Sinuous narrow ridges about 50– 60 km long and
0.5– 1.5 km wide represent another type of ridge within
Isidis. These ridges appear as continuous structures and
typically inserted in a narrow depression. This type of ridge
occurs at the periphery of the Isidis floor (particularly, near
the contact of Isidis and Syrtis) and commonly they are
radial to the center of the basin. By their morphology, these
ridges strongly resemble the ridges at the mouth of Tiu
Vallis in Chryse Planitia and have been interpreted either as
squeeze-ups between two ice floes [Lucchitta et al., 1986]
or as esker-like features [Kargel and Strom, 1992; Kargel et
al., 1995].
[14] The high-resolution MOLA topographic data have
revealed that the floor of Isidis Basin is complicated by a
system of broad sinuous to arcuate ridges [Head et al.,
2002] that represent the third type of ridge structures within
Isidis Basin (Figure 2). The ridges are about 50 m high with
a few structures as high as about 150 m and several tens of
km wide. They occur throughout the whole area of the Isidis
floor, and divide it into series of secondary basins about
150– 180 km across [Hiesinger and Head, 2002; H. Hiesinger and J. W. Head, The Syrtis Major volcanic province,
Mars: Synthesis from Mars Global Surveyor data, manuscript in preparation, 2003 (hereinafter referred to as Hiesinger and Head, manuscript in preparation, 2003)]. The
floor of these basins is typically flat or slightly concave
downward so the apparent depth of the basins mostly
depends on the height of the bordering ridges.
[15] Ridges with the same topographic characteristics
typically occur within the northern plains where they are
covered by materials of the Vastitas Borealis Formation.
There the ridges make a broad circumferential pattern
around the Tharsis Rise and have been interpreted as buried
wrinkle ridges analogous to those exposed elsewhere on
Mars within the Hesperian ridged plains (Hr) [Head et al.,
2002]. These ridges, both in Isidis and within the northern
plains, have almost no morphologic expression on the
surface in Viking images, however, and have not been
included into the global data set of wrinkle ridges by
Chicarro et al. [1985]. Both the mascon within Isidis Basin
and the presence of the broad ridges (probable wrinkle
ridges) on its floor are consistent with the interpretation of
lava filling of the central portions of Isidis Basin from Syrtis
Major Planum.
[16] The eastern portion of Syrtis Major Planum (Figure 1)
is on the inner wall of the Isidis Basin where the regional
slope toward the basin is about 0.7 –1.0. In the medium to
low-resolution VO images which are available for this area,
the surface of Syrtis appears as morphologically smooth and
relatively featureless and only in some places are a few
short and morphologically pristine lava flows visible. The
eastern portion of Syrtis Major is significantly different
from the rest of the Planum because it displays almost no
morphologic or topographic signatures of wrinkle ridges
while these are common structures elsewhere in Syrtis
Major Planum [Schaber, 1982; Hiesinger and Head,
2002; Hiesinger and Head, manuscript in preparation,
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IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
17 - 5
Figure 2. Detrended topographic map of Isidis Planitia. The sinuous topographic ridges (contrasting
bright features) make a radial-concentric pattern the center of which is shifted toward the western portion
of Isidis. The center of the map is at 13N, 272W. Simple cylindrical projection. The high-pass filter
core is 50 km. From Head et al. [2002].
2003]. The disappearance of wrinkle ridges within the
eastern portion of Syrtis Major may mean that they are
covered and that this area has been flooded with lava
relatively recently in its evolution.
[17] We performed crater counts within the eastern portion of Syrtis in the area where wrinkle ridges are absent.
The counts show that the number of craters larger than 5 km
per 106 km2 is one hundred sixty five (±25, Figure 3). This
number of craters is very close to, and statistically indistinguishable from, the crater density for the rest of Syrtis
Major [Hiesinger and Head, 2002; Hiesinger and Head,
manuscript in preparation, 2003] and for the surface of
Amenthis trough to the southeast of the basin [Maxwell and
McGill, 1988; Grizzaffi and Schultz, 1989]. The crater
density in eastern Syrtis and for the rest of it as well is
equivalent to an Early Hesperian crater retention age
[Tanaka, 1986]. Thus, if the late episode of volcanic
resurfacing in eastern Syrtis indeed took place, it appears
to correspond to a rather short phase of volcanism that was
close in time to the final stages of the formation of Syrtis
Major Planum.
[18] The high-resolution MOLA data reveal that the
eastern portion of Syrtis hosts a number of very prominent
topographic ridges (Figure 4). The length of the ridges
varies from 100 up to 200– 250 km. Their mean crosssectional area is about 3 km2 (one sigma standard deviation,
±1 km2). In several important aspects these ridges are
different from typical wrinkle ridges in the rest of Syrtis
Major Planum and in other areas of ridged plains on Mars:
(1) The ridges in eastern Syrtis are oriented in W-E direction
along the higher topographic gradient and the distribution of
wrinkle ridges in the rest of Syrtis is less correlative, if at all,
with the topography; (2) The ridges in eastern Syrtis are less
sinuous than typical wrinkle ridges in Syrtis and elsewhere
on Mars; (3) The ridges in eastern Syrtis do not produce an
anastomosing pattern of structures that is typical of wrinkle
ridges in other parts of Syrtis Major [Hiesinger and Head,
2002; Hiesinger and Head, manuscript in preparation, 2003]
and elsewhere [Head et al., 2002]; (4) There is no evidence
for the ridges in eastern Syrtis to be arranged en echelon
while this pattern appears to be characteristic of wrinkle
ridges in the rest of Syrtis Major Planum; (5) The ridges in
eastern Syrtis are systematically higher (100– 400 m high)
and broader (25 – 50 km wide) than the wrinkle ridges
nearby the eastern portion of the Syrtis lava plateau (50 –
80 m high, 15– 30 km wide); (6) Some of the ridges in
eastern Syrtis become broader down the regional slope;
(7) The two largest ridges in the eastern Syrtis have a
Figure 1. (opposite) The distribution of topography within Isidis Planitia (blue and purple colors) and adjacent Syrtis
Major Planum (red-orange-yellow colors). The width of the map is about 1500 km. The floor of Isidis below 3.5 km is
flat with variations in the relief less than 500 m over an area about 800 800 km. The eastern portion of Syrtis Major
displays a steady slope toward Isidis with a mean slope of about 0.3– 0.4. MOLA gridded topography map with spatial
resolution 1/64 of a degree. Contour interval is 500 m. Map projection is simple cylindrical. Red boxes indicate areas of
detailed study. Color-coded topography the areas of detailed study, each about 240 km in width. a) Northern area. The
arcuate scarp between 281W and 282W and two flow-like features that start at about 16N, 281.5W and are oriented in
eastern and northeastern directions are seen. The southern flow crosses a topographic ridge within Isidis Planitia (at about
16N, 280.5W) without deflection. Note that both flows spread on top of two topographic ridges (buried wrinkle ridges
[Head et al., 2002], right side of the map above the center). Red arrow indicates a mesa (remnant of Syrtis plateau). b) The
major topographic features within the southern area are longitudinally oriented troughs within the HNu unit (between
279W and 280W) and equidimensional (at 12.5N, 278.5W) and arcuate (at 11N, 278.2W) depressions at the eastern
boundary of this unit. Where a topographic ridge (a lava tube?) in the eastern portion of Syrtis Major comes to the plateau
edge, there is a distinct topographic ridge within the HNu unit (center of the map).
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IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Figure 3. Results of crater counting in the eastern portion of Syrtis Major Planum and in the western
portion of Isidis Planitia where the ridged member of the Vastitas Borealis Formation is exposed. The
number of craters larger than 5 km in diameter is 165 (Early Hesperian) and 118 (Late Hesperian) for the
eastern Syrtis and western Isidis, respectively. In both regions the crater count was done on the MDIM
photobase with a resolution about 200 m/px within a region of the same area (about 0.23 106 km2) and
shape. For clarity, error bars are shown as light gray (Syrtis) and hatched (Isidis) areas.
triangle-shaped cross-section and a narrow (<1 km) summit
depression. Distinct lava flows appear to emanate from
these depressions.
[19] The above distinctions strongly suggest a different
mode of origin for the ridges in eastern Syrtis and wrinkle
ridges within the rest of Syrtis. The many distinctive
characteristic features of the eastern Syrtis ridges support
the interpretation that they are either lava flows or tubes or
perhaps both [Schaber, 1982]. Direct evidence for superposition of lava flows associated with the ridges onto the
surrounding terrains (Figure 5) suggests that the ridges are
among the youngest features in eastern Syrtis Major.
[20] The contact between the lava plains of Syrtis Major
and the floor of Isidis Basin (the transition zone) is the key
area of our study. This territory illustrates the most important aspects of the interaction between lavas from Syrtis and
materials on the floor of Isidis Planitia and the major
episodes of the sequence of events during this interaction.
The transition zone in the area of our study is sharp to
gradational and is characterized either by a prominent single
scarp or by a network of narrow troughs. A very characteristic feature of the transition zone is the knobby terrain that
consists of numerous small knobs and mesas. They are
typically arranged in elongated and equidimensional clusters and, in places, in flow-like features. A significant
portion of the transition zone is occupied with unit HNu
(undivided materials of the plateau and high-plains assemblage) [Greeley and Guest, 1987]. Dense collections of
mesas, small knobs, and plates make up the surface of this
unit and, in fact, it represents a large and contiguous area of
knobby terrain. The details of the structure of the transition
zone are described in the following section.
3. Transition From Syrtis Major Planum to
Isidis Basin
[21] We analyzed in detail the transition zone from Syrtis
Major Planum to the Isidis Basin in two neighboring areas
that appear to portray the key features of the transition. The
northern area is from 14 to 17.5N and the southern area
is from 10N to 14N.
3.1. Northern Area
[22] A significant portion of the transition there is characterized by a scarp that extends as a wide arc convex
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
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Figure 4. MOLA shaded relief map of the transition zone from Isidis Planitia (left) to Syrtis Major
Planum. Width of image is 600 km. The appearance of the coarse and fine topographic details is
emphasized by different directions of illumination. The sinuous and arcuate ridges are the most
prominent topographic features within Isidis (right side of the maps). The eastern portion of Syrtis Major
(center and left side of the maps) is characterized by a series of topographic ridges oriented normal to the
general strike of the Isidis/Syrtis contact (central portions of the maps). To the west and south, an
anastomosing pattern of narrow and sinuous wrinkle ridges in Syrtis Major is seen (right and lower
portions of the maps). The maps are based on the MOLA (1/64 of a degree) topographic map, projection
is simple cylindrical.
toward Syrtis Major for about 125– 300 km from 14N to
about 15.5N (Figures 1a, 6a, and 6b). The MOLA topographic map de-trended with a hi-pass filter about 22 km
wide (Figure 7) shows that the zone of the scarp occurs
where the surface of Syrtis Major forms a gentle depression
confined between two topographic ridges at about 16N and
14N (Figure 8). This characteristic of the plateau topography weakly depends on the size of the filtering windows
and appears within their broad range from about 15 up to 50
km [Head et al., 2002]. The floor of Isidis Basin adjacent to
the scarp is among the lowest areas within the entire basin
(Figures 1a, 6a, and 6b) and is topographically flat and
slightly sloped to the south.
[23] We have measured the elevation of the scarp base
and rim in nine points corresponding to the major breaks in
slope along five MOLA orbits that correspond to the MOC
images available for the scarp area. The difference between
the rim and the base elevation gives the height of the scarp
at a given scarp exposure. The mean height of the scarp is
about 530 m with variations from 370 up to about 650
meters (Table 1). The elevation of the scarp baseline is close
to 3800 m (mean elevation is about 3830 m) and the
variations of the elevation are as much as about 260 m over
a distance of 125 km (Table 1).
[24] Almost along its entire strike, the scarp cuts the
surface of Syrtis Major cleanly with little or no evidence for
progressive plateau breakup (Figure 8). A typical characteristic of the scarp is that its edges are scalloped at
different scales. The largest scale is represented by the
arc-like shape of the scarp itself and has the width about
100 km (Figure 8a). The next scale of the sinuosity of the
scarp is due to broad (about 20 km wide) alcoves at its edge
(Figure 8b). The alcoves are cut by smaller niches that are
about 5 km wide and those are deformed by scallops with
typical width about 1.5 kilometers (Figure 8c). Due to this,
the scarp appears as a sinuous feature in plan view and its
general morphology does not favor a tectonic explanation of
its origin. In many cases, numerous large (about 700– 800
and up to 1000 m across), equidimensional, and angular
blocks are present on the floor of Isidis Basin near the base
of the scarp (Figures 8a, 8c, and 8d). It is likely that only a
portion of the blocks is exposed and that their actual size is
larger and may correspond to the dimension of small-scale
scallops at the scarp rim. In this case, the blocks may be due
to destruction of the scarp by small-scale landsliding and
fall of large individual blocks. Direct evidence for fallen
boulders and blocks (a few meters up to a few tens of
meters) is seen in each MOC image of the scarp area. There
is no evidence, however, on the floor of Isidis Basin
adjacent to the scarp for the larger contiguous bodies of
landslide that may be responsible for the formation of the
larger-scale niches and alcoves of the scarp.
[25] At about 14.5N a large highstanding mesa is visible
on the floor of Isidis Basin (Figures 1a, 6a, and 6b). The
mesa is separated from the mainland of Syrtis Major
Planum by a gap about 11 km wide (Figure 8d). Morphologically, the surface of the mesa is indistinguishable from
the surface of Syrtis Major lava plateau and continues the
topographic trend of the neighboring surface of Syrtis.
These characteristics of the mesa strongly suggest that it
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IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Figure 5. The surface of the eastern portion of Syrtis
Major Planum where one of the most prominent topographic ridges occurs (center of the image from left to
right). Width of image is 110 km. The crest area of the
ridge is occupied by a rille-like elongated depression (at the
left edge of the image). Distinct small lava flows (arrows
indicate lava fronts) emanate from the ridge which was
interpreted [Schaber, 1982] as a lava tube. Fragment of the
VO image 377S58 (resolution 218 m/px).
Figure 7. Detrended topographic map of the northern
portion of the transition zone from Isidis Planitia to Syrtis
Major Planum. Width of image is about 300 km. The area
within Syrtis Major bounded by the arcuate scarp is
confined by two topographic ridges (arrows) from the north
and the south. The flow-like features in the northern part of
the transition zone are the extensions of the northern
topographic ridge (presumably, a lava tube) into Isidis
Basin. The high-pass filter core is 22 km. See Head et al.
[2002] for details of the technique.
Figure 6. a) The morphology of the transition zone in the northern area with superposed contour lines,
b), showing the variations in elevation along specific features of the transition zone (contour interval is
50 m). An arrow indicates a mesa; the solid lines crossing the image are tracks of MOLA profiles. Width
of area is about 240 km. See text for details. Figures a) and b) are fragments of the MDIM photobase.
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
17 - 9
Figure 8. Morphology of the large arcuate scarp in the northern area. a) A large alcove characterizes
this segment of the scarp. The typical size of these feature along the scarp is a few tens of km. A grabenlike feature within Syrtis Major (left lower portion of the image) is cut by the scarp at the base of which
large angular blocks are seen on the floor of Isidis Basin (upper portion of the image). Fragment of the
VO image 152S05 (26 m/px). b) Niches at the scarp (lower left corner) have typical width of about 5 km.
Fragment of the VO image 151S01 (26 m/px). c) Scalloped edge of Syrtis Major Planum (left half of the
image) along the arcuate scarp. The scallops have a typical width of about 1 –2 km. Angular and
elongated blocks of about the same size as the size of the scallops are seen on the floor of Isidis Basin
(right half of the image). Fragment of the VO image 152S06 (25 m/px). d) Gap between the main portion
of Isidis Basin (lower left corner of the image) and a large mesa within Isidis (upper right corner of the
image). The edges of both the main plateau and the mesa are scalloped and a large number of rectangular
and angular blocks are seen on the floor of the gap. Fragment of the VO image 152S07 (26 m/px). All
images show the area about 13 13 km.
is a large piece of Syrtis Major. The floor of the gap
between the mesa and the plateau is peppered with numerous large and small blocks (Figure 8d) and the edges of both
the mesa and the Syrtis plateau are etched with larger niches
and smaller scallops.
[26] MOLA orbit 13813 crosses the southern portion of
the scarp almost normal to its strike (Figure 9). Such
geometry allows for direct measurements of the topographic
gradients at the scarp. MOC image M11-02034 (resolution
2.94 m/px) corresponds to the MOLA orbit 13813 and
shows the fine details of the scarp structure. The most
important feature of the scarp is that it consists of two
distinct parts, lower and upper, separated by a relatively
narrow (about 300 m wide) shelf-like feature (Figure 9a).
The surface of both parts of the scarp displays a low
brightness and crisp morphology with evidence for layering; this clearly represents the exposed cross-section of the
Syrtis Major lava plateau. The surface of the shelf is
brighter and featureless and likely represents a debris
apron. The slope within the lower part of the scarp is
about 10.5, within the shelf it is about 7, and the slope is
as high as about 25 for the upper portion of the scarp
(Figure 9b).
[27] The uppermost portion of the upper part of the scarp
appears to be slightly different (it is darker and has less
subdued morphology) from the rest of the scarp and may
represent either a single lava flow or a stack of thin flows
indistinguishable at this resolution. There is a simple
17 - 10
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Table 1. Elevations at the Base and Brim and Scarp Height Along the Arcuate Scarp and the Flow-Like Features in the Northern Areaa
Break in Slope at the Scarp Base
MOLA Orbit
Latitude (N)
Longitude (W)
Elevation, m
12329
13813
11260
11260
11260
16605
16605
12656
12656
MEAN:
13.99
14.00
14.03
14.28
15.57
14.58
15.42
14.73
15.29
281.04
281.12
281.39
281.42
281.59
281.58
281.69
281.79
281.87
3877.29
3882.85
3811.85
3772.25
3832.38
3831.18
3868.17
3819.74
3756.49
3828.02
11260
13813
12329
13846
15209
MEAN:
281.59
281.35
281.28
281.04
280.15
15.57
15.71
15.75
15.82
15.76
11260
13813
12329
13846
15209
MEAN:
281.65
281.38
281.31
281.06
280.21
11260
13813
12329
13846
15209
MEAN:
13813
12329
13846
15209
MEAN:
Break in Slope at the Scarp Brim
Latitude (N)
Longitude (W)
Elevation, m
Scarp Height, m
281.03
281.11
281.39
281.41
281.60
281.57
281.70
281.78
281.87
3506.15
3443.16
3167.38
3212.06
3265.85
3279.81
3309.01
3228.95
3289.96
3300.26
371.14
439.69
644.47
560.19
566.53
551.37
559.16
590.79
466.53
527.76
Southern Flow-like Feature, Southern Scarp
3832.38
281.60
15.64
3786.45
281.35
15.74
3789.48
281.28
15.78
3725.94
281.04
15.85
3857.84
280.16
15.79
3798.42
3265.85
3396.30
3305.35
3369.89
–
3334.35
566.53
390.15
484.13
356.05
–
449.22
15.96
15.91
16.02
15.99
16.21
Southern Flow-like Feature, Northern Scarp
3779.14
281.64
15.89
3739.59
281.38
15.89
3807.70
281.30
15.94
3832.54
281.06
15.96
3762.05
280.21
16.18
3784.20
2922.59
3292.49
3406.83
3672.83
–
3323.685
856.55
447.10
400.87
159.71
–
466.06
281.66
281.43
281.33
281.12
280.29
16.17
16.27
16.17
16.41
16.77
Northern Flow-like Feature, Southern Scarp
3790.95
281.68
16.21
3783.10
281.43
16.30
3819.11
281.34
16.20
3764.38
281.13
16.45
3772.45
280.32
16.93
3786.00
3230.62
3253.87
3353.32
3402.97
–
3310.19
560.33
529.23
465.79
361.41
–
479.19
281.46
281.39
281.17
280.34
16.47
16.57
16.78
17.10
Northern Flow-like Feature, Northern Scarp
3726.76
281.45
16.44
3704.97
281.38
16.52
3678.53
281.17
16.76
3771.87
280.32
16.93
3720.53
3284.29
3315.38
3540.91
–
3380.19
442.47
389.59
137.62
–
323.23
Scarp
13.92
13.97
14.08
14.23
15.64
14.53
15.47
14.65
15.35
a
Note: The MOLA orbit 15209 crosses the flow-like features where they do not have distinct scarp.
geometric relationship between width of a layer as it is seen
in a map (Lv) and the true thickness of this layer (Ht):
Ht ¼ ðLv = cosðaÞÞ* sinða gÞ;
where a and g are angles of the slope that cut the layer and
the dip of the layer, respectively. If an assumption is made
that the uppermost layer (or layers) within a lava plateau is
parallel to the surface of the plateau, then it is possible to
estimate the true thickness of lava flows and stacks. For the
upper layer seen in the MOC image M11-02034 the mean
thickness is about 95 m (±11 m). In our study we assume
that the steep slopes (25) occur in other places along the
scarp in its upper sections and estimated the true thickness
of lava stacks (relatively thick coherent layer with little
evidence for internal layering) or individual flows in four
other localities along the scarp (Table 2). The individual
layers exposed in the scarp have a thickness about 10 –15 m
and the mean thickness of the stacks is about 80 m varying
from about 40 up to 115 m (Table 2).
[28] To the north of the scarp, the transition from Syrtis
to the Isidis Basin appears as two elongated flow-like
features (Figures 1a, 6a, and 6b). They are oriented normal
to the general strike of the contact, and extend for 110– 150
km into the basin. In plan view, the features have a
mushroom-like shape and are about 8– 12 km across where
they are close to Syrtis and about 30– 60 km at their distal
ends. These features represent a continuation of a topographic ridge (interpreted to be of volcanic origin) at the
northern margin of Syrtis Major. Near Syrtis, along about
one third of the length of the features, both their sides are
bounded by distinct scarps 300– 450 m high on the average.
There, the surface of the features is morphologically smooth,
coherent, and similar to the surface of Syrtis Major at the
resolution of both the VO and MOC images (Figure 10). In
the direction from the main volcanic plateau of Syrtis toward
the east inside Isidis Basin, however, there is clear observational evidence for the progressive plateau breakup and
transformation into knobby terrain, consisting of numerous
knobs, plates, and mesas (Figures 10b –10d). The typical
17 - 11
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Figure 9. The high-resolution image (MOC image M11-02034, 2.94 m/px) and the corresponding
MOLA orbit 13813 show details of the structure of the arcuate scarp. See text for description. Fragment
of MOC image shows area about 1.2 3.4 km.
size of the features of the knobby terrain is about 2 km
(±0.8 km).
[29] As in the case of the arcuate scarp to the south, the
edges of the flow-like features are scalloped at different
wavelengths. The larger alcoves in the bounding scarps
have an average width of about 12 km, twice as small as
the typical width of alcoves at the arcuate scarp. In places,
large angular blocks appear to be rubble within and in close
Table 2. True Thickness of Uppermost Lava Stacks at the Eastern Edge of Syrtis Major Planum
MOC
Image
Resolution,
m/px
Latitude, N
Longitude, W
Profile
Layer/Stack
M07-02742
4.42
14.37
281.1
PR1
PR1
PR1
stack1
stack2
stack3
M11-02034
2.94
14.23
281.17
PR1
PR2
PR3
PR4
M08-02589
2.95
14.93
281.82
PR1
PR1
PR2
PR2
PR3
PR3
M10-01296
2.95
15.08
281.91
PR1
PR2
Lv, m
168.8
196.9
209.6
MEAN:
stack1
226.4
stack1
220.5
stack1
173.5
stack1
199.1
MEAN:
stack1
82.6
stack2
103.3
stack1
82.6
stack2
135.7
stack1
102.9
stack2
102.9
MEAN:
stack1
215.4
stack1
247.8
MEAN:
TOTAL MEAN:
Ht, m
78.8
92.0
97.9
89.6
105.7
103.0
81.0
93.0
92.3
38.6
48.2
38.6
63.4
51.0
51.0
55.1
100.6
115.7
108.2
77.2
17 - 12
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Figure 10. Progressive changes of texture of the surface of Syrtis Major Planum along the southern
flow-like feature in the northern portion of the transition zone. Width of each image is about 14 km.
a) The surface of Syrtis Major within the plateau is morphologically intact. Material on the surface of the
plateau embays old cratered terrain. Fragment of the VO image 155S19 (28 m/px). b) The area where the
flow-like feature begins. The surface of the flow appears to be morphologically intact and analogous to
the surface of Syrtis Major. At the northern side of the feature (center of the image), however, a large
alcove with additional smaller scallops cuts the flow. Large angular blocks are rubble on the floor of
Isidis Basin within the alcove. Fragment of the VO image 153S15 (27 m/px). c) The surface of the flow at
about its middle (large diagonal feature in the lower left portion of the image) where the flow is disrupted
into plates. Fragment of the VO image 153S17 (28 m/px). d) At the distal end of the southern flow, it
appears as a cluster of knobs, plates, and mesas. Fragment of the VO image 152S14 (27 m/px). All
images show the area about 13 13 km.
proximity to alcoves (Figure 10b) on the floor of Isidis
providing direct evidence for the collapse of the flow
edges.
[30] We have measured the elevations of the rims and
bases on both sides of the flow-like features in places where
they are crossed by five MOLA orbits corresponding to the
available MOC images for this area (Table 1). As it is seen
from the table, the elevation of the rims of each bounding
scarp becomes progressively lower basinward whereas the
elevations of the scarp base are much more even. This is
because the surface of the flow-like features continues the
topographic trend of Syrtis Major and leads to progressive
diminishing of the height of the bounding scarps. Due to
this, both flow-like features appear to be thinning out
toward the interior of the Isidis Basin. Although the
elevation of the baseline of the features is more stable, it
shows significant variation from baseline to baseline and
from feature to feature. The maximum differences in elevation along the bounding scarps are about 100 m over a
distance of about 80 kilometers.
[31] A specific characteristic of the flow-like features is
that their surface texture changes along strike. Near Syrtis,
where the flows are coherent and bounded by the scarp
features their surface has about the same slope as the surface
within Syrtis. The regional slope within Syrtis in the
vicinity of the flows is about 0.3 over a distance of about
40 km and the slope within the flows is about 0.2 over the
same distance. At some point, however, at 16N, 280.7W
for the southern flow and 16.5N, 281.1W for the
northern flow, the topographic profile of the flows becomes
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
17 - 13
Figure 11. Topographic profile along the southern flow-like feature. The surface of the flow, where it is
coherent, continues the topographic trend of the surface of Syrtis Major to the west. Starting from the
point where the coherent surface of the flow breaks up and appears as a cluster of knobs and mesas the
overall topographic profile of the flow becomes almost horizontal. Points along the profile indicate
individual measurements of elevation taken from the MOLA topographic map with resolution 1/64 of a
degree. Width of image is about 170 km. The image showing the position of the profile is a fragment of
the MDIM photobase.
nearly horizontal (Figure 11). Starting from this point, the
surface of the flow-like features breaks up and consists of
numerous tightly spaced knobs (Figure 10d) that become
progressively less dense outward. The total height of the
features within these disrupted parts is about 100 – 200 m
and remains approximately the same along the strike of the
features. It is important to note that the height of the flowfeatures in their horizontal part is either almost equal or
double the thickness of the uppermost lava stacks within the
arcuate scarp to the south.
[32] MOLA orbit 15209 crosses the southern flow-like
feature where it shows the dense cluster of knobs and mesas
and MOC image M15-00469 (resolution is 2.95 m/px)
corresponding to this orbit shows a single small (about
1.9 km across and about 140 m tall) mesa. A debris apron
surrounds the mesa but on its northern slope at least two
thick layers or layer stacks are exposed. The fine-scale
texture of the mesa surface is the same as within the lava
plateau of Syrtis Major leaving little doubt that the mesa is a
piece of the plateau. The surface of the mesa slopes slightly
(at about 1.3) to the north and appears to be broken within
its northern third. The high-resolution MOLA topography
and MOC images allow us to estimate the thickness of the
uppermost layer (or possible stack of layers) exposed at the
mesa walls. The estimated thickness is about 75– 80 m,
which is very close to the estimates of the thickness of
layers exposed within the arcuate scarp to the south.
3.2. Southern Area
[33] The southern area (Figures 1b and 12) is dominated
by material of the unit HNu [Greeley and Guest, 1987]
that consists of a great number of knobs, plates, and mesas
(Figure 12). These features have typical sizes of about 3 –
5 km across (and up to a few tens of km), and thus they are
17 - 14
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Figure 12. The morphology of the transition zone in the southern area of detailed study, a), with
superposed contour lines, b), showing the variations in elevation along specific features of the transition
zone (contour interval is 50 m). Width of image is about 260 km. See text for details. Figures a) and b) are
fragments of the MDIM photobase. The solid line crossing the image is the track of the MOLA orbit
13989.
noticeably larger than knobs characterizing the knobby
terrain in the northern area. The largest plates in the
southern area occur either at the edge of Syrtis Major or
within the northeastern portion of the unit HNu where the
floor of Isidis Basin is relatively high (3700, 3600m).
The unit HNu, in fact, represents a large and contiguous
area of knobby terrain at the transition from Syrtis Major to
Isidis Basin. This area runs along the edge of Syrtis for
about 350 km in the S-N direction and extends for about
150 km from the edge of Syrtis into Isidis Planitia.
[34] To the west, the knobby terrain display a gradational
transition to Syrtis Major following, however, the regional
break in slope along the edge of the coherent portion of
Syrtis that is characterized by a steady slope (about 1.3)
toward the Isidis Basin (Figure 13). The break in slope
occurs at different elevations, which are close, on the
average, to the 3700 m contour line (Figure 1b). The
diffuse contact with the Vastitas Borealis Formation defines
the eastern edge of the unit HNu that roughly coincides with
an elevation of about 3900 meters (Figure 1b). The highresolution topographic map shows, however, that the variations in the elevation of the unit baseline along its eastern
boundary are about 200 m over a distance many tens of
kilometers.
[35] The topographic profiles across the area of knobby
terrain show that its surface is flat on the average and
follows the regional slope of the floor of Isidis Basin toward
the southwest. Although the individual MOLA topographic
profiles show that the surface of the unit HNu is highly
irregular, the typical height of individual knobs and mesas
within it is about 100 m with a few knobs that are taller, up
to 250 m (Figure 14). The value of 100 m, which defines the
visible thickness of the HNu unit, is close to that for the
knobby terrain in the northern area of our study. In a manner
similar to the northern area, there are no pitted mounds or
their chains visible in close proximity to unit HNu.
[36] The important characteristic of unit HNu visible in
the topographic map (Figure 1b) is that its territory is
complicated by elongated and equidimensional depressions
tens of kilometers across. The depth of the depressions is
about 50– 100 m and, in places, up to 150 m and they
appear to be organized in linear and arcuate troughs. The
interior of the troughs has more subdued morphology and
noticeably less abundant knobs (Figure 12). The two most
prominent enter the area of knobby terrain from the south
and north along about 279.5W. Due to this, the central
portion of unit HNu (between about 12– 13N and 279–
280W) appears as a short and broad topographic ridge
extending in the east direction from the edge of Syrtis Major
into Isidis Basin for about 60 km (Figure 1b). It is important
to note that two of the largest lava flows (or possibly lava
tubes) within Syrtis come to the edge of the plateau at about
12N where the broad ridge within the knobby terrain
begins. This suggests that the ridge within the unit HNu
is a continuation of the lava flows in Syrtis. Another large
(about 60 km across) equidimensional depression within
unit HNu occurs at the eastern edge of the unit (between
about 12 – 13N and 278 – 279W). The surface of the
depression is also characterized by few knobs and mesas
and those that are visible appear to be smaller, less crisp
morphologically, and more rounded in plan view.
[37] One of the most obvious differences between the
southern area and the northern one is that no prominent
single scarp outlines the main plateau of Syrtis Major in the
southern area. All scarps there are relatively low features
that either outline individual mesas or occur in series at the
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
17 - 15
Figure 13. Topographic profile across the southern area of the detailed study. Width of image is about
240 km. The morphologically coherent surface of Syrtis Major Planum slopes toward Isidis Basin at
steady topographic gradient about 0.4. Where the plateau is breaking up (at the contact with the knobby
terrain of the unit HNu to the East) the gradient abruptly diminishes (0.1) and the surface of the unit
HNu is essentially horizontal. The profile is taken from the MOLA topographic map with resolution 1/64
of a degree. The image showing the position of the profile is a fragment of the MDIM photobase.
edge of Syrtis Major (Figure 15). Along the contact between
the unit HNu and the Syrtis lava plateau there is abundant
evidence for plateau breakup (Figures 15a and 15b). For
example, at about 13.5N, 280.5W the morphologically
smooth and coherent surface of Syrtis is cut by a series of
curvilinear graben. The larger graben separate big elongated
fragments of the plains off the main body of the plateau and
subordinate graben further divide these fragments into
smaller pieces. At about 12.2N, 280W a large triangleshaped block of the Syrtis plateau is outlined from the west
by curvilinear narrow troughs and the surface of the block is
almost horizontal and lower while the edge of the plateau
immediately to the west of the block is higher and sloped
toward the east. This suggests that the block is a detached
piece of Syrtis. The eastern edge of the block is broken into
smaller pieces. The progressive fragmentation of larger
pieces of the Syrtis plateau into smaller small knobs and
mesas is the typical feature of the transition zone from Syrtis
Major to Isidis Basin in the southern area of our study
(Figures 15c and 15d).
[38] There are varieties of surface textures within the
knobby terrain of the HNu unit, which depend on the size
and arrangement of knobs and mesas. In the central portion
of the unit, at about 12.2N and between 279 – 280W, the
approximately rectangular plates about 3.5 km across are
arranged in bands that are about 4 – 4.5 km wide and 30–
40 km long (Figure 15e). These bands characterize the
surface of the broad topographic ridge that appears as a
continuation of the largest lava flows (possible lava tubes)
within Syrtis Major. At the southern edge of the knobby
terrain area (at 10– 10.5N, 278– 279W) a similar arrangement of plates into bands occur (Figure 15f ). There is a
series of subparallel curvilinear narrow troughs there that are
characterized by distinct narrow rims on both sides and some
of the troughs are transformed into ridges (with medial
depression, in places) along their strike (Figure 15f ). The
17 - 16
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Figure 14. Part of MOLA orbit 13989 that crosses the eastern portion of knobby terrain of the unit HNu
in the southern area of the detailed study. The typical height of the knobs is about 100– 150 meters. Track
of the orbit is shown in Figure 12a. Latitude is in degrees North.
troughs continue into the plateau for about 30– 60 km and
cut it into slices about 8 –12 km wide. At the edge of the
plateau the slices are abruptly disrupted and continue into
Isidis Basin as bands of tightly spaced plates that are
characterized by softened morphology and cut by narrow
and apparently shallow elongated depressions in two directions. This creates a hummocky texture of the surface that is
characteristic of the southern edge of the knobby terrain.
4. Discussion
[39] The specific large- and small-scale features of the
transition zone from Syrtis Major to Isidis Basin that have
been described in the previous sections allow us to address
two major questions about the character of interaction
between the volcanic province of Syrtis Major and the
lowland of Isidis Basin: (1) What is the timing and sequence
of events during the emplacement of materials present at the
contact of Syrtis Major and Isidis Planitia?; (2) What is the
possible nature of the Vastitas Borealis Formation materials
that cover the floor of Isidis Basin?
4.1. Timing and Sequence of Events
[40] The crater count within Syrtis Major Planum [Maxwell and McGill, 1988; Hiesinger and Head, 2002; Hiesinger and Head, manuscript in preparation, 2003] shows
that about 150 to 165 craters larger than 5 km in diameter
occur in 106 km2 of the territory of Syrtis. This crater
density corresponds to an Early Hesperian age [Tanaka,
1986]. Our crater count (Figure 3) within the eastern portion
of Syrtis (area is about 230,000 km2) that has evidence for
relatively late resurfacing shows the density about 165
craters larger than 5 km per 106 km2. Taking into account
the error bars of the crater count, our results do not differ
from the previous data meaning that the eastern portion of
Syrtis Major is indistinguishable from the rest of the Planum
by the crater statistics. Grizzaffi and Schultz [1989] determined the same crater density for the surface of plains
within the Amenthis trough that cuts the southeastern
portion of the rim of Isidis Basin. The crater counts within
the basin [Grizzaffi and Schultz, 1987, 1989; Maxwell and
McGill, 1988] shows that the surface of Vastitas Borealis
Formation on the floor of the basin is younger (Late
Hesperian) than the surface of Syrtis Major. We have
counted craters on the floor of Isidis Basin near the
transition zone from Syrtis to Isidis within the territory that
is equivalent both in area and shape to the territory where
we have counted craters in the eastern Syrtis. Our data (118
craters >5 km per 106 km2, Figure 3) show that the crater
retention age of the surface of Vastitas Borealis Formation
near the transition zone also correspond to the Late Hesperian time. Grizzaffi and Schultz [1987] have pointed out,
however, that the plains within Isidis Basin interior possibly
exhibit two ages related to the formation and modification
of the plains.
[41] Both the crater statistics data and characteristic structures visible in MOLA topography imply that the floor of the
Isidis Planitia basin was covered with lava plains before the
emplacement of materials of Vastitas Borealis Formation
[Grizzaffi and Schultz, 1987, 1989; Maxwell and McGill,
1988]. The recent results from the high-resolution MOLA
data [Head et al., 2002] show that a system of large sinuous
ridges complicates the floor of Isidis Basin (Figure 2).
These ridges appear to be similar in plan view to the
common wrinkle ridges deforming the Hesperian lava
plains (unit Hr) elsewhere on Mars. Several observations
strongly suggest that the large ridges in Isidis are, in fact,
buried wrinkle ridges [Head et al., 2002]: (1) The basin of
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Figure 15. Plateau breakup and progressive fragmentation of the Syrtis Major materials within the
southern area of the detailed study. a) and b) The initial stages of the breakup when series of linear and
curvilinear graben cut the morphologically coherent surface of the Syrtis Major plateau (central portions
of both images). c) and d) At these stages of the breakup, large fragments of the plateau (plates) are
completely separated from the Syrtis Major mainland. e) and f) At these stages of the breakup, the large
plateau blocks are further disrupted into smaller knobs, plates, and mesas. All images are fragments of the
VO image 377S58 (218 m/px) and each covers an area about 55 55 km.
17 - 17
17 - 18
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Isidis Planitia is a large (up to 400– 500 mGal) mascon
[Smith et al., 1999] known since the Viking mission
[Sjogren, 1979; Sjogren and Wimberly, 1981]. The presence
of a mascon suggests lava filling of Isidis Basin and
deformation associated with subsidence. Furthermore, wrinkle ridges are common structures of the Hesperian lava
flows elsewhere on Mars [e.g., Scott and Tanaka, 1986;
Greeley and Guest, 1987]. (2) The lowland of Isidis Basin
neighboring the large lava province of Syrtis Major, the
surface of which is deformed by wrinkle ridges [Schaber,
1982; Greeley and Guest, 1987; Hiesinger and Head, 2002;
Hiesinger and Head, manuscript in preparation, 2003]. (3)
The large ridges in Isidis are geometrically similar to the
true wrinkle ridges exposed elsewhere on the surface of
Mars. (4) The large ridges in Isidis appear to be similar in
width, length, and sometimes in arrangement to the
‘‘stealth’’ wrinkle ridge-like structures that make up a
circum-Tharsis pattern in the northern lowlands of Mars
[Head et al., 2002].
[42] The most prominent topographic features within
eastern Syrtis (Figure 4) are lava flows and possibly lava
tubes [Schaber, 1982]. Although the eastern portion appear
to be of the same age (the Early Hesperian) as the rest of the
plateau, the prominent lava flows there occupy a small
portion, about 10 –12% of the area of our crater counting.
Such a small total area of these features prevents reliable
estimates of their age with the available images. Observational evidence such as small lava flows emanating from the
larger features and superposed on the surrounding surface
suggests, however, that these lava flows and tubes are
manifestations of the latest volcanic activity in the region.
The flows extend down the regional slope of Syrtis Major
toward Isidis and enter the basin. In the northern area of our
detailed study the two large flow-like features within Isidis
occur as continuations of a large topographic ridge (presumably, a lava tube) within Syrtis. In the southern area, the
highest portion of the unit HNu is in the area where the most
prominent lava flows (putative tubes) in Syrtis come to the
edge of the plateau.
[43] There is abundant evidence for internal layering
within the large scarp at the edge of Syrtis Major lava
plateau similar to the layering characterizing scarps of
Valles Marineris and interpreted as lava flows [McEwen et
al., 1999; Malin and Edgett, 2001]. The layering means that
lava from Syrtis has been delivered into Isidis Basin not as a
single unit but by discrete events. Our estimates show that
the thickness of layers within the Syrtis lava suite varies
from a few tens of meters up to 100 –115 m and is about 80
m on the average. The measurements of unembayed wrinkle
ridges in the Lunae Planum/eastern Solis Planum region
[Head et al., 2002] show that the mean height of these
features is about 65 m, which is less than the mean
measured thickness of the lava stacks (Table 2). Wrinkle
ridges are absent in the eastern portion of Syrtis but are
common features in the rest of the Planum [e.g., Greeley
and Guest, 1987; Hiesinger and Head, 2002; Hiesinger and
Head, manuscript in preparation, 2003]. There, the ridges
appear to be narrower and lower (less prominent) than in the
Lunae Planum area. We interpret this to mean that the
characteristic lack of wrinkle ridges in eastern Syrtis is
consistent with their burial by later lava flows of sufficient
thickness.
[44] An important observation in this respect is that the
large wrinkle-type ridges in Isidis apparently did not control
the flow path of the flows from Syrtis (Figure 1a). This is
consistent with the suggestion that the materials of the
Vastitas Borealis Formation covered the possible wrinkle
ridges in Isidis by the time of the late volcanic activity and
the later flows from Syrtis were emplaced on top of these
materials.
[45] The knobby terrain appears to be one of the most
common features along the transition from Syrtis Major to
Isidis Basin. The knobby terrain in both areas of our detailed
study (the knobby component of the unit Aps [Greeley and
Guest, 1987] in the northern area and the unit HNu in the
southern area) share three important characteristics: (1) This
type of terrain begins basinward after the regional break in
slope at the contact between Syrtis Major and Isidis Basin
(Figures 11 and 13); (2) The overall surface of the occurrences of knobby terrain is horizontal on average and
parallel to the surrounding surface (Figure 14); (3) The
visible thickness (the average height of individual knobs
and mesas) of knobby terrain is about 100 – 200 meters
(Figures 6b and 12b). These characteristics of the knobby
terrain are also consistent with the emplacement of latest
portions of lava from Syrtis on the already existing flat and
horizontal surface of the Vastitas Borealis Formation.
[46] Another typical feature of the transition zone are
scarps that occur in the northern area preferentially as high
single structures (Figure 8) and as a series of lower
structures in the southern area (Figure 15). The scarps are
high (a few hundred meters) structures that maintain
approximately the same height if oriented parallel to the
strike of the transition zone or become progressively lower if
oriented normally to the general orientation of the transition
zone. The typical feature of the larger scarps is that they are
sinuous at different scales and are characterized by sets of
alcoves, niches, and scallops of a variety of widths, from a
few up to tens of kilometers. At some niches the evidence for
rock avalanches is presented and, typically at the base of
scarps, numerous angular blocks are visible (Figure 10).
There is direct evidence for small boulders and blocks (from
a few meters up to a few tens of meters across) slumped from
the large scarps in the MOC images for the northern area of
our study (Figure 9). All these features of the scarps are
inconsistent with their tectonic origin but readily explained
by scalloping of the scarp edges during listric faulting and
mass-wasting that occurs at different scales. In contrast to
this, in the southern area there are no large single scarps but,
instead, abundant evidence for Syrtis Major plateau breakup
is seen (Figure 15). The plateau breakup typically occurs
exactly at the major break in slope from the steady-sloped
surface of Syrtis to the almost horizontal floor of Isidis
Basin. The formation of scarps and the network of faults
cutting and breaking up the plateau represent the youngest
process in the sequence of events that shaped the transition
zone from Syrtis Major Planum to Isidis Basin.
[47] The contiguous areas of knobby terrain and scarps
show significant variations in elevation over relatively short
distances (typically, a hundred meters to a hundred km,
Figures 6b, 12b, and 16). Such variations, which are two
orders of magnitude larger than the variations at the mouths
of the large circum-Chryse outflow channels [Ivanov and
Head, 2001], appear to be inconsistent with the late
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
17 - 19
Figure 16. Elevations of the baseline of the southern flow-like feature in the northern area of detailed
study. The elevations have been determined for the major breaks in slope at the base of the flow where it
is crossed by five MOLA orbits that correspond to the available MOC images for this area. The MOLA
tracks are shown in Figure 6. The difference in the elevations is as large as about 100 m per 100 km of the
feature length. The numbers in the lower part of the plot indicate individual MOLA orbits.
embayment of the knobby terrain or scarps by the materials
of the Vastitas Borealis Formation. Tanaka and Banerdt
[2000] and Watters [2003] have proposed that the broad
tilting of the floor of Isidis Basin is due to loading in the
northern lowlands. We see no evidence that such broadscale tilting has influenced the formation of small-scale
morphologic and topographic features characterizing the
transitional zone from Syrtis to Isidis.
[48] The above characteristics of the typical features of
the transition zone such as knobby terrain and bordering
large single scarps suggest that these features represent
highly modified remnants of the late lava flows from Syrtis
Major that were superimposed on the surface of materials of
the Vastitas Borealis Formation. Thus the generalized
sequence of events in the area of transition from the highstanding lava plains of Syrtis Major to the lowland of Isidis
Basin appears as follows (Figures 17a and 17b): (1) Lava
flows from Syrtis Major extended into Isidis Basin and
partly filled its volume. Apparently, the volcanic activity
within the eastern portion of Syrtis Major continued longer
than the volcanism in the rest of the plateau; (2) The lava
plains on the floor of Isidis Basin were deformed by wrinkle
ridges that have separated the floor of the primary basin into
secondary basins; (3) Materials of the Vastitas Borealis
Formation filled the basin blanketing the surface of the
preexisting lava plains and partly covered the wrinkle
ridges; (4) The very late lava flows from the eastern Syrtis
were emplaced on top of the Vastitas Borealis Formation
materials, were disrupted, and presently appear as knobby
terrain.
[49] In order to estimate the largest possible area of the
knobby terrain we outlined it along the most distant knobs
and mesas. The area within this contour includes the voids
where no knobs or mesas are seen and is about 50,000 km2.
The mean height of the scarp in the northern area, about 0.5
km, is likely an upper limit of the thickness of the late lava
flows from the eastern Syrtis. The visible thickness of
knobby terrain, about 100 m, likely represents a lower
estimate of the thickness of the eastern Syrtis lava flows.
Thus the total volume of the late volcanic material from
Syrtis delivered into Isidis Basin is likely from 5,000 to
25,000 km3.
[50] The total cross-section of all topographically prominent ridges within eastern Syrtis, assuming that all of them
are lava flows and tubes that delivered lavas to the transitional zone, is about 13 km2. This is a maximum estimate
because some ridges may not be lava flows and this would
reduce the effective cross-section of the lava feeders. We
also assumed a range of velocities of the lava flows to be
from 0.1 up to 10 km per hour, which should incorporate all
reasonable velocities of lava flows. At the above numbers,
the necessary volume of lava (from 5,000 to 25,000 km3)
would be emplaced on the floor of Isidis during a very short
time, from about 2 days to about 800 days. It is clear that
such duration is far beyond the ‘‘resolution’’ of the age
determination by the standard technique based on the
images available for the area of our study.
4.2. Possible Nature of Vastitas Borealis Formation
[51] There are three principal situations in which lava
flows of Syrtis Major are in contact with other materials: (1)
The surface of the unit Hs that makes up the Syrtis Major
Planum is characterized by flow fronts and margins. There
is good evidence within the eastern portion of Syrtis Major
for the superposition of lava flows on the surface of the
plateau (Figure 5). Also, the stacked lava flows of different
17 - 20
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Figure 17. a) The interpreted cross-section of the transition zone from Isidis Basin to Syrtis Major
Planum (not to scale) and b) the suggested correlation chart of units and structures for this area. See text
for details.
thickness are exposed in any scarp that cut the surface of
Syrtis; (2) Along the southern, western, and northern edges
of Syrtis Major the lava flows from the plateau are in
contact with heavily cratered Noachian terrains (Figure 18a)
and occur there at a variety of topographic gradients. For
instance, where lava flows from Syrtis enter the crater
Antoniadi (Figure 18b) through the narrow gap in the crater
rim [Hiesinger and Head, 2002; Hiesinger and Head,
manuscript in preparation, 2003], the topographic gradient
is changed from about 0.07 within Syrtis to about 0.5 in
the gap, and back to almost horizontal on the crater floor;
(3) Along the eastern edge of the plateau, in the area of our
study, the lava flows are in contact with the materials of
Vastitas Borealis Formation on the floor of Isidis Basin.
[52] Neither in case (1) nor (2) is there any evidence for
plateau breakup or progressive disruption of lava flows into
small knobs, plates, and mesas. These specific features
occur exclusively at the eastern edge of Syrtis Major where
its lava flows entered into Isidis Basin (Figures 6 and 12).
Such contrasting characteristics of the contact of the Syrtis
lavas with their surroundings suggest that either the lava
flow sequence within Syrtis Major itself, or rocks of the
Noachian highlands, provided firm and resistant basement
for the formation of the morphologically intact suite of lava
plains. In contrast, materials of Vastitas Borealis Formation
on the floor of Isidis Basin served as a weak, non-resistant
basement for the lava flows from Syrtis Major. In order to
provide this, the basement as a whole or, more likely, some
part of it should be mobilized and evacuated during the
interaction with superposing lava flows. The phenomena
observed at the transition zone from Syrtis Major Planum to
Isidis Basin is readily explained by interaction of hot lava
with materials enriched with volatile components (H2O,
CO2, or both) on the floor of the basin. During (and perhaps
following) the interaction with lavas, these components
mobilized and escaped, which softened the non-volatile
residuum, and allowed the lava flows to subside, break
up, and eventually to be completely destroyed [Squyres et
al., 1987; Hoffman, 2000]. The superposition of hot volcanic materials from Syrtis Major on top of volatile-bearing
sediments of the Vastitas Borealis Formation within Isidis
Basin may explain the key features of the contact such as
the breakup of the Syrtis Major plateau, formation of
knobby terrain along the contact and the other specific
features.
[53] The large sinuous ridges that are covered within
Isidis Basin with the material of Vastitas Borealis Formation, regardless of their origin, form secondary basins on the
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
17 - 21
Figure 18. Character of interaction of lava flows from Syrtis Major Planum with the surrounding
materials. a) Syrtis lavas are in contact with the relicts of ancient cratered uplands in the southern portion
of Syrtis Major. The lava embays the old terrains without any evidence for breakup and/or fragmentation,
which is the characteristic of the contact of the Syrtis lavas with the materials on the floor of Isidis Basin.
b) Lava flows from Syrtis Major enter the crater Antoniadi at the northwestern portion of Syrtis. The lava
flows remain morphologically intact within the Syrtis plateau (a), within the breached rim of the
Antoniadi crater where the flows are on steep slopes (arrow), and on almost horizontal floor of the crater
(b). Both images are fragments of the MDIM photobase and show areas about 140 140 km. See
Hiesinger and Head [2002] and Hiesinger and Head (manuscript in preparation, 2003) for details of these
relationships.
floor of Isidis. These basins, which are 150 –180 km across
[Hiesinger and Head, 2002; Hiesinger and Head, manuscript in preparation, 2003] should introduce a major
inhomogeneity in the distribution of the Vastitas Borealis
Formation material. Its varying thickness could play a key
role in the appearance of specific features of the transition
zone from Syrtis Major to Isidis Basin. For example, the
large flow-like features that characterize the northern area of
our study extend into Isidis on top of two low and broad
topographic ridges separating the floor of Isidis into a series
of secondary basins (Figure 1). The scarps at the edges of
the flows (Figure 10) are facing toward the deeper portion
of the secondary basins (Figure 1). The large arcuate scarp
that cuts the edge of Syrtis Major to the south of the flowlike features also outlines the western portion of a secondary
basin within Isidis (Figure 1). The thickness of the materials
of the Vastitas Borealis Formation should be larger in such
basins. Potentially, a larger amount of volatiles could be
stored there, enough to destroy superimposing lava flows,
undermine them, cause the scarp retreat, and create high
bounding scarps at the edges of the flows where the layer
enriched in volatile components is pinching out.
[54] In contrast to the northern area, in the southern area
of our study there are no single prominent scarps and broad
and shallow topographic troughs and equidimensional
depressions characterize the area of unit HNu (Figure 1),
which dominates the southern area. This difference could be
due to the smaller thickness of the volatile-bearing layer of
the Vastitas Borealis Formation within the southern area or
smaller amount of volatile components or both. Another
explanation of the characteristic features within the southern
area (Figure 15, but still in the framework of the interaction
of lavas with the volatiles) is the most rapid and/or more
voluminous emplacement of volcanic materials there. This
is supported by the presence of the two largest lava flows
(tubes) within the Syrtis Major plateau converging on the
central area of the extension of knobby terrain in the
southern area (unit HNu). The other important features
that occur in the southern area and could be indicative of
the lava/volatile interaction are the softened morphology of
the knobs and plates and the narrow troughs with rims at the
southern edge of the unit HNu. One of the plausible
explanations of the unusual morphology of these features
is the interaction of hot lava flows with an underlying
volatile-rich substratum, rapid release of the volatiles and
their ventilation through weak zones in a relatively thick
layer of lavas.
[55] The interaction of hot lava flows with the volatilerich layer must lead to diminishing of the amount of the
volatile components and maybe also to their complete loss if
the volatiles are not very abundant. Thus the superposition
of lava flows from Syrtis Major on the materials of Vastitas
Borealis Formation may explain the typical lack of small
pitted mounds and cones on the floor of Isidis Basin within
and in the vicinity of the knobby terrain if the mounds
represent either pseudocraters [Frey et al., 1979] or cryovolcanic cones [Hoffman et al., 2001; Hoffman and Tanaka,
2002].
5. Summary and Conclusions
[56] In our study, we analyzed in detail the transition zone
from Syrtis Major Planum to Isidis Basin in order to address
the following questions: (1) What is the general sequence of
events in the transition zone and what is the place and role
of the Hesperian ridged plains that make up the majority of
17 - 22
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
Syrtis Major? (2) How does this sequence correlate with the
proposed origin of the Vastitas Borealis Formation [Grizzaffi and Schultz, 1989; Scott et al., 1995; Tanaka and Kolb,
2001; Tanaka et al., 2000, 2001a, 2002b]? (3) What is the
possible nature of the Vastitas Borealis Formation?
[57] Our results and interpretations are summarized in the
schematic cross-section of the transition zone from Isidis
Basin to Syrtis Major Planum (Figure 17). In this scenario,
in the Early Hesperian, volcanic plains are emplaced in
Syrtis Major (the lower part of the Syrtis Major Formation,
Hs, lower) and wrinkle ridges deform their surfaces soon
thereafter. Concurrently, volcanic plains are emplaced on
the floor of the Isidis Basin and were deformed by a
network of wrinkle ridges. The apparent simultaneity of
these units may mean that Syrtis Major was the source of
many of the flows in the Isidis Basin. In the early part of the
Upper Hesperian, subsequent to formation of most of
wrinkle ridges, the Vastitas Borealis Formation (VBF) was
emplaced in the Isidis Basin and elsewhere in the
northern lowlands [e.g., Tanaka and Scott, 1987]. Following the emplacement of the VBF, flows of the upper
part of the Syrtis Major Formation were emplaced, erupting
from the eastern margins of Syrtis Major Planum and
flowing down into the westernmost part of the Isidis Basin
on top of the recently emplaced VBF. Modification of the
superposed lavas by degradation of apparently volatile-rich
material of the VBF formed the scarps and unusual morphology of the unit HNu that characterizes the transitional
zone. The Vastitas Borealis Formation continued to undergo
modification and evolution during the Late Hesperian to
create the presently observed facies (ridged, knobby, etc.)
and deposit. Our interpretation is largely based on stratigraphic relations documented in this paper, which include 1)
the detection of underlying Hesperian-aged ridged plains
below the Vastitas Borealis Formation in the Isidis Basin
[Head et al., 2002], 2) the crater ages of major units, and 3)
the detailed documentation of the characteristic features and
stratigraphic relationships within the transition zone from
Syrtis Major to the Isidis Basin described above.
[58] In a recent paper, Tanaka et al. [2002b] outlined a
detailed three-stage model for the evolution of the Hellas
Basin rim and the filling of the basin interior which they
also applied to Syrtis Major Planum and the Isidis Basin
interior [see also Tanaka et al., 2000]. In this model they
envisioned the following steps. 1) Prior to emplacement of
Syrtis Major lavas, sills intrude shallow friable rocks of the
basin rim and mobilize volatile-rich impact breccias.
2) Surface rocks are fluidized and flow into the adjacent
basin, removing massifs from the basin margin, and producing volatile-rich sedimentary deposits across the basin
floor. 3) Subsequent volcanic eruptions cover the eroded
basin rim surface and basin inner slopes with lava plains
forming Syrtis Major Planum, but leave the volatile-rich
deposits in the basin interior floor exposed and largely
undisturbed.
[59] The results of our study suggest an alternative
sequence of events (Figure 17) that is not consistent with
the model outlined by Tanaka et al. [2000, 2001a, 2001b,
2002b]. The unit presently exposed on the basin floor (the
VBF) postdates both stratigraphically and from crater count
data the Early Hesperian volcanic ridged plains in Syrtis
Major Planum [Hiesinger and Head, 2002; Hiesinger and
Head, manuscript in preparation, 2003] and in the subsurface of Isidis Planitia [Head et al., 2002]. Stratigraphically,
it predates the latest phases of Late Hesperian volcanism
originating from the edge of Syrtis Major Planum and
extending down onto the western edge of the Isidis Basin
floor. These findings suggest that material of the VBF is not
derived from intrusions of pre-Syrtis Major sills. Thus the
specific morphology and stratigraphic relationships typical
of the transitional zone from Syrtis Major to Isidis Basin
suggest a different source for the material of the Vastitas
Borealis Formation and later age than proposed by Tanaka
et al. [2002a, 2002b]. We do agree, however, that the VBF
represents a volatile-rich deposit on the floor of the basin.
[60] Grizzaffi and Schultz [1989] proposed that a thick
(up to 2 km) glacier-like air fall deposit was emplaced on
the floor of Isidis Basin. In this model, the remnants of the
deposit, the ridged member of the Vastitas Borealis Formation [Greeley and Guest, 1987], form the youngest materials
within Isidis and represent a lag deposit left after the
sublimation of the volatile component. Although this model
offers a possible source for the volatile-rich material, it does
not explain why the air fall occurred precisely within the
basin. Also, in the model of Grizzaffi and Schultz [1989], all
lava plains from Syrtis Major are considered to be relatively
old and predate the air fall deposit. Our findings, however,
place the formation of material of the Vastitas Borealis
Formation between two major episodes of volcanism in
Syrtis Major Planum.
[61] The model of Scott et al. [1995] based on detailed
mapping of a range of morphologies within Isidis Basin
suggests the existence of a large lake-like body of water
within the basin. This model provides another explanation
for the formation of a volatile-rich deposit (the Vastitas
Borealis Formation) and its stratigraphic relationships with
the lavas from Syrtis Major. The stratigraphic order of the
VBF described by Scott et al. [1995] corresponds closely to
our scheme of the sequence of events at the transitional zone
from Syrtis to Isidis. The model of Scott et al. [1995],
however, does not specify the sources for water that could
accumulate in the basin and the map showing the areal
distribution of channels and possible paleolakes clearly
indicates the lack of feeding channels around Isidis.
[62] Another possible explanation of the volatile-rich
nature of material of the VBF on the floor of Isidis Basin
is the existence of a large standing body of water (an ocean)
within the northern lowlands, the water of which could fill
Isidis through its lowered northeastern rim [Parker et al.,
1989, 1993]. This hypothesis is consistent with the MOLA
topography data [Head et al., 1999; Ivanov and Head,
2001; Kreslavsky and Head, 2002], the sequence of events
found in the transitional zone from Syrtis to Isidis, and
readily explains the nature of material of the VBF. In this
scenario, water emplaced on the floor of the basin is likely
to have frozen and sublimed away geologically rapidly
[e.g., Kreslavsky and Head, 2002] and the VBF is interpreted to be the sublimation residue of the frozen water in
the basin. The late stage Syrtis Major lava flows superimposed on top of the VBF material prior to its advanced
sublimation interacted with its volatile component and
caused its sublimation and escape. The evacuation of the
volatiles left behind a soft non-cemented deposit that was
unable to support the load of lava flows. The flows were
IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT
broken, disrupted, and underwent an enhanced mass wasting due to both the relaxation of the underlying deposit and
enforced ventilation of volatiles. These processes ultimately
formed the unit HNu and other specific features of morphology and topography in the transitional zone from Syrtis
Major to Isidis Basin.
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J. W. Head III and M. A. Ivanov, Department of Geological
Sciences, Brown University, Box 1846, Providence, RI 02912, USA.
([email protected])