JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. E1, 5004, 10.1029/2000JE001445, 2002
Northern lowlands of Mars: Evidence for widespread volcanic
flooding and tectonic deformation in the Hesperian Period
James W. Head III
Department of Geological Sciences, Brown University, Providence, Rhode Island, USA
Mikhail A. Kreslavsky
Department of Geological Sciences, Brown University, Providence, Rhode Island, USA
Astronomical Observatory, Kharkov National University, Kharkov, Ukraine
Stephen Pratt
Department of Geological Sciences, Brown University, Providence, Rhode Island, USA
Received 8 December 2000; revised 2 July 2001; accepted 16 August 2001; published 29 January 2002.
[1] Recent Mars Orbiter Laser Altimeter (MOLA) data have provided a new picture of the
Martian northern lowland basin topography and surface roughness. In order to assess detailed
topographic structure important in understanding the formation and evolution of the northern
lowlands, we have removed regional slopes from the topography to produce a series of maps
highlighting the local topographic and geologic structure. We find that (1) the northern
lowlands are underlain by a regional unit containing a basin-wide system of subparallel
wrinkle ridges; (2) this unit contains highly modified craters, the number of which suggests an
Early Hesperian age; (3) this unit is laterally contiguous with Early Hesperian-aged ridged
plains in the southern uplands; (4) the orientation and location of the wrinkle ridges in the
North Polar Basin complete a global circum-Tharsis ridge system forming a band 7000 km
wide and extending over the whole circum-Tharsis region; and (5) several subareas of the
northern lowlands show individual wrinkle-ridge patterns (e.g., Isidis and Utopia are basinlike). The recognition of this unit and the superposed very degraded craters permits us to
assess the stratigraphy and geometry of subsequent units and structure. We find that (1) the
present spacing and height of wrinkle ridges and geometry of buried craters suggest that the
Vastitas Borealis Formation is a thin sedimentary unit superposed on regional ridged plains
(Hr) and that its minimum average thickness is 100 m; (2) the polar terrain completely
obscures the wrinkle-ridge system and some circumpolar deposits partially obscure it,
supporting the interpretation that some circumpolar deposit thicknesses exceed several hundred
meters and that wrinkle-ridge formation was not active in the Amazonian; (3) Late Hesperianaged outflow channels entering Chryse Planitia are controlled by the orientation and
topography of wrinkle ridges deep into the basin, indicating that wrinkle ridges had largely
formed by this time; (4) Amazonian-aged smooth plains units, particularly in Amazonis
Planitia, further bury and obscure the underlying wrinkle ridges, and (5) fretted terrain,
particularly in the Deuteronilus Mensae region, formed subsequent to the Early Hesperian-aged
ridged plains, and remnants can be seen to extend beneath the Vastitas Borealis Formation.
The recognition of these units and their stratigraphic relationships permits us to outline a new
perspective on the history of the northern lowlands: (1) in the Early Hesperian the majority of
the northern lowlands was filled with volcanic plains similar to those presently exposed in the
southern uplands; outcrop patterns of buried Noachian-aged terrain and volcanic flooding
models suggest an average thickness of 800– 1000 m; (2) these plains were deformed soon
thereafter by Tharsis-circumferential and basin-related wrinkle ridges; (3) circum-Chryse
outflow channels were emplaced in the Late Hesperian, forming subdued channels whose
course was largely controlled by wrinkle-ridge orientation and height, and these channels
deposited water and sedimentary material in the basin; (4) loss of the outflow channel water
resulted in formation of the Vastitas Borealis Formation as a residual sedimentary deposit on
top of the ridged plains; little evidence is seen for the presence of any massive near-surface
residual ice deposits remaining from the outflow channel effluent; (5) Amazonian volcanic
plains were emplaced, primarily in Amazonis Planitia and Utopia Planitia, further obscuring
the Hesperian ridged plains; and (6) Late Amazonian polar and circumpolar deposits formed,
further obscuring the structure of the underlying Hesperian ridged plains. The widespread
Copyright 2002 by the American Geophysical Union.
0148-0227/02/2000JE001445$09.00
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emplacement of the Hesperian-aged ridged plains of apparent volcanic origin is interpreted to
mean that the volcanic phase represented by this unit was global in nature and resurfaced the
northern lowlands, in addition to the 10% of the planet previously known, for a total
resurfacing of 30% of Mars. This remarkable event increases dramatically the amount of
water and other volatiles that might have been degassed into the atmosphere during the Early
Hesperian and further underlines the global significance of the contractional deformation
typical of this period.
INDEX TERMS: 6225 Planetology: Solar system objects: Mars, 5480
Planetology: Solid surface planets: Volcanism, 5475 Planetology: Solid surface planets:
Tectonics, 5470 Planetology: Solid surface planets: Surface materials and properties;
KEYWORDS: Mars, volcanism, tectonism, northern lowlands, wrinkle ridges, oceans
1. Introduction and Background
1.1. Northern Lowlands
[2] The northern lowlands of Mars cover about one third of the
surface of the planet and are the central part of a larger drainage
basin that has composed about two thirds of the planet’s surface for
most of its history [Smith et al., 1998; Zuber et al., 1998]; the
northern lowlands may have contained a large standing body of
water in earlier Mars history [Parker et al., 1989, 1993; Baker et
al., 1991] and have been proposed to be the location of plate
tectonic activity in earliest Mars history [Sleep, 1994].
[3] In ascending stratigraphic order the exposed surface units of
the northern lowlands [Scott and Tanaka, 1986; Tanaka and Scott,
1987; Greeley and Guest, 1987] are dominated by (1) Noachianaged remnants, including a small unit dissected by channels in
Acidalia Planitia, old degraded crater rims, and Scandia Colles, a
collection of domes interpreted to be old degraded Noachian crust,
(2) the Hesperian-aged Vastitas Borealis Formation, an unusual
unit interpreted to be degraded lava flows and sediments, (3)
Hesperian-aged outflow channel deposits at the margins of the
lowlands in Chryse Planitia, (4) various local Amazonian-aged
plains units, (5) Elysium channel and flow deposits, (6) the north
polar cap, consisting of Late Amazonian ice and layered terrain
deposits, and (7) Amazonian-aged circumpolar mantling material.
The predominantly sedimentary nature of many of these northern
plains units combined with modification processes since their
formation has obscured the character and age of underlying terrain
and thus its origin and evolution. Two fundamental issues remain
unresolved: (1) when and how did the distinctive northern lowlands form, and (2) what were the processes involved in their
evolution?
1.2. Mars Orbiter Laser Altimeter Data
[4] The Mars Orbiter Laser Altimeter (MOLA) experiment on
board the Mars Global Surveyor mission has provided a new
global characterization of the topography and slopes of Mars
[Smith et al., 1998, 1999; Aharonson et al., 1998; Zuber et al.,
1998; Garvin et al., 1999; Kreslavsky and Head, 1999, 2000].
The combination of gravity and topography has provided
important new insights into crustal structure and thermal evolution [Smith et al., 2001; Zuber et al., 2000]. These data (Figures
1a and 1b) have shown that (1) the northern lowlands are
extremely flat [Smith et al., 1998]; (2) they are irregular in
shape and consist of two basins, the ancient, circular, heavily
modified Utopia Basin of impact origin [McGill, 1989] and the
irregularly shaped North Polar Basin [Head et al., 1999]; (3) the
dichotomy boundary (the distinctive morphologic and local
topographic slope boundary between the southern heavily cratered uplands and the northern lowlands) does not coincide
exactly with the crustal thickness trends [Zuber et al., 2000];
and (4) the surface of the northern lowlands is exceptionally
smooth at scale lengths from hundreds of meters to tens of
kilometers [Aharonson et al., 1998; Kreslavsky and Head, 1999,
2000]. As a further step in the analysis of the origin and
evolution of the northern lowlands, we have used recent MOLA
data to analyze basin topography and surface roughness charac-
teristics, removing regional slopes to assess subtle topographic
variations.
1.3. Representation of Topography
[5] MOLA vertical accuracy is 0.3 m [Smith et al., 2001].
This means that 1-m-high features can be detected and mapped in
smooth regions. Topographic variations across the northern lowlands, excluding deep impact craters, are 3000 times greater, of
the order of 3 km. This high dynamic range resulting from the
excellent MOLA data quality actually produces some problems for
the global visual presentation of the data and their interpretation for
geomorphological analysis.
[6] Contour presentation of topography (as actual contour lines
or color shaded renditions) is the manner in which topographic
information is commonly portrayed for geomorphological and
geological analyses (Figure 1b). This technique is not suitable,
however, for obtaining a simultaneous overview of a wide range of
spatial scales. A gray-scale image can present only major topographic variations, because the human eye can distinguish only
15 – 20 shades of gray. Use of color-coded images increases the
perceptible dynamic range a few times. The color-coded images are
less suitable, however, for several aspects of visual geomorphologic analysis. This is because the same small features displayed as
variations of dark gray and light gray are automatically accepted by
the human eye as the same, while the same features displayed as
shades of different colors are not. Gray scales or color scales can be
repeated a few times. This increases the perceptible dynamic range
but decreases the suitability for geomorphological analysis.
[7] The use of derivatives, that is, imaging of slopes rather than,
or in addition to, mapping of elevations, provides a set of powerful
methods for visual presentation of topographic data of a wide
dynamic range. Such methods are based on a common property of
planetary topography: smaller features usually have steeper slopes.
Among these methods the most suitable for geomorphological
analysis is the presentation of simulated shaded relief (‘‘gradient
maps’’) showing steep (small-scale) features overlain with colorcoded topography showing regional-scale features. Figure 2a
shows an example of artificial topography with a set of features
of different scale and steepness. In the images shown in this
example and in this paper, the brighter shades denote higher
relative elevations. Simulated shaded relief for this example
(Figure 2b) demonstrates an important characteristic of this type
of data presentation: it can emphasize or deemphasize some
features depending on the mutual orientation of the slopes and
the simulated light source.
[8] There are numerous topographic features on the northern
plains of Mars whose lateral scale is of the order of tens of
kilometers but whose characteristic slopes are not much steeper
than those of the regional relief. Slope-imaging methods do not
give the best visual presentation of such features (Figure 2b). Our
aim is to present the topographic data in a form uniformly showing
topographic features with gentle slopes, lateral scales of tens of
kilometers, and vertical scales of tens of meters in the northern
plains of Mars. To do this, we applied a set of filters that remove
long-wavelength (regional) topography from 16 pixels per degree
and 32 pixels per degree gridded topography data.
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
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Figure 2. Comparison of different visual presentations of an
artificial topography model to illustrate the detrended topography
in relation to other techniques. The model was created by
superposing (1) a gentle smooth slope from lower right down to
upper left, (2) a steep tall dome at the lower right, (3) a steep scarp
at the lower left, (4) three very subtle ridges, two in the center and
one (about two times higher) along the right side, and (5) noise. (a)
Gray-scale presentation of the artificial topography. Brighter
shades denote higher elevations. (b) ‘‘Gradient map,’’ showing
simulated shaded topography. Simulated illumination is from the
top. Note that the ridge along the right side is almost invisible
because the illumination direction is not favorable to revealing it.
Ridges on the slopes of the dome are barely distinguishable. (c)
Topography filtered by a linear high-pass filter. Window (core) size
was the same as the size of the circle around the letter c. The gray
scale is stretched 6 times in comparison to Figure 2a. The lowest
and highest areas are saturated. The area between the ridges and the
dome base is not seen. (d) Topography filtered by the median highpass filter (‘‘detrended topography’’). Window (filter core) size and
the gray-scale stretch are the same as for Figure 2c. The
topographically subtle, but real, ridges are seen as are the more
distinctive features of the scarp and the dome.
[9] Application of any filter modifies topography and can
produce artifacts in the filtered images. Linear high-pass filters
produce distortions related to sharp breaks in regional slopes; for
example, deep troughs appear in the filtered topography along the
polar cap base or the dichotomy boundary. Figure 2c shows an
example of the application of a simple linear high-pass filter: for
each point the mean elevation in a circular vicinity of the point
(filter window or core) was subtracted from the elevation at this
point. It is seen that small-scale details along the base of a steep
large-scale slope are obscured. Other kinds of linear high-pass
filters give similar results. To reduce such unwanted effects, instead
of linear filters we applied a ‘‘median high-pass filter’’: for each
point, the median elevation in a circular window was subtracted
from the elevation at this point. Our artificial example (Figure 2d)
shows that in this case it is possible to trace subtle features (two
low ridges) through the break in the large-scale slope.
[10] In this paper we study the northern lowlands using topography to which was applied ‘‘median high-pass filters’’ with
circular windows of 50, 100, and 200 km in diameter. We refer to
these data as ‘‘detrended’’ topography. We present it as gray-scale
images (Figure 1c). We stretched the shade scale to show features
tens of meters high. Steep features at the tens of kilometer scale,
e.g., impact craters, are saturated: their higher parts are ‘‘whiter
than white,’’ and their lower parts are ‘‘blacker than black.’’ Knobs
and grooves, as well as the previously detected 3-km-scale background topography of the Vastitas Borealis Formation [Kreslavsky
and Head, 1999, 2000], are not completely resolved in the data
used and sometimes are not seen in the detrended images. For
example, in regions of polygonal terrain a number of linear
depressions are seen, but they do not form a continuous network,
while higher-resolution images clearly show a polygonal groove
pattern.
[11] All smooth topographic features much larger than the
window size are filtered out and are not seen in the images at
all. Examples of such completely removed features are gentle
global slopes and regional slopes along the dichotomy boundary
and in the Utopia basin. The filtering procedure leaves unchanged
all topographic features where both dimensions are sufficiently
smaller than the window. Understanding of the filtering procedure
and comparison of the detrended topography produced with different window sizes allow one to distinguish between real topographic features and the types of candidate artifacts described
above (see also Figure 2). We are confident that all of the features
discussed in this contribution are real.
1.4. Scope of Analysis
[12] Removal of the regional slopes has allowed us to detect and
assess subtle topographic variations and geologic structure in the
northern lowlands [Head and Kreslavsky, 2000], and we report on
these analyses here. We have used these techniques to explore the
regionally very flat and smooth topography of the northern lowlands [e.g., Smith et al., 1998, 1999] in order to detect important
topographic elements that might be obscured by regional topographic trends in normal contour map portrayals. Specifically, we
use these data to analyze (1) the presence and distribution of linear
ridges and wrinkle-ridge-like structures, (2) the presence of fretted
terrain and its extension into the northern lowlands, (3) the traces
of outflow channels down into the northern lowlands, (4) the
Figure 1. (opposite) The northern hemisphere of Mars. (a) Shaded relief map of the topography of the northern hemisphere derived
from Mars Orbiter Laser Altimeter (MOLA) data, showing the location of major features and regions. Latitudes are marked at 20°, 40°,
60°, and 80°N. This and other maps in Figure 1 are in Lambert equal area projection. The western longitude convention is used to
facilitate comparison to published U.S. Geological Survey (USGS) maps. (b) Topography of the northern hemisphere from MOLA data
shown as a gray-scale image and contour map. The contour interval is 1 km, and contours above 12-km elevation are not shown (white).
The positions of the Utopia Basin and the North Polar Basin are clearly seen in the northern lowlands, with Utopia centered near 250°W,
43°N and the North Polar Basin extending across the region of the north pole from about 180°W, 40°N to 30°W, 30°N. (c) Detrended
topographic map of the northern hemisphere. The filter core radius is 100 km. Locally, bright is high and dark is low. The abundant and
pervasive systems of ridges in the northern lowlands are clearly seen. (d) Location map showing the main features in the northern
hemisphere discussed in the text, overlain on the detrended topographic map. (e) Major structural trends and domains in the northern
lowlands. Solid straight and curved lines with short orthogonal bar show orientations representing the major trends of ridges; dashed lines
show boundaries between domains. Detailed orientations are shown in Figures 5 and 10 – 14. (f ) Location map of profiles and several
subsequent figures. Letters refer to locations of images and profiles shown in Figure 5.
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
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Figure 3. Global distribution of ridges on the surface of Mars displayed in Lambert azimuthal equal-area projection
centered on the equator at (left) 0° and (right) 180°. All Martian ridges (16,000) mapped by Chicarro et al. [1985]
are shown, and the density of black shows the density of the individual features at this scale. Note the paucity or
absence of ridges in the northern lowlands. Modified from Chicarro et al. [1985].
nature of structural patterns on the floors of the Isidis, Utopia, and
North Polar Basins, (5) the presence of subtle and buried impact
craters, (6) the role of volcanic plains in the history of the northern
lowlands, (7) the nature of the Vastitas Borealis Formation, and (8)
the relationship of these units, structures, and features to the
stratigraphy and history of the northern lowlands.
2. Analysis
2.1. Northern Lowlands: Geologic Characteristics and
Structure
[13] Examination of Figures 1a and 1b shows the basic topography of the northern lowlands, with the two major basins
(Utopia and North Polar) dominating the region. These basins
are floored predominantly by the Vastitas Borealis Formation of
Middle to Late Hesperian age, superposed Amazonian deposits in
the Utopia Basin (the Elysium Formation), and later Amazonian
polar and circumpolar deposits in the North Polar Basin (Figure
1d). The topography of these basins is unusually smooth at a wide
range of scales [Smith et al., 1998; Kreslavsky and Head, 1999,
2000], and Viking Orbiter images revealed little in the way of
tectonic structure in these regions (e.g., see maps of Chicarro et al.
[1985], Watters [1993], Scott and Tanaka [1986], Tanaka and Scott
[1987], and Greeley and Guest [1987]) (Figure 3). Comparison of
Figures 1a and 1b (topography) and Figure 1c (detrended topography) reveals, however, an abundance of linear features in the
northern lowlands that are not visible in the topography data alone
[see also Withers and Neumann, 2001]. These can be subdivided
into several major trends (Figures 1d – 1f) as follows.
2.1.1. North Polar Basin trend. [14] The dominant trend in
the North Polar Basin (Figures 1c, 1d, 1e, and 4) is formed by a
very distinctive set of linear parallel ridges striking across the basin
in a broadly arcuate pattern between Tharsis and Utopia (Figures
1a – 1c). The trend is first seen in the lowlands in Chryse Planitia
and extends from there for a distance of 1500 km through Acadalia
Planitia, where it is interrupted by the island composed of a
Noachian-aged dissected unit. It then stretches for another 2000
km deep into the North Polar Basin until it reaches the vicinity of
the North Pole, where it is obscured by overlying polar and
circumpolar deposits [Tanaka and Scott, 1987; Fishbaugh and
Head, 2000] for a distance of 1250 km. It emerges from beneath
these deposits north of Alba Patera and then extends through
Arcadia Planitia for 1750 km to the edge of Amazonis Planitia,
where it is clearly covered by smooth units of the Amazonian-aged
Arcadia Formation for a distance of 1500 km. The trend
reemerges from beneath the Arcadia Formation in Amazonis [see
Plescia, 1993] and extends for a distance of 1000 km into the
highlands region in the vicinity of Medusae Fossae. The total
distance of this trend in the North Polar Basin is 9000 km, and
the pattern is clearly arcuate around the northern part of Tharsis
(Figures 1a, 1c, 3, and 4).
[15] To a first order these data show that the almost complete
lack of structure in the northern lowlands [e.g., Chicarro et al.,
1985; Watters, 1993; Scott and Tanaka, 1986; Tanaka and Scott,
1987; Greeley and Guest, 1987] (Figure 3) was apparently due to
the inability of Viking image data to reveal structure. This may
have been due to the commonly hazy atmosphere above the
lowlands, superposition of younger units, such as the Vastitas
Borealis Formation, or the eolian modification and smoothing of
much of the surface. The distinctive and pervasive structure
revealed by the MOLA detrended data appears to complete much
of the regional circumferential trend around Tharsis previously
mapped outside the northern lowlands [e.g., Chicarro et al., 1985;
Watters, 1993].
[16] In general, the map patterns of the surface structures in the
northern lowlands can best be described as linear to anastomosing
(Figures 4 and 5). Individual features are broadly parallel, but they
wax and wane in their distinctiveness and width along strike and
have lengths ranging up to several hundreds of kilometers. The
vast majority of the occurrences would be described as parallel, in
which linear to somewhat anastomosing parallel ridges dominate.
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Figure 4. (a) Detrended topographic map of the North Polar Basin. Map projection is Lambert polar equal-area. The
filter core radius is 50 km. (b) Sketch map showing major features and structures.
In some places (e.g., the eastern part of Vastitas Borealis (0°)
(Figure 5d); the eastern part of Acidalia (Figure 5e); and Chryse
Planitia (Figure 5f)) the pattern is somewhat more polygonal. The
similarity of shapes in map view and patterns of distribution
between the linear features in the northern lowlands and wrinkle
ridges elsewhere on Mars [Watters, 1993] is striking and lends
support to the interpretation of these ridges as wrinkle ridges.
[17] Some of the characteristics of these individual features can
be determined from their geometry and relationships in these
detrended maps. In order to investigate the spacing and heights
of these features, we compiled six profiles of the detrended data
spaced across their zone of occurrence (Figures 1f and 5a – 5f).
Each profile was oriented normal to the trend of the ridges and
extended for 4000 km in length. These profiles were placed
underneath the detrended image and compared; the peak corresponding to each linear feature was identified where it crossed the
profile, and these data provided the basis for compiling a histogram
of linear feature heights and spacing, taking into account and
correcting for the projection and orientation of features relative to
the profile. On the basis of 162 measurements across a total linear
distance of 14,000 km, the mean spacing of these features is
83 km (Figure 6).
[18] The features that comprise the rest of the circum-Tharsis
structural trend outside the northern lowlands (Figure 3) are predominantly wrinkle ridges [e.g., Watters and Maxwell, 1983, 1986;
Chicarro et al., 1985; Davis and Golombek, 1990; Golombek et al.,
1991; Watters, 1993; Allemand and Thomas, 1995] and are of
contractional origin. Allemand and Thomas [1995] presented data
for the spacing between wrinkle ridges on the eastern flank of
Tharsis, showing that the mode was 50 km, with a minimum of 10
km and a maximum spacing of 100 km. In an analysis of the
broader Tharsis rise, Watters [1991] found similar results for 15
different domains of ridged plains in the circum-Tharsis region
south of the northern lowlands. His results show the circum-Tharsis
spacing frequency distribution, indicating that typical spacing is in
the 30- to 50- km range, similar to the compilation of previous
measurements described by [Zuber and Aist 1990, Table 1].
[19] Using the detrended MOLA data, we made similar measurements of wrinkle-ridge spacing in Lunae Planum and Syria
Planum (Figures 5g and 6b). On the basis of six profiles crossing
69 ridge structures, we found that the mean spacing was 50 km,
consistent with the results of Allemand and Thomas [1995] and
Watters [1991]. Comparison of the mean spacing between ridges in
the circum-Tharsis rise region and the northern lowlands (Figure 6)
shows that the mean northern lowlands spacing (83 km) is almost
1.7 times that of ridges in the Eastern Solis Planum and Lunae
Planum (50 km).
[20] The shapes and patterns of distribution of the linear
features are also seen and can be compared to wrinkle ridge
patterns in Lunae Planum and Syria Planum (Figure 5). Wrinkle-
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
Figure 5. Sections of detrended topographic maps in the northern lowlands and profiles of detrended data. See
Figure 1f for location of individual areas and profiles. Profiles are presented and oriented in sequence across the
northern hemisphere as shown in Figure 1f, and thus the proflies of Figures 5a, 5b, and 5c are oriented with south
toward the top. Profiles are data number (dn) values stretched from 0 to 256 and show the relative elevations of
features across the center line (white line) of each detrended image. To obtain spacing and altitude data, ridges were
mapped on these images, located in detrended profiles, and then located in corresponding altimetric profiles. (a)
Amazonis Planitia. (b) Arcadia Planitia. (c) Vastitas Borealis (180°). (d) Vastitas Borealis (0°). (e) Acidalia Planitia.
(f) Chryse Planitia. (g) Compare orientations and patterns of linear features in the northern lowlands (Figures 5a – 5f)
to wrinkle ridges in Lunae Planum formed in Hesperian-aged ridged plains (Hr).
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Figure 5.
(continued)
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
Figure 5.
(continued)
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Figure 5.
ridge patterns can be described in three main categories: (1)
parallel, in which linear parallel ridges dominate (e.g., Syria
Planum, Lunae Planum, and Coprates), (2) orthogonal, in which
there is a major linear parallel trend and an orthogonal minor trend,
and (3) polygonal, in which the linear parallel trends are interrupted by variable angular orientations, crater rim structures, and
other features which degrade parallel or orthogonal trends (e.g.,
Hesperia Planum). Polygonal trends often occur in regions where
more heavily cratered terrain or degraded highland terrain lies at
shallow depths below the surface (e.g., Hesperia Planum [Watters,
1993, Figure 6] and Arcadia Planitia [Plescia, 1993]).
[21] The detailed topographic structure of these features can be
determined using individual MOLA altimetric profiles and gridded
altimeter data (in this area the grid data point density is 1/32
degree). We examined the height of all 162 structural elements
along the individual profile traverses where the spacing measurements were made. These data show that the linear elements are
subdued ridges that have a mean height of 48 m (Figure 6c). As
shown in selected profiles (Figure 7), their shape is very similar to
wrinkle ridges elsewhere on Mars [e.g., Watters, 1993]. Comparison of this mean height (48 m) to measurements of ridges in the
detrended topography in the Eastern Solis Planum and Lunae
Planum region (Figure 6d) (mean 65 m) shows that the ridges
are typically 1.4 times higher outside the North Polar Basin.
[22] These comparisons show that there is a tendency for the
ridges we measured in the northern lowlands to be more widely
spaced and somewhat lower than wrinkle ridges in Hesperian-aged
ridged plains in Eastern Solis Planum and Lunae Planum. The
differences are not dramatic (mean separation distances 83 km
versus 50 km, or 1.7 times; mean heights 48 m versus 65 m, or 1.4
times), as shown in Figures 6a – 6d.
(continued)
[23] The characteristics of these linear features that we have
mapped in the northern lowlands in terms of their spacing, shapes,
and patterns in map view, heights, and cross-sectional profiles,
together with their general continuity with the circumferential
trends around Tharsis (compare Figures 1e, 3, and 4), support
the interpretation that these features are wrinkle ridges. This
interpretation is further supported by previous mapping that shows
that wrinkle ridges in the circum-Tharsis area outside the northern
lowlands lie predominantly in the Hesperian-aged ridged plains
(Hr) and that this unit underlies younger stratigraphic units and
deposits of the northern lowlands (primarily the Vastitas Borealis
Formation) [e.g., Scott and Tanaka, 1986; Tanaka and Scott, 1987;
Greeley and Guest, 1987; Plescia, 1993; Rotto and Tanaka, 1995].
[24] If these features are so similar to wrinkle ridges, why were
they not detected before? Ridges on Hesperian ridged plains often
consist of a wide arch and a narrow sharp ridge [e.g., Watters,
1991]. Ridges observed in the detrended data in the northern
lowlands are usually just arches and do not typically have sharp
narrow ridges on top. Elsewhere, the sharp narrow ridges are
commonly much more readily observed in images owing to their
steeper slopes. In the northern lowlands the atmosphere is usually
less transparent owing to hazes and the polar hood. Because of this,
when the Sun is low, scattered light obscures surface topography,
and when the Sun is high, the gently sloping topography of arches
is not seen and the steeper ridges are subdued.
[25] The trend toward broader spacing and more subdued
heights of the ridges in the northern lowlands (Figure 6), together
with the sedimentary and volcanic nature of many of the subsequent units, suggests that blanketing, burial, and subdual are also
among the explanations. Examination of individual areas and
profiles (Figure 5) further documents some of the candidate
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
30
a
North Polar Basin
Ridge System
All structures (n=154)
Max: 318.3
Min: 11.1
Mean: 83.1
Median: 70.3
Frequency
25
20
15
10
5
0
0
40
80
120
160
Spacing (Km)
200
240
25
b
Lunae Planum /
Eastern Solis Planum
Ridge Systems
All Structures (n=65)
Max: 103
Min: 18.5
Mean: 49.9
Median: 48.1
Frequency
20
15
10
5
0
0
40
80
120
160
200
240
Spacing (Km)
30
c
North Polar Basin
Ridge System
Frequency
25
20
All Structures (n=162)
Max: 340
Min: 0
Mean: 48.2
Median: 40.0
15
10
5
0
0
50
100
150
200
Height (m)
12
Frequency
Lunae Planum /
Eastern Solis Planum
Ridge Systems
All Structures (n=69)
Max: 202
Min: 5
Mean: 65.2
Median: 47
d
10
8
6
4
2
0
0
50
100
150
200
Height (m)
Figure 6. Histograms of spacing and heights of linear features
and wrinkle ridges in the North Polar Basin and, for comparison,
Lunae Planum and Syria Planum. The mean spacing of ridges in
(a) the North Polar Basin is greater than the mean spacing of
wrinkle ridges in (b) Lunae Planum/Eastern Solis Planum. The
mean height of ridges in (c) the North Polar Basin is less than the
mean height of wrinkle ridges in (d) Lunae Planum/Eastern Solis
Planum. See Figure 1f for the location of individual areas and
Figures 4a – 4f for the location of individual profiles.
3 - 11
explanations. In Amazonis Planitia (Figure 5a), ridge spacing
typical of the west central region increases, and heights decrease
to the east, where the ridges are partly, and then wholly, buried by
Amazonian-aged plains of the Arcadia Formation. To the west in
this profile, a similar trend is seen as the ridges are buried by
Amazonian-aged flows from Elysium Mons [see also Plescia,
1993]. In Arcadia Planitia (Figure 5b; lower left part of image)
and Vastitas Borealis (180°) (Figure 5c; left part of image), Late
Hesperian flows from Alba Patera embay and subdue the ridges. In
the Vastitas Borealis (180°) and (0°) areas (Figures 5c and 5d),
polar and circumpolar deposits subdue and cover the ridges. In the
Chryse Planitia region (Figures 5d and 5f), superposed Late
Hesperian channels and channel deposits have eroded and subdued
the ridges.
[26] Amazonian modification of the surface is also seen in
abundance in Mars Orbiter Camera (MOC) images. We examined
numerous MOC images covering key wrinkle ridges in Lunae
Planum and seen in the detrended altimetric data. The narrow
width of the MOC images means that most high-resolution strips
cover only a part of the ridge, and virtually no evidence of the
wrinkle ridges can commonly be seen in images of the northern
lowlands. Instead, one observes a variety of very lightly cratered
deposits and textures that suggest the recent reworking of a
significant part of the surface [Kreslavsky and Head, 2000].
Clearly, this serves as an important factor in obscuring the wrinkle
ridges in the images.
[27] The orientation of these features can also be determined
from angular measurements made along these profiles (Figure 8).
This plot also emphasizes the parallelism of the linear trends seen
in the northern lowlands and shows that they curve in an arcuate
pattern around Tharsis, essentially completing the arc to the north,
with a center of curvature located at about the highest part of the
Tharsis rise (8°S, 95°W). This trends stands out distinctly
(Figure 1c) from others in the Utopia Basin, in Isidis Planitia,
and in the area north of Deuteronilus Mensae (near Lyot crater),
which we discuss further below.
[28] On the basis of these data and comparisons, we conclude
that the North Polar Basin contains extensive linear features that
are most plausibly interpreted as wrinkle ridges and that the
arcuate trend documented here provides a major missing section
of the previously mapped circumferential trend around Tharsis
[e.g., Chicarro et al., 1985; Watters, 1993] (Figures 3 and 8). The
exposed circum-Tharsis wrinkle ridges previously mapped [e.g.,
Chicarro et al., 1985; Watters, 1993] are predominantly on the
ridged plains unit (Hr) of Early Hesperian age. The generally
obscured and more subdued nature of these features in the
northern lowlands appears to be attributable to modification by
later Hesperian (Vastitas Borealis Formation; Alba Patera Formation; channel deposits) and Amazonian-aged sedimentary and
volcanic units. Comparison of the ridges located in these two
areas illustrates this and suggests that sedimentary and volcanic
modification of a surface that was originally similar to the
Hesperian-aged ridged plains (Hr) could produce the observed
differences in ridge spacing and height (Figure 9). For example,
in the central part of the Figure 5a image, prominent wrinkle
ridges of Hesperian ridged plains (Hr) are exposed and strike
normal to the profile and are seen as distinctive peaks and
troughs in the profile. To the left of this area, the wrinkle ridges
are progressively buried by younger volcanic and fluvial deposits
in Amazonis Planitia (in this area, note the exposed wrinkle
ridges at the top of the image and the buried remnants along the
profile and below). This type of configuration is similar to the
modification scenario illustrated in Figure 9 (middle) in which
volcanic flooding dominates. In comparison, examination of
Figures 5c and 5d shows that the ridges deeper in the North
Polar Basin are more subdued than those seen in the Hesperian
ridged plains in the middle of Figure 5a but are not surrounded or
completely buried as suggested by the relationships in Amazonis
3 - 12
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
Eastern Solis Planum
Vertical Ex. =370
4.0
250 m
3.5
0
3.0
0
200
400 km
2.5
1.0
Elevation (km)
Lunae Planum
0.5
250 m
0.0
0
-0.5
-1.0
-4.0
Vastitas Borealis (180°)
-4.5
250 m
0
-5.0
-5.5
-6.0
Figure 7. Altimetric profiles illustrating the cross-sectional shapes of circum-Tharsis wrinkle ridges in (top) Eastern
Solis Planum and (middle) Lunae Planum compared to those in (bottom) the northern lowlands (Vastitas Borealis
(180°); Figure 5c).
Planitia seen in the left part of Figure 5a. The type of configuration seen deeper in the North Polar Basin (Figure 5c and 5) is
similar to the modification scenario illustrated in Figure 9
(bottom), in which sedimentary deposition dominates and ridges
become subdued and partly obscured. On the basis of these
examples we interpret much of the difference in ridge height
and spacing characteristics (Figures 5 and 6) to be due to
postformation volcanic and sedimentary modification processes,
as illustrated in Figure 9.
[29] Montési and Zuber [2001] have suggested that differences
in ridge spacing between the ridged plains and the northern
lowlands reported in this study are primary and reflect a local-
ization instability due to different crustal and lithospheric structure between the two areas. We believe that the relatively small
differences in mean heights and spacings between the two
populations (Figure 6) are reasonably interpreted in terms of
the subdual of a northern lowland population of wrinkle ridges by
subsequent sedimentary and volcanic deposition (e.g., Figure 9).
A more comprehensive analysis of the morphometry of northern
lowlands ridges is necessary to determine if there is a statistically
significant difference in primary ridge spacing, as required by the
interpretation of Montési and Zuber [2001].
2.1.2. Isidis Basin trends. [30] Isidis is the site of a large
impact basin [Schultz and Frey, 1990]. The Isidis Basin interior is
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
3 - 13
Figure 8. Orientations of linear features in the northern lowlands compared to those in the circum-Tharsis ridged
plains outside the northern lowlands (see also Figure 3): (a) the detrended topographic map in a Lambert azimuthal
equidistant projection centered on the highest point of the Tharsis rise (8°S, 95°W) and (b) sketch map of the major
trends revealed in this study. The map shows three major areas: the Tharsis volcanic province, the northern lowlands,
and the southern cratered uplands. All other enclosed areas not labeled otherwise are occurrences of Hesperian ridged
plains unit (Hr) as mapped by previous workers [Scott and Tanaka, 1986; Tanaka and Scott, 1987; Greeley and Guest,
1987]. Note that the North Polar Basin trends mapped in this analysis complete the circum-Tharsis trend of wrinkle
ridges missing in previous analyses [e.g., Chicarro et al., 1985; Watters, 1993] (see Figure 3) and form a broad
annulus of wrinkle ridges circumferential to Tharsis and spanning a radius of 7000 km. The concentric circles,
starting from the middle of the map, represent radii of 1763, 3527, 5290, and 7054 km.
characterized by Hesperian-aged Vastitas Borealis Formation (Hvr,
ridged member), Amazonian smooth plains (Aps), and Hesperian
ridged plains (Hr) (along its northeastern margin) [Greeley and
Guest, 1987]. Wrinkle ridges were not generally mapped in the
interior of the Isidis Basin in previous regional and global studies
[e.g., Chicarro et al., 1985; Watters, 1993]. The detrended
topographic map of the Isidis Basin, however, reveals a
remarkable configuration of ridges (Figure 10). In the outer
portions of the interior of the basin, a series of irregular radial
spoke-like features dominates the trend. In the central interior part
of the basin (the inner 300 – 400 km), the ridges display a more
random orientation. Superposed on these trends is a general WNWESE ridge trend extending through the middle of the basin. Finally,
it is clear that circular ridge configurations are more common near
the margins of the basin interior, with diameter distributions similar
to impact craters in the adjacent cratered terrain. This last
observation is interpreted to mean that wrinkle ridges are
influenced by the rim crest topography of buried craters, a
phenomenon that is more common in shallowly buried regions
such as basin margins [e.g., Allemand and Thomas, 1995; DeHon
and Waskom, 1976].
[31] We thus interpret the Isidis Basin ridges detected in the
detrended topography to be a system of wrinkle ridges, showing
many of the same characteristics of wrinkle ridges as seen in the
North Polar Basin. In Isidis, however, the pattern is largely
controlled by the circular configuration of the basin, in contrast
to the more linear nature of the North Polar Basin. The pattern in
Isidis is similar to those seen in wrinkle ridges in the lunar maria
(see maps of Solomon and Head [1980]). The lunar wrinkle ridges
are attributed to lithospheric loading by mare basalt accumulation
within the basins, with superposition of a global thermal stress that
shifted to compressional as the Moon changed from net expansion
to net contraction in its early history [Solomon and Head, 1980;
Watters, 1993, Figure 6].
[32] Why were these wrinkle ridges not detected in earlier
analyses [e.g., Chicarro et al., 1985; Watters, 1993]? Apparently,
the veneer of deposits represented by Hvr and Aps was sufficient to
obscure both the morphologic and topographic expression of these
features in Viking Orbiter images. Specifically, Grizzaffi and
Schultz [1989] mapped hillocky and small-scale ridged terrain
throughout the interior of Isidis which they interpreted as a
transient air fall deposit of ice and dust which obscured the
underlying units and subsequently underwent sublimation and
partial removal [see also Tanaka et al., 2000]. Such an overlying
residual blanket could readily modify the morphology of wrinkle
ridges in much the same way as it appears to have done in the
North Polar Basin. Contiguous outcrops of the Hesperian-aged
ridged plains in the northeastern part of the basin [Greeley and
Guest, 1987] strongly suggest that this unit underlies the subsequent units (e.g., Hvr and Aps) and is the unit on which the wrinkle
ridges detected in the basin interior are formed.
2.1.3. Utopia Basin circumferential and radial trends.
[33] The Utopia Basin was previously predicted to be the site of a
large and degraded impact basin on the basis of geological
structure [McGill, 1989], a hypothesis confirmed by MOLA data
[Frey et al., 1999; Thomson and Head, 2001]. The interior of the
Utopia Basin is characterized by the Hesperian Vastitas Borealis
Formation (all four members) overlain by Amazonian deposits
associated with Elysium (lava flows and other lobate and smooth
deposits) [Greeley and Guest, 1987]. Along the southern margin of
3 - 14
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
Figure 9. Interpretive diagram showing the ridge spacing and height in recently emplaced and deformed plains and
an impact crater superposed on these plains (1, top) compared to modifications that would occur through
emplacement of a subsequent deposits, such as lava flooding (2, middle) or sedimentation (3, bottom). The result of
the emplacement of the volcanic flow unit (2, middle) is embayment of the taller ridges and burial of the shorter
ridges, causing the lowering of average ridge height and an increase in ridge spacing due to burial of the lower ridges.
The rim of the impact crater is breached, the crater interior is filled, and the rim crest is covered. The result of
sedimentation (3, bottom) is the broad mantling of the underlying ridged topography and the subdual of sharp shortwavelength wrinkles on top of the ridges. Some preferential filling of the lows between ridges would be expected, and
impact craters (closed depressions) would serve as catchment basins for sediments. The effects of blanketing
sedimentation would be to decrease ridge heights and to tend toward increased ridge spacing, depending on the level
of subdual of shorter ridges. Compare to the relationships in Figure 6.
Figure 10. (a) Detrended topographic map of Isidis Planitia. The map is centered at 13°N, 272°W; north is at the
top. Map projection is simple cylindrical. The filter core radius used is 50 km. (b) Sketch map showing the location
and distribution of linear features.
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
3 - 15
Figure 11. (a) Detrended topographic map of the Utopia Basin. The map is centered at 44°N, 252°W; north is at the
top. Map projection is stereographic centered at the center of the map. The filter core radius used is 200 km. (b)
Sketch map showing the location and distribution of linear features.
the Utopia Basin a band of Hesperian ridged plains (Hr) about
2000 km long and 500 km wide is exposed [Greeley and Guest,
1987]. Although a few structural elements were mapped within
Utopia, the general distribution of structures and wrinkle ridges
was minor to nonexistent [e.g., Chicarro et al., 1985; Watters,
1993].
[34] The detrended topographic map of the Utopia Basin
(Figure 11), however, reveals abundant ridges and a distinctive
configuration apparently related to processes similar to those seen
in Isidis and the North Polar Basin. To the north of the basin is
seen the general linear trend of the North Polar Basin wrinkle
ridges (compare top of Figure 11 and Figures 1c and 1e). As one
approaches the margins of the basin, the ridges become more
circumferential, and this broad pattern gives way to a series of
generally radially oriented ridges with some zones of more
randomly oriented ridges in the southwestern part of the basin
Figure 12. (a) Detrended topographic map of the North Pole region. The map is centered at the pole; map projection
is stereographic. The filter core radius used is 200 km. (b) Sketch map showing the location and distribution of linear
features.
3 - 16
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
Figure 13. (a) Detrended topographic map of Northern Arabia Terra and Deuteronilus Mensae. The map is centered
at 47°N, 339°W; north is at the top. Map projection is stereographic centered at the center of the map. The filter core
radius used is 200 km. (b) Sketch map showing the location and distribution of linear features.
adjacent to the heavily cratered terrain. In one such area of
the basin (west central) a buried crater forms a circular structure
with ridges extending away like spokes, suggesting that the ridgeforming activity was partly controlled by the presence of impact
craters. The Hesperian-aged deposits of the Vastitas Borealis
Formation contain the most distinctive and abundant ridges.
The Amazonian-aged deposits of the Elysium Formation contain
few ridges, and the unit appears to embay those units (primarily
Hv) with ridges [Tanaka et al., 1992b; Thomson and Head, 2001].
The morphology, exposure patterns, and distribution of the
majority of the ridge structures are consistent with those of
wrinkle ridges, as previously described in other areas of the
northern lowlands. The ridges forming the radial system appear
wider, less sinuous, and more asymmetric in profile in comparison
to many wrinkle ridges. Some concentric ridges may be related to
aqueous processes or to modification of tectonic features by water
and ice processes [e.g., Head et al., 1999; Thomson and Head,
2001]. The lack of detection of the vast majority of these
structures in previous studies can also be attributed to the
modifying and subduing effect of the Vastitas Borealis Formation
and the virtually complete burial by the Amazonian-aged deposits
of the Elysium Formation [e.g., Thomson and Head, 2001]. The
exposure of the pre-Vastitas Borealis Hesperian-aged ridged plains
(Hr) at the southern edge of the basin (and the wrinkle ridges seen
there) [Greeley and Guest, 1987] strongly suggests that the unit
on which the wrinkle ridges are formed is the Hesperian ridged
plains (Hr), covered and obscured by later units (particularly the
Vastitas Borealis Formation).
2.1.4. Polar and circumpolar deposits. [35] Polar deposits
of Amazonian age (permanent ice, Api, and layered terrain, Apl),
as well as Amazonian circumpolar mantling deposits, are
superposed on the Vastitas Borealis Formation in the region of
the North Pole [Dial, 1984; Tanaka and Scott, 1987; Fishbaugh
and Head, 2000]. Structural elements associated with the Vastitas
Borealis Formation (other than the features defining the various
facies) were not observed prominently or in abundance in
regional maps [Dial, 1984; Tanaka and Scott, 1987]. The
MOLA detrended topography (Figure 12; see also Figures 1,
4, and 5c) shows, however, that abundant linear ridges can be
seen in the Vastitas Borealis Formation in the vicinity of the
North Pole. Ridges interpreted to be wrinkle ridges in earlier
sections cross the North Polar Basin in the vicinity of these
deposits and are further covered and obscured by the circumpolar
deposits and completely buried where they underlie the polar
deposits. This suggests that by the time of the emplacement of
these Late Amazonian-aged deposits, wrinkle ridging had ceased,
supporting the interpretation that the majority of wrinkle ridging
took place relatively soon after the emplacement of the
Hesperian plains (Hr).
2.2. Northern Arabia Terra-Deuteronilus Mensae
[36] The highland-lowland boundary in northern Arabia Terra
(Figure 1a) is one of the most complex along the dichotomy
boundary. Noachian-aged plains of the plateau sequence have been
resurfaced by Noachian (Nplr) and Hesperian ridged plains (Hr),
and these in turn have been cut by several stages of Hesperian
channel development to produce the distinctive texture of the
fretted terrain [Greeley and Guest, 1987; McGill, 2000]. Fretted
terrain erosion occurred most likely in the Early Hesperian and
involved liquid water [McGill, 2000]. These deposits are embayed
and overlain by the Late Hesperian Vastitas Borealis Formation
(Hvr and Hvk) in Acidalia Planitia. A significant part of the region
has been modified by Amazonian-aged mass wasting, smooth
plains emplacement, and formation of debris flows on the channel
floors [McGill, 2000], as well as by ejecta from the crater Lyot. The
orientation of wrinkle ridges in the upland Noachian and Hesperian
ridged plains is NNW to NW; no wrinkle ridges are mapped in the
Vastitas Borealis Formation in the adjacent lowlands [Chicarro et
al., 1985; Greeley and Guest, 1987; Watters, 1993].
[37] Examination of the detrended topography in this area
(Figure 13) reveals a remarkable transition across the dichotomy
boundary. To a first order, the crater Lyot and its ejecta dominate
part of the region, but the distinctive texture of the fretted terrain
can be traced several hundred kilometers into the lowland areas
mapped as the Vastitas Borealis Formation. The individual bumps
of Hvk, the knobby member of the Vastitas Borealis Formation,
dominate the upper left-hand part of Figure 13, but near this can be
seen broad linear ridges extending in a NW direction, ridges that
extend toward the North Pole and converge with the circumTharsis trend in the North Polar Basin (see Figure 1). These trends
are parallel to the linear trends represented by the wrinkle ridges of
Nplr and Hr in northern Arabia Terra [Greeley and Guest, 1987]
and were apparently continuous across the complex transition zone
now represented by the fretted terrain.
[38] Thus, on the basis of these new data (Figure 13), we
conclude that (1) the complex transition zone of the fretted terrain
extends several hundred kilometers out into the lowlands beneath
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
3 - 17
Figure 14. (a) Detrended topographic map of Chryse Planitia and Acidalia Planitia. The map is centered at 32°N,
36°W; north is at the top. Map projection is sterographic centered at the center of the map. The filter core radius used
is 200 km. (b) Sketch map showing the location and distribution of linear features.
the Vastitas Borealis Formation, (2) wrinkle ridges predating the
formation of the fretted terrain extend from the uplands across the
transition zone and into the basement of the northern lowlands, and
(3) on the basis of the stratigraphy of this region [Greeley and
Guest, 1987; McGill, 2000] and the relationships revealed by these
new data, it is likely that Hesperian ridged plains (Hr) exposed in
the adjacent uplands extend well into the northern lowlands in this
region below the Vastitas Borealis Formation (Figures 1 and 13).
2.3. Hesperian Outflow Channels of Chryse Planitia
[39] A series of major Hesperian-aged outflow channels extends
from the cratered uplands and across Hesperian-aged ridged plains
(Hr) into the northern lowlands in Chryse and Acidalia Planitia
[Scott and Tanaka, 1986; Rotto and Tanaka, 1995] (Figure 1a). The
northern lowlands in this area is characterized predominantly by
the Vastitas Borealis Formation (all four members), a kipuka of
Noachian dissected plains, and several Amazonian units. Wrinkle
ridges of the Hesperian ridged plains (Hr) trend in the broad
circum-Tharsis mode (e.g., Lunae Planum, parts of Xanthe Terra)
or more NNW (northern Terra Meridiani). Only a very few wrinkle
ridges have been mapped into the northern lowlands [Scott and
Tanaka, 1986; Chicarro et al., 1985; Watters, 1993].
[40] The detrended topography in this area (Figure 14) reveals
an abundance of linear ridges that are continuous with the wrinkle
ridge trends of the adjacent uplands and shows how the traces of
the channels have continued into the northern lowlands, beyond the
major change in channel morphology [e.g., Ivanov and Head,
2001]. The detrended topography data also show that virtually all
of the channels were influenced in their flow patterns by the
presence of wrinkle ridges, confirming previous interpretations
[Scott and Tanaka, 1986; Rotto and Tanaka, 1995] that the
channel-forming events postdated the emplacement of Hr and the
formation of the wrinkle ridges on them [e.g., Head and Kreslavsky, 2000]. On the basis of these new data, we conclude that Chryse
Planitia and Acidalia Planitia were floored by extensive deposits of
Hesperian-aged ridged plains continuous with those observed in
the adjacent uplands and overlain by outflow channel deposits and
the Vastitas Borealis Formation.
2.4. Amazonis Planitia
[41] This region, located west of Olympus Mons and east of
Elysium (Figure 1), is one of the smoothest areas in the northern
lowlands, smoother even than the Vastitas Borealis Formation
[Aharonson et al., 1998; Kreslavsky and Head, 1999, 2000]. The
region is characterized by members of the Arcadia Formation, an
Amazonian-aged unit with visible flow fronts in Viking images,
and interpreted to be volcanic in origin [Scott and Tanaka, 1986].
Recent MOC images of the Elysium Planitia to the west have
shown evidence of extensive smooth plains of volcanic origin [e.g.,
Keszthelyi et al., 2000] of recent Amazonian age (within the last
few tens of millions of years) [Hartmann and Berman, 2000].
Additional detrended topography data from MOLA provide evidence that the deposits of Amazonis Planitia are laterally continuous with those of Elysium Planitia [Head and Kreslavsky, 2001;
Head et al., 2001a]. Underlying these Late Amazonian-aged
deposits in the northern lowlands are various members of the
Vastitas Borealis Formation and Hesperian-aged ridged plains (Hr)
east of Phlegra Montes and the Elysium Rise [Scott and Tanaka,
1986]. The wrinkle ridges of Hr are associated with peaks or crater
rims, suggesting relatively shallow burial of a cratered surface by
volcanic plains, and subsequent deformation [e.g., Plescia, 1993].
The trends of these features form part of the circum-Tharsis system
mapped in the northern lowlands (Figures 1e, 5, and 8). The
detrended topographic data reveal the presence of ridges in
Amazonis Planitia (Figure 5a, center left; compare to Figures 5c
and 5d), but these ridges are more irregular, more widely spaced,
and lower than others in the North Polar Basin. We interpret these
data to indicate that Hesperian-aged ridged volcanic plains (Hr)
underlie both the Vastitas Borealis Formation and Amazonian-aged
volcanic plains units (Aa) in this area and that the superposition of
3 - 18
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
Median differential slope, deg
0.30
0.20
0.10
0.05
Hr H
Hs
0.03
Hr S
Hr L
0.02
Hvk
0.01
1
10
100
Baseline length, km
Figure 15. Vastitas Borealis Formation roughness characteristics
and comparison to those of Hesperian-aged ridged plains. Shown is
the dependence of the median absolute value of differential slope
on the baseline length for the Vastitas Borealis Formation and
several ridged plains units. The median absolute value of
differential slope for a given baseline length characterizes the
surface roughness at the spatial scale of the baseline length (see
Kreslavsky and Head [2000] for details). Hvk, the knobby member
of the Vastitas Borealis Formation. Ridged plains: Hs, the Syrtis
Major Formation; Hr L, Lunae Planum; Hr S, the eastern part of
Sinai Planum; Hr H, Hesperia Planum. Unit boundaries are taken
from 1:15,000,000 USGS geological maps of Mars [Scott and
Tanaka, 1986; Tanaka and Scott, 1987; Greeley and Guest, 1987].
Addition of a layer of 100 m thickness can convert the roughness
observed in Hesperian ridged plains (Hr) type units to that
observed in the Vastitas Borealis Formation (Hv).
the Amazonian-aged volcanic plains on the Vastitas Borealis
Formation has further increased ridge spacing and reduced ridge
height (e.g., Figure 9) [Head et al., 2001a].
2.5. Vastitas Borealis Formation
[42] About 45% of the northern lowlands are presently occupied
by the Late Hesperian-aged Vastitas Borealis Formation [Tanaka
and Scott, 1987], which in turn is covered by the Early Amazonian-aged materials of the Elysium Formation in the Utopia Basin,
by the Arcadia Formation in Arcadia and Amazonis, and by Late
Amazonian-aged polar and circum-polar deposits [Scott and
Tanaka, 1986]. Superposition relationships described above clearly
show that the northern lowlands wrinkle-ridge system predates the
Elysium Formation (Figure 11), the Arcadia Formation (Figure 5),
and the polar and circumpolar deposits (Figure 12). The topography of the wrinkle ridges is observed in the Vastitas Borealis
Formation, although in this unit ridge spacing tends to be greater
and topographic expression more subdued than wrinkle ridges
exposed elsewhere (Figures 6 and 17). A variety of data show that
the surface of the northern lowlands is unusually smooth at all
scale lengths compared to terrains exposed elsewhere on Mars
[Smith et al., 1998; Kreslavsky and Head, 1999, 2000; Aharonson
et al., 1998; Head et al., 1999].
[43] Study of kilometer-scale roughness [Kreslavsky and Head,
2000] has shown that the Vastitas Borealis Formation is characterized by a distinctive dependence of roughness on scale. The
dependence of the median differential slope used as a measure of
roughness by Kreslavsky and Head [2000] on baseline length has a
pronounced maximum at 3 km baseline (Figure 15). Virtually no
other terrains on Mars possess such a kilometer-scale roughness
signature. This characteristic 3-km-scale surface roughness is
represented mostly by gently sloping knobs. This knobby pattern
is mostly undersampled in the gridded topography maps. A specific
texture of the Vastitas Borealis Formation, however, is already seen
in 32 pixel per degree detrended topography images (see upper left
part of Figure 13). Owing to this specific texture, the boundaries of
the Vastitas Borealis Formation are usually clearly seen in the
detrended topography maps. The roughness map [Kreslavsky and
Head, 2000] allows one to outline the boundaries even more
reliably but at lower resolution. At many locations the topography
and roughness maps enable one to outline the Vastitas Borealis
Formation more reliably than the images. In some areas there are
discrepancies between the boundaries seen in the roughness and
topography maps and the boundaries mapped by Scott and Tanaka
[1986] and Greeley and Guest [1987]. It is interesting that the
1400-km-long boundary between the Vastitas Borealis Formation
and the Alba Patera Formation (Figure 4) is expressed as a
distinctive topographic step 10 – 30 m high. This step clearly shows
that the Vastitas Borealis Formation material is superposed on top of
the Alba Patera Formation and that its thickness, at least near the
edge, is several tens of meters [Kreslavsky and Head, 2001a].
[44] Geological mapping discussed above shows that Hesperian
ridged plains (Hr) are abundant in the uplands surrounding the
northern lowlands [Greeley and Guest, 1987; Tanaka and Scott,
1987; Scott and Tanaka, 1986]. In addition, the stratigraphic
columns in these maps place the Hesperian ridged plains (Hr) as
Early Hesperian in age and the Vastitas Borealis Formation as Late
Hesperian in age, essentially contemporaneous with the formation
of outflow channels and their deposits. Stratigraphic evidence
derived from the new detrended data and cited above strongly
suggests that Hesperian ridged plains (Hr) underlie units presently
exposed in the northern lowlands in Chryse Planitia, Amazonis
Planitia, Acidalia Planitia, and Utopia Planitia. This raises the
question of whether the unusual roughness characteristics of the
Vastitas Borealis Formation [Kreslavsky and Head, 1999, 2000]
could be related to a combination of features associated with this
unit and an underlying unit (Hr).
[45] To address this question, we examined the median differential slope of Hr exposed outside the northern lowlands
(Figure 15) and compared it to that of the Vastitas Borealis
Formation. Hesperian ridged plains are generally rougher than
the Vastitas Borealis Formation, but the Vastitas Borealis Formation is very similar to Hr at intermediate scale lengths (2 – 5 km)
and has lower differential slopes at shorter (<1 km) and longer (>8
km) scale lengths. How thick would a layer of overlaying material
have to be to provide enough smoothing to make Hr appear like
Hv? To address this question, we used Hesperia Planum as a
typical example of Hr.
[46] Smoothing at a larger spatial scale requires addition of a
thicker layer. In addition, the difference between roughness of
ridged plains and the Vastitas Borealis Formation increases with
increase of baseline length (Figure 15). Hence the result of
simulations crucially depends on the largest spatial scale that can
be considered as a scale of roughness. We found that for an 80-km
baseline the median differential slope is almost the same for any
chosen MOLA profile across Hesperia Planum (as well as for
shorter baselines), while for a baseline twice as long the median
differential slope differs from profile to profile by more than a
factor of 2. We conclude that for large patches of ridged plains,
slopes at 80-km baseline still reflect the intrinsic surface roughness,
while slopes at larger baselines are defined by regional topography.
[47] Finding the minimal amount of material necessary to reach a
given smoothness is a difficult mathematical problem, and so we
addressed this question using simulations. We chose several MOLA
profiles across Hesperia Planum to characterize the ridged plains, Hr.
The total number of points in the segments chosen exceeded 10,000,
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
3 - 19
Figure 16. Crater characteristics and crater age estimates. (a) Cumulative diameter-frequency distribution for stealth
craters (thin line) and superposed (real) craters (thick line) within the Vastitas Borealis Formation outline. (b)
Diameter frequency of stealth and superposed (real) craters for a conventional set of diameter bins. (c) Depth-diameter
relationships of stealth craters (crosses) and superposed (real) craters (triangles). (d) Detrended topographic map (100km filter core radius) showing numerous examples of stealth (dark) and real (brighter) craters. Stealth craters shown
by wide arrows may influence the ridge pattern. A real crater shown by the narrow arrow might be thought to
influence faint ridges to the west and to the south, but its ejecta definitely overlies and obscures the NW-SE trending
ridges. The scene is centered at 65°N, 223°W, and north is at the top. (e) Areal distribution of stealth craters (open
circles) and superposed (real) craters (solid circles).
which provided a robust roughness estimation. Then we modified
the profiles by ‘‘adding some material’’ in a way that provided an
‘‘economical’’ smoothing of the profiles. We tried several heuristic
algorithms for smoothing by ‘‘adding some material.’’ The most
effective algorithm among those tried successively replaces randomly chosen concave segments of profiles with straight segments.
We feel that the amount of added material in this algorithm is rather
close to the rigorous minimum. Our simulations showed that
application of a layer with a mean thickness of 90 m can reduce
the roughness at 80-km baseline from the level of Hr in Hesperia
Planum to the level of the Vastitas Borealis Formation. The algorithm applied produces a very smooth surface at short scales, and an
additional 10-m-thick layer is necessary to provide the characteristic
3-km-scale topography (Figure 15). The resulting average thickness
of 100 m provides a lower boundary for the mean thickness of the
Vastitas Borealis Formation material. Obviously, the thickness can
be somewhat higher, because the emplacement process probably
does not work like an effective smoothing algorithm.
[48] Thus we conclude that on the basis of stratigraphic relations and surface roughness analyses, the Vastitas Borealis For-
3 - 20
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
Figure 16. (continued)
mation could plausibly represent a thin veneer of 100-m minimum thickness superposed on Hesperian ridged plains (Hr) in the
northern lowlands.
[49] What might be the nature of such a veneer? Among the
contemporaneous units in the northern lowland region are the Late
Hesperian channel deposits (Hc), representing the formation of the
outflow channels and their debouching into the northern lowlands.
Although the detailed nature of these events is controversial (e.g.,
see review by Carr [1996]), all agree that their formation involved
emplacement of sediment into the northern lowlands. Carr et al.
[1987] estimated that around the Chryse basin (the area of the most
significant channel input into the northern lowlands), the volume of
sediment eroded and emplaced into the lowlands was about 4 106 km3. If the Vastitas Borealis Formation represents a sedimentary unit emplaced over the northern lowlands with a minimum
average thickness of 100 m, then its minimum total volume
would be about 3 106 km3, an amount less than, but close to, this
value. On the basis of these several considerations, we thus adopt
as a working hypothesis the concept that the Vastitas Borealis
Formation may represent at least part of a sedimentary layer
emplaced during outflow channel formation [see also Tanaka
and MacKinnon, 1999]. In the following sections we assess this
working hypothesis.
2.6. Impact Craters
[50] Abundant impact craters with a large variety of morphologic characteristics are observed on Mars (see review by Strom et
al. [1992]). The density and size-frequency distribution can be
used to date the surfaces of geologic units and place them in the
context of the global stratigraphy of Mars [e.g., Tanaka et al.,
1992a]. The morphologic characteristics of craters can be used to
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
assess their mode of formation, influence of the substrate, and
aspects of the cratering process (e.g., see review by Strom et al.
[1992]). Analysis of the detrended topographic data clearly shows
the distribution and morphology of the relatively fresh craters
mapped in the northern lowlands [e.g., Greeley and Guest, 1987;
Scott and Tanaka, 1986; Tanaka and Scott, 1987; Barlow, 1988;
Garvin et al., 2000a, 2000b; J. B. Garvin et al., Geometric
characteristics of Martian impact craters from the Mars Orbiter
Laser Altimeter (MOLA), submitted to Journal of Geophysical
Research, 2001] (Figures 1d, 5, 10 – 14, and 16). Details of the
structure of large craters, especially their fluidized ejecta patterns,
are clearly seen in the maps. Also revealed in the detrended data,
however, is a large number of circular features with a range of
diameters and a very subdued morphology (Figure 16d). We
interpret these as hidden, or ‘‘stealth,’’ circular depressions, most
likely to be subdued and buried impact craters (Figure 16). In this
section we analyze the detrended topographic data to assess the
morphology and number of impact craters and stealth features and
their relationship to the different units observed in the area.
2.6.1. Stealth craters. [ 51 ] There are many smooth,
shallow, flat-floored circular depressions in the northern lowlands
(Figure 16). Their perfect circular form and apparently random
scattering around the surface indicate their impact origin. We called
such objects stealth craters. Some of them can be recognized in
Viking low-resolution mosaics, while many of them are too smooth
to be visible in the images, as is the case with most of the wrinkle
ridges. In the detrended topography maps the stealth craters clearly
differ from fresh craters: all fresh craters have distinctive rims and
ejecta patterns. Stealth craters appear only within the boundaries of
the Vastitas Borealis Formation (Figure 16e). We found no stealth
craters in the Hesperian ridged plains of Lunae Planum, suggesting
that such craters did not form during the emplacement of Hr.
Rather, these craters were probably formed before or during the
Vastitas Borealis Formation emplacement and then heavily
modified and obscured by the Vastitas Borealis Formation material.
[52] We systematically studied the stealth crater population
and compared it to the population of real craters within the
Vastitas Borealis Formation. For the purposes of this study we
took the boundaries of the Vastitas Borealis Formation from the
geological maps [Scott and Tanaka, 1986; Tanaka and Scott,
1987; Greeley and Guest, 1987]. For several segments of the
outer boundary, where the edge of the characteristic roughness
signature in the roughness map [Kreslavsky and Head, 2000]
clearly deflected from the mapped boundary, we adjusted the
boundary. We included the circumpolar mantling (unit Am of
Tanaka and Scott [1987]) and large craters mapped as separate
units into the Vastitas Borealis Formation outline. The total area
within this outline is 16.9 106 km2.
[53] We identified and mapped all stealth craters in the northern
lowlands using the detrended topography maps (Figure 16e)
[Kreslavsky and Head, 2001a]. We calculated the diameter of each
stealth crater as the diameter of a circle having the same area as the
mapped depression. The largest stealth crater is 169 km in diameter
(this object is located on the western slope of Utopia basin and was
mentioned in section 2.1.3); the second largest stealth crater is 75
km. The smallest identified stealth craters are 6 km. Identification
of stealth craters smaller than 10 – 15 km is not completely reliable
owing to the limited resolution of the topographic data. There are
several clusters of irregularly shaped smooth depressions in the
region under study. Large stealth craters can be easily distinguished
from such depressions owing to their perfect circular shape.
However, the shape of small depressions cannot be reliably
assessed because of the limited actual resolution of gridded topography data. The same reason causes the diameter estimation of
the small craters to be inaccurate. For quantitative characterization
of the stealth craters population we used only craters larger than 16
km. There are 140 such stealth craters, and their size-frequency
distribution is shown in Figures 16a and 16b. The mean density of
3 - 21
the stealth craters within the Vastitas Borealis Formation boundaries is 8.3 per 106 km2.
[54] Straightforward use of these data for age estimation is not
possible, because our diameter measure is different from that used
to establish the age scale on Mars [Tanaka, 1986]. Rim crest-to-rim
crest diameter is traditionally used as a size measure in all crater
population studies, but this cannot be simply applied to stealth
craters because they do not have distinctive rims. Strictly speaking,
we do not know the details of the crater modification process;
hence we cannot unambiguously tie our measured diameters to any
morphometric characteristics of pristine craters. We proceed by
making the reasonable assumption that the size of a stealth crater is
equal to the size of inner depression of a pristine crater. We
identified all real craters within the same outline using the
detrended topography maps and measured their sizes in the same
manner as we did for the stealth craters. Of course, we included
central peaks and peak rings in the inner depression. The total number
of real craters larger than 16 km is 80. Their density is 4.7 per 106 km2.
[55] We compared the lists of craters obtained in this study with
the crater catalog kindly provided to us by N. Barlow and
described by Barlow [1988]. All objects identified as real craters
in our study except one were present in the catalog. Crater sizes
measured through the area of the inner depression turned out to be
5 – 30% smaller than the rim-to-rim diameters listed in the catalog.
Among 140 stealth craters, only 14 were present in the catalog; 13
of them are situated near the outer boundaries of the Vastitas
Borealis Formation, mostly in western Elysium Planitia and southeastern Utopia Planitia.
[56] The size-frequency distribution of the stealth craters turned
out to be remarkably similar to that of real craters (Figures 16a and
16b). It is probable that we see almost all craters formed on the
substrate before or during the emplacement of the Vastitas Borealis
Formation. If some craters were totally erased by the Vastitas
Borealis Formation emplacement, we would expect that smaller
craters would be more likely to be erased than large craters; hence
we would find a noticable relative paucity of small stealth craters.
Thus we can use the total number of stealth and real craters (220,
which gives a density of 13.0 per 106 km2) to estimate the age of
the substrate, as discussed below.
[57] We measured the depth of the craters as the difference
between the elevation of the surroundings and the elevation of
the crater floor. The elevation of the crater floor was calculated
as the median elevation within a circle around the crater center
with the diameter twice smaller than the crater diameter. We
checked that for the distinctive, nonstealth craters, the elevation
calculated in this way usually reflects the elevation of the floor
rather then the elevation of the central peak or peak ring. The
elevation of the surroundings was calculated as the median
elevation within a ring between 4 and 4.25 crater diameters
from the center. For the real craters, such a ring is usually
outside the ejecta. The measured depth is plotted against crater
size in Figure 16c. Several pedestal craters have negative depths:
their floor is higher than their surroundings. The depth of real
craters is widely scattered, though large craters are generally
deeper. The depth of the stealth craters is scattered around 100
m, and no dependence of the depth on crater size is observed. Only
two stealth craters are deeper than 200 m, and one of them is the
largest stealth crater. The very shallow nature of the stealth craters
is consistent with their extensive erosion and infill after their
formation on an underlying unit (most likely Hr). The depth
distribution of the fresher craters suggests that they have not
undergone the same level of erosion and infilling. The fresher
craters are superposed on the Late Hesperian Vastitas Borealis
Formation. The contrast in crater depths between these two
populations suggests that the stealth craters underwent modification and shallowing during Late Hesperian, most likely coincident
with the emplacement of the Vastitas Borealis Formation. Among
the candidate processes of modification of these unusual craters are
3 - 22
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
(Figure 9) (1) erosion and deposition associated with the emplacement of the outflow channels (which might degrade the rims and
infill the crater interiors), (2) impact into a standing body of water
[e.g., Lindström et al., 1996] (which might involve catastrophic
backwash, tending to cause erosion of the rims and infilling of the
interior), (3) glacial planing [e.g., Kargel et al., 1995] (which might
remove the rim and infill the crater), (4) enhanced sedimentation in
standing bodies of water (extreme sedimentation rates of 50 cm/yr
are known in terrestrial submarine craters [e.g., Thatje et al., 1999]),
and (6) eolian redistribution of sediment (which has not affected the
subsequent craters in the same manner).
2.6.2. Estimation of ages of surfaces. [58] Crater densities
for the chronostratigraphic series on Mars were defined by Tanaka
[1986] for craters larger than 1, 2, 5, and 16 km in diameter.
Densities for craters larger than 16 km were established for the
Noachian only, because for younger terrains the density is too
small to be useful in most cases. For the whole region of the
Vastitas Borealis Formation, however, the total number of craters
>16 km in diameter is large enough for statistical interpretation.
[59] For smaller craters the Hesperian spans a factor of 3 in
crater density [Tanaka, 1986]. We assume that the density of
craters >16 km changes in approximately the same way from the
lower to the upper boundary of the Hesperian. The density of real
craters within the Vastitas Borealis Formation is 2.75 times lower
than the density of real + stealth craters. Thus, if we postulate the
Vastitas Borealis Formation to be Upper Hesperian and close to the
Amazonian boundary, the substrate will be Lower Hesperian and
close to the Noachian boundary.
[60] To use the value of crater density itself rather than the
density ratio, we need to compensate for the bias due to the
difference in the crater size definition. N. Barlow’s catalog contains
167 craters larger than 16 km for the rim-to-rim diameter within the
unit outline. Among them, 26 craters do not have noticeable rims
or ejecta patterns in the detrended topography, including 14 stealth
craters larger than 16 km, according to our survey described above.
We interpret that the rest (167
26 = 141) of the craters in the
catalog postdate the Vastitas Borealis Formation. This number is
1.76 times greater than our number of real craters. This difference
is due to the difference in crater size definition, and the factor 1.76
can be used to compare our crater densities with the commonly
used system. Applying this factor to our density of real + stealth
craters, we obtain 23 per 106 km2, just a little less than the
Hesperian/Noachian boundary defined by Tanaka [1986] at 25
per 106 km2. This result is in agreement with that which we
obtained above using the density ratio. A population of larger
subdued quasi-circular depressions described by Frey et al. [2001]
and interpreted to represent an underlying Noachain-aged surface
is discussed in more detail later.
2.6.3. Interaction of ridges and craters. [61] Allemand
and Thomas [1995] studied crater-ridge relationships for the
Lower Hesperian ridged plains in the Coprates region. They
found a number of examples of ridge-crater intersections, where
(1) ridges postdate craters, (2) ridges predate craters, and (3)
ambiguous crosscutting relationships occur. An analogous study
for the northern lowlands is made difficult by the subdued
character of all features and the relatively low map resolution;
most of crater-ridge intersections would be classified as
ambiguous. However, one of the intersection types where ridges
postdate craters, namely, ‘‘ridges stopped or modified by crater
intersection,’’ is clearly identifiable in the detrended topography
maps. There are several examples where the ridge pattern definitely
‘‘knows’’ about the existence of a stealth crater. The best example
is the previously mentioned spoke-like ridges near the largest
stealth crater in Utopia (section 2.1.3; Figure 11). Several other
stealth craters serve as terminations of the ridges. There are a few
sites where ridges split or cross each other at stealth craters. These
observations show that ridge formation occurred (or continued)
some time after the deposit of the substrate material. The
observations do not provide good constraints on this time;
however, it is clear that it could be a measurable part of the
Hesperian. There are no convincing examples of ridges postdating
real craters. We found two sites (61°N, 272°W and 60°N, 224°W)
where ridges might be thought to be influenced by real craters, but
these examples are ambiguous. There are numerous examples
where ridges are covered and obscured by crater ejecta. This
suggests that wrinkle ridge formation had ceased by some time
in the Late Hesperian, although the exact duration and time of
cessation is unknown.
3. Discussion and Synthesis
3.1. Northern Lowlands Wrinkle-Ridge System
[62] Detrended topographic data provide substantial evidence
for the presence of a comprehensive system of structural features
throughout the northern lowlands (Figure 1c) that was essentially
undetected in previous data sets (Figure 3) [Chicarro et al., 1985;
Watters, 1993]. Evidence supporting the interpretation of these
features as wrinkle ridges includes their patterns in map view
(Figures 1c and 5), cross-sectional topography (Figure 7), spacing
(Figure 6), and continuity with wrinkle ridges in the Tharsis region
(Figure 8) and elsewhere (Figures 10 – 14).
3.2. Subdivisions of the Northern Lowlands Wrinkle-Ridge
System
[63] Trends of wrinkle ridges in the northern lowlands (Figures
1c and 1e) can be subdivided into (1) Isidis Basin, a generally
equidimensional system (Figure 10), (2) Utopia Basin, a system
exhibiting both concentric and radial ridges (Figure 11), (3)
circum-Tharsis, a system that stretches for 7000 km in an arcuate
pattern around the Tharsis rise (Figures 4 and 8), and (4) Northern
Arabia Terra, a system that extends NW from Arabia, crosses the
fretted terrain into the northern lowlands, and merges with that of
the North Polar Basin (Figure 13). The Isidis and Utopia systems
are similar to patterns observed in lunar impact basins due to
loading of the lithosphere by mare basalt impact basin filling and
flexural deformation [e.g., Solomon and Head, 1979, 1980]. The
circum-Tharsis system (Figure 8) is more comparable to the
extensive linear and arcuate patterns observed in Oceanus Procellarum on the Moon [Whitford-Stark and Head, 1980] and completes the broad pattern of wrinkle ridges mapped in the circumTharsis region [Chicarro et al., 1985; Watters, 1993] forming a
pattern with a radius of 7000 km. This pattern is also broadly
similar to the circum-Aphrodite wrinkle-ridge pattern on Venus
[e.g., Bilotti and Suppe, 1999].
3.3. Nature of the Substrate Deformed by Wrinkle Ridges
[64] Several lines of evidence suggest that the Vastitas Borealis
Formation represents a veneer on a preexisting unit deformed by
wrinkle ridges. We interpret this unit to be Hesperian ridged plains
(Hr) on the basis of the following points.
3.3.1. Stratigraphy. [65] Examination of the distribution of
circum-Tharsis units shows that unit Hr (wrinkle-ridged plains of
Early Hesperian age) forms part of the annulus around Tharsis.
Where this unit abuts the northern lowlands (e.g., Chryse Planitia),
the wrinkle ridge patterns and orientations in Hr appear continuous
with those observed in the immediately adjacent overlying Vastitas
Borealis Formation, suggesting that Hr underlies the Vastitas
Borealis Formation (Figures 8, 11, 13, and 14).
3.3.2. Surface roughness. [66] The characterization of
surface roughness and slopes by Kreslavsky and Head [1999,
2000] shows that the northern lowlands and the Vastitas Borealis
Formation are unusually smooth at all scale lengths. Characterization of Hr (Hesperian ridged plains) indicates that these
units are also smooth, but rougher at several scale lengths than the
Vastitas Borealis Formation. We have shown above that addition of
3 - 23
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
GEOLOGICAL UNITS AND MAJOR EVENTS
SYSTEM
UPPER HESPERIAN
LOWER HESPERIAN
Hr
Npl
PLATEAU
SEQUENCE
RIDGED
PLAINS
Hch
Hal
?
?
?
PERIOD OF
WRINKLE RIDGE FORMATION
Ael
Hv
ALBA PATERA FORMATION
ELYSIUM
FORMATION
CHANNEL SYSTEM
MATERIALS
HESPERIAN
Ad
Am
CIRCUMPOLAR DUNE AND
MANTLING DEPOSITS
Aa3
Aa1
NOACHIAN
Ap NORTH POLAR
DEPOSITS
VASTITAS BOREALIS
FORMATION
AMAZONIAN
ARCADIA FORMATION
Figure 17. Correlation of map units and geologic events. The stratigraphy and geological history of the northern
lowlands of Mars is modified from 1:15,000,000 USGS geological maps of Mars [Scott and Tanaka, 1986; Tanaka
and Scott, 1987; Greeley and Guest, 1987]. Noachian unit Npl is the plateau sequence and includes several subunits
(Npl1 (cratered unit), Npl2 (subdued cratered unit), Npld (dissected unit), and Nplr (ridged unit)). Hesperian unit Hch
is channel system materials and consists of two subunits (Hch (channel material) and Hchp (channel floodplain
material)). The Arcadia Formation consists of several members, two of which are shown here. Aps, smooth plains of
Amazonian age, and HNu, an undivided unit forming hills and mesas several to more than 10 km across, are not
shown. The interpreted period of formation of wrinkle ridges is shown by the vertical line; the solid line indicates
more certainty, and the dashed line and question marks indicate less certainty.
100 m of material to units with typical Hr roughness would be
enough to produce a roughness scale length plot comparable to that
of the Vastitas Borealis Formation (e.g., Figure 15).
3.3.3. Models for wrinkle-ridge formation. [67] Wrinkle
ridges are thought by most workers to be contractional tectonic
features. There is, however, no commonly accepted mechanical
model for their formation. A comprehensive review of such models
has been recently presented by [Schultz 2000, section 4]. Most
models for the formation of wrinkle ridges [e.g., Zuber and Aist,
1990; Watters, 1988, 1991, 1993; Zuber, 1995; Mouginis-Mark et
al., 1992; Mangold et al., 1998; Montési et al., 2000] show that
wrinkle-ridge formation is favored in a deforming medium in which
a strong layer overlies a weaker layer (e.g., lava/megaregolith) and
that they are inconsistent with a deforming medium where a weak
layer overlies another weak layer (e.g., sediment/megaregolith).
Thus the occurrence of wrinkle ridges in the northern lowlands is
interpreted to be more consistent with their formation in an Hr-like
volcanic plains unit than in a wholly weak layer (e.g., sediment,
megaregolith).
3.3.4. Wrinkle ridge height and spacing. [68] CircumTharsis wrinkle ridges in the northern lowlands tend to be more
widely spaced and more vertically subdued than in previously
mapped [Chicarro et al., 1985; Watters, 1993] circum-Tharsis
exposures of Hr (e.g., Lunae Planum, Syria Planum) (Figures 6
and 7). One explanation for this is that wrinkle ridges in the
northern lowlands have been subdued and partly buried by
subsequent emplacement of the Vastitas Borealis Formation, thus
tending to increase ridge spacing and decrease ridge height (e.g.,
Figure 9). Wider spacing could also be due to a localization
instability related to different crustal and lithospheric structure
between the two areas [Montési and Zuber, 2001], but a more
comprehensive analysis of the ridge morphometry is necessary to
determine if there is a statistically significant difference in primary
ridge spacing, as required by this interpretation.
3.4. Age of Northern Lowlands Substrate Deformed by
Wrinkle Ridges
[69] Superposed impact craters and stratigraphic relationships
indicate a Late Hesperian age for the Vastitas Borealis Formation
[Scott and Tanaka, 1986]. Our analysis of the detrended topography data, however, shows that there are a number of previously undetected stealth craters in the northern lowlands. We found
140 such subdued craters >16 km in diameter, and these craters
appear to form part of the wrinkle-ridged surface that we interpret
to predate the Vastitas Borealis Formation. Using the detrended
altimetry data, we also counted craters whose morphology indicated that they were relatively fresher than the stealth craters. The
combined population of stealth and relatively fresher craters
provides numbers that are consistent with an Early Hesperian
age for the substrate, an age essentially identical to the Early
Hesperian ridged plains (Hr) (Figures 16a, 16b, and 17). We thus
conclude that these data support the interpretation that the vast
areas of the northern lowlands deformed by wrinkle ridges repre-
3 - 24
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
sent regions underlain by, and in several cases laterally equivalent
to, Hr, the wrinkle-ridged plains of Early Hesperian age.
3.5. Stratigraphy and History of the Northern Lowlands
[70] On the basis of these observations and interpretations, we
review the stratigraphy and history of the northern lowlands as
previously mapped and as reconstructed from these new results
(Figure 17). Previous mapping [Scott and Tanaka, 1986; Tanaka
and Scott, 1987; Greeley and Guest, 1987] has shown that the
northern lowlands are surrounded to the south (south of the
dichotomy boundary) by three major units in order of decreasing
abundance (measured in length of boundary), Noachian cratered
terrain (Npl1), Hesperian ridged plains (Hr), and Noachian ridged
plains (Nplr).
[71] According to this mapping, the oldest unit presently
exposed within the northern lowlands is Npld, a dissected unit
similar to the heavily cratered unit in the southern uplands but
more highly modified by small channels and troughs. The only
occurrence of this is in Acidalia Planitia, where a 300-km-diameter
outcrop occurs which is annular in planform and stands 300 – 800
m above the surrounding plains. The second oldest unit is HNu,
which is an undivided unit that spans the Noachian and Early
Hesperian and forms highstanding hills and irregular mesas that
have diameters of several kilometers to more than 10 km. Exposures of this unit form Scandia Colles and the degraded rims of
large craters in the vicinity [Tanaka and Scott, 1987]. These are
interpreted by Tanaka and Scott [1987] as remnants of ancient
material heavily eroded by mass wasting that are projecting
through younger units. In these previous maps, there are no surface
exposures of the units so abundant in the adjacent uplands in the
vicinity of the boundary with the northern lowlands (e.g., Noachian
cratered terrain (Npl1), Noachian ridged plains (Nplr), or Hesperian
ridged plains (Hr) [Scott and Tanaka, 1986; Tanaka and Scott,
1987; Greeley and Guest, 1987]).
[72] The next younger unit is Hal, the lower member of the
Alba Patera Formation, which spans much of the Hesperian. This
unit occurs on the northern flanks of Alba Patera and consists of
degraded lobate scarps, fractures, and graben; these are interpreted
to be structures and flows from Alba Patera [Tanaka and Scott,
1987]. The upper part of Hal is laterally equivalent to Hchp
(channel floodplain materials) and Hv (the Vastitas Borealis Formation). Our data show that in part, Hv postdates Hal [Kreslavsky
and Head, 2001a]. A unit interpreted to be channel floodplain
materials from fluvial channels south of the area (Hchp) occurs
along the western edge of Acidalia Planitia and Chryse Planitia and
is continuous with Hch (channel materials) in Chryse Planitia.
These channel deposits are shown by Tanaka and Scott [1987] and
Scott and Tanaka [1986] to be laterally equivalent to the Vastitas
Borealis Formation (Hv), which is Late Hesperian in age. Hv forms
vast subpolar plains deposits and consists of several members
(Hvm, mottled; Hvg, grooved; Hvr, ridged; and Hvk, knobby
[Scott and Tanaka, 1986]). This unit and its subunits are thought
[Scott and Tanaka, 1986; Tanaka and Scott, 1987; Greeley and
Guest, 1987] to be formed by several (multiple?) processes,
including eolian, fluvial, periglacial, alluvial, volcanic, tectonic,
and desiccation.
[73] Overlying these units (Figure 17) are several units of
Amazonian age, including the Arcadia Formation (Aa, smooth
plains of lava flow and sedimentary origin), the Elysium Formation
(Ael, a variety of units of Early Amazonian age which are of
volcanic origin and associated with the Elysium rise and related
deposits in the Utopia Basin), smooth plains (Aps), which may be of
eolian origin, and Late Amazonian polar and circumpolar deposits.
[74] Among the most notable characteristics of this previously
mapped stratigraphic sequence [Scott and Tanaka, 1986; Tanaka
and Scott, 1987; Greeley and Guest, 1987] (Figure 17) are (1) the
extremely low abundance of Noachian-aged units in the northern
lowlands (These units make up 48% of the surface of Mars
outside the northern lowlands but <3% of the northern lowlands.),
(2) the lack of occurrence of previously mapped Hr (ridged plains)
in the northern lowlands, while outside the northern lowlands Hr is
a major unit making up 11% of the surface of Mars, (3) the
common abundance of these units in the southern uplands adjacent
to the northern lowlands, and (4) the unique occurrence of the
Vastitas Borealis Formation, occurring only in the northern lowlands and making up 45% of the area of this region.
[75] These observations raise several questions that we have
partly addressed in previous sections: Could the northern lowlands
have been the site of extensive flooding by Early Hesperian
volcanic plains which were subsequently deformed by wrinkle
ridges? Could such flooding explain the paucity of Noachian-aged
units in the northern lowlands? Could the Vastitas Borealis Formation be a veneer superposed on such Hesperian-aged ridged
plains?
[76] In order to analyze further these questions, we performed
experiments in which we used MOLA topography data to simulate
the emplacement of regional plains (Hr) in several typical areas of
the southern uplands. We took a series of randomly chosen profiles
across Noachis Terra. This region is dominated by Npl1, the
widespread cratered unit of Early to Middle Noachian age [Scott
and Tanaka, 1986; Greeley and Guest, 1987], which makes up
16% of the area outside the northern lowlands. We then simulated application of a layer of material in the same manner as
outlined in section 2.5, where we assessed the application of a
deposit on the ridged plains and compared it to the characteristics
of the Vastitas Borealis Formation. We found that the minimal
mean thickness of the layer necessary to reduce the 80-km-scale
roughness from the level of Noachis Terra to the level typical for
the ridged plains is 350 m. This lower limit of Hr material
thickness is certainly far below the real thickness, because volcanic
resurfacing is not likely to operate in a manner similar to the
optimal smoothing algorithm, and also because the 100-km-scale
topographic pattern resulting from the simulation does not resemble the observed topography of northern plains.
[77] We then simulated the regional volcanic flooding of these
regions to determine how much lava would be required to bury
typical southern upland terrain to a depth that would completely
bury it or that would form outcrop patterns similar to those
presently seen in the northern lowlands. First, we examined an
area of 1 106 km2 in Noachis Terra, west of the Hellas basin
(Figure 18). This region is dominated by Npl1, the widespread
cratered unit of Early to Middle Noachian age [Scott and Tanaka,
1986; Greeley and Guest, 1987], which makes up 16% of the
area outside of the northern lowlands. We used the digital MOLA
topography as a basis (Figure 18) and flooded it by creating
equipotential surfaces on the existing topography in three 500-m
increments. This technique is known as a ‘‘porous’’ flooding model
[see Head, 1982] and is equivalent to having vents virtually
everywhere in the area. A separate approach is to have a smaller
number of vents in specific areas and to assess how much lava
might build up locally before it overtops local topography and
begins to flood adjacent lowlying areas. In the approach we used,
the 500-m increments are added from the top of the preexisting low
point (or equipotential surface). The sequence of flooding steps
shows that 41% of the area is resurfaced after 500 m of flooding
and 65% after 1000 m and that after 1500 m of flooding, over
90% of the area has been resurfaced. The topography of the Npl1
unit remaining exposed forms patterns dominated by arcuate crater
rim segments and a single plateau about 100 250 km in
dimension. These patterns are strikingly similar to those seen in
the northern lowlands (Figure 1) in association with outcrops of
Npld and HNu (see description above and Scott and Tanaka
[1986], Tanaka and Scott [1987], and Greeley and Guest
[1987]). Note that although the maximum thickness of volcanic
deposits would be 1500 m, the range is from 0 to 1500 m, and the
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
Figure 18. Lava flooding simulation of a portion of the southern uplands of Mars. Noachis Terra, west of the Hellas
Basin, composed predominantly of Npl1, the widespread cratered unit of Early to Middle Noachian age [Scott and
Tanaka, 1986; Greeley and Guest, 1987]. The first step (upper left) begins at 1000 m above mean planetary radius
(MPR). The 1000-m equipotential level is raised by 500 m in each succeeding step, progressively flooding the terrain
until only a few crater remnants remain. See color version of this figure at back of this issue.
3 - 25
3 - 26
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
average thickness of volcanic plains over the whole area under
consideration (1 106 km2) would be 920 m.
[78] Next, because unit Npld (the Early to Middle Noachianaged dissected unit which is similar to Npl1 but more dissected by
small valleys) makes up a significant percentage of the area south
of the northern lowlands (11%), we chose an area of this terrain
in northern Terra Tyrrhena, just west of Hesperia Planum (the type
area of Hr, the Hesperian-aged ridged plains), and performed the
same type of flooding experiment. Beginning with the initial step
in Figure 19, we flooded the terrain in 500-m increments. This
initial flooding reduces the underlying topography exposed in this
area to 64%, while an additional 500 m reduces the area exposed to
34%, and the last 500-m increment reduces the area exposed to 6%
(Figure 19). Again, note that although the maximum thickness of
volcanic deposits is 1500 m, the range is from 0 to 1500 m, and
the average thickness of volcanic plains in this case over the whole
area considered would be 820 m. The terrain remaining exposed
after this last step shows outcrop patterns of portions of crater rim
crests and a few linear to rectangular intercrater areas. The addition
of another 500 m of lava (not shown in Figure 19) reduces the
outcrops to <2% of the area and removes most of the small plateau
areas, leaving a few isolated arcuate crater rim crests as kipukas.
These patterns are again similar to those few outcrops of HNu and
Hpld observed in the northern lowlands (Figure 1).
[79] In summary, we flooded typical Noachian-aged cratered
and dissected terrain (units which together make up 27% of the
area outside of the northern lowlands) by a maximum of 1500 m
of volcanic material and an average of 820 – 1000 m. This
flooding produced an area with a small percentage of Noachianaged terrain surrounded by volcanic plains, with patterns of
Noachian unit outcrops similar to those seen in the northern
lowlands. These results suggest that if the characteristics of the
Noachian-aged local to regional topography of the northern lowlands were similar to those of typical areas of the southern uplands,
then Early Hesperian volcanic flooding with thickness of the order
of 800 – 1000 m of lavas could explain the present distribution of
units. In addition, a population of large subdued quasi-circular
depressions described by Frey et al. [2001] was interpreted by
them to represent an underlying Noachain-aged surface. The
emplacement of the Hesperian-aged ridged plains could be a major
factor in the burial and subdual of the original craters to produce
the now-subdued quasi-circular depressions described by Frey et
al. [2001], as discussed in more detail later. We previously raised
the following questions: Could the northern lowlands have been
the site of extensive flooding by Early Hesperian volcanic plains
which were subsequently deformed by wrinkle ridges? Could such
flooding explain the paucity of Noachian-aged units in the northern
lowlands? On the basis of these analyses, we conclude that these
questions can be plausibly answered in the affirmative.
[80] We now return to the question of whether the Vastitas
Borealis Formation could be a veneer superposed on such Hesperian-aged ridged plains. Previous workers have shown that the
Vastitas Borealis Formation is time-correlative with the Hesperianaged channel deposits (Figure 17) and suggested that it may have
several origins and represent the operation of several processes
(including eolian, fluvial, periglacial, alluvial, volcanic, tectonic,
and desiccation) [Scott and Tanaka, 1986; Tanaka and Scott, 1987;
Greeley and Guest, 1987]. Our data are consistent with the
emplacement of extensive deposits of Hesperian-aged ridged plains
(Hr) in the northern lowlands and its subsequent degradation by
smoothing processes to produce the presently observed smoothness
characteristics [Kreslavsky and Head, 1999, 2000] and wrinkle
ridge characteristics (tendency toward greater spacing and lower
heights in the northern lowlands; Figures 6, 9, and 15). The close
association in space and time of the Hesperian-aged outflow
channel deposits and the Vastitas Borealis Formation in the northern lowlands (Figure 17) supports the conclusion that the channels
were largely emplaced on top of the Hesperian-aged ridged plains
(a relationship seen along the margins of the uplands in places like
Lunae Planum [e.g., Scott and Tanaka, 1986]). Most workers agree
that the emplacement of the outflow channels involved a significant sediment load (see review by Carr [1996]), although the exact
percentage is a matter of controversy.
[81] We conclude that the emplacement of the outflow channels
provided sufficient water and sediment to the northern lowlands to
veneer significant parts of the basin (Figure 9, bottom) [Kreslavsky
and Head, 2001b]. Processes in the period subsequent to the Late
Hesperian (a period possibly as much as 3.3 – 3.5 billion years long
[Tanaka et al., 1992a; Hartmann and Neukum, 2001]) could have
redistributed some of this material by eolian and other processes. In
addition, other processes have clearly operated on deposits in the
northern lowlands to produce additional Amazonian-aged units,
including (1) mass wasting and lateral transport of material from
the margins of the dichotomy boundary into the northern lowlands,
(2) volcanic activity, (3) polar deposit emplacement and modification to produce the circumpolar deposits, and (4) periglacial
processes [e.g., Rossbacher and Judson, 1981; Squyres and Carr,
1986; Kreslavsky and Head, 2000].
3.6. Implications for the Evolution of Mars
[82] On the basis of the above discussions, we interpret the
wrinkle-ridge distribution documented in this paper to represent the
emplacement of a significant volume of Early Hesperian-aged
volcanic plains in the northern lowlands that subsequently were
deformed by wrinkle ridges. If this interpretation is correct, then
there are some important implications for the history of Mars and
the northern lowlands.
[83] It is likely that the northern lowlands were already topographically low by the Middle to Late Noachian [e.g., Wilhelms
and Squyres, 1984; McGill and Squyres, 1991; Zuber et al., 1998;
Smith et al., 2001], caused at least in part by the formation of the
Utopia Basin [McGill, 1989], but some evidence exists for slightly
later formation [e.g., McGill and Dimitriou, 1990]. The surface
characteristics of the northern lowlands outside the Utopia Basin
may have consisted of Noachian-aged cratered terrain and Noachian-aged plains [e.g., McGill and Dimitriou, 1990], as in the
southern uplands, or possibly Noachian-aged sedimentary or ocean
deposits [e.g., Clifford and Parker, 2001].
[84] Subsequent to the completion of our analysis, Frey et al.
[2001] described a population of circular arcs and structures in the
northern lowlands revealed in MOLA data which they interpreted
to be buried impact basins. They found 644 quasi-circular depressions in the northern lowlands whose diameters are >50 km and
concluded that ‘‘the smooth and sparsely cratered Hesperian and
younger lowland plains are likely only a thin veneer overlying a
much older crust.’’ They further concluded that the buried lowlands surface is at least as old as the exposed Noachian highland
surface [Frey et al., 2001]. The vast majority of stealth craters
mapped in our analysis are <50 km in diameter, and thus the
population mapped by Frey et al. [2001] is complementary in size
distribution to the populatiom of stealth craters that we mapped
here (Figure 16). On the basis of this comparison, we interpret the
population of quasi-circular depressions mapped by Frey et al.
[2001] to predominantly represent features superposed on the
Figure 19. (opposite) Lava flooding simulation of a portion of the southern uplands of Mars. Northern Terra Tyrrhena, west of Hesperia
Planum, composed predominantly of Npld, the extensive Early to Middle Noachian age dissected unit [Scott and Tanaka, 1986; Greeley and
Guest, 1987] which is similar to the widespread cratered unit (Npl1) but more dissected by small valleys. The first step (upper left) begins at
1500 m above mean planetary radius (MPR). The 1500-m equipotential level is raised by 500 m in each succeeding step, progressively
flooding the terrain until only a few crater remnants and small plateaus remain. See color version of this figure at back of this issue.
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
underlying Noachian-aged surface that was subsequently flooded
by Hesperian-aged ridged plains, as outlined in this study. The
population of craters represented by the stealth structures (Figure
16) was then superposed on the ridged plains surface during the
Early Hesperian and modified before or during the emplacement of
the Vastitas Borealis Formation. The population of relatively fresh
craters superposed on the Vastitas Borealis Formation (Figure 16)
represents those that accumulated on it subsequent to its emplacement in the Late Hesperian.
[85] Outside the northern lowlands there is an observed early
planetwide volcanic emplacement phase of Late Noachian-Early
Hesperian age. For example, Late Noachian ridged plains (Nplr)
comprise 3% of the presently exposed surface of Mars and
Hesperian-aged ridged plains (Hr) make up another 10%. The
thickness of the Hesperian ridged plains (Hr) has been estimated to
be generally 300 – 600 m in Lunae Planum [Frey et al., 1991] and
0.5 – 1.5 km in eastern Tharsis [DeHon, 1982], and recent observations of layers interpreted to be volcanic flows in the walls of
Valles Marineris have led to the suggestion that thicknesses could
be even greater [e.g. McEwen et al., 1999].
[86] Thus, if Hesperian-aged ridged plains underlie the northern
lowlands, then this would mean that the total percentage of the
planet resurfaced by Hesperian-aged volcanic plains would be the
10% presently exposed in the southern uplands and the 20%
making up the northern lowlands, for a total of 43.5 106 km2,
or 30% of the surface of Mars. Tanaka et al. [1988, 1992a]
estimated that Early Hesperian volcanic plains covered 19.3 106 km2 of Mars (13% of the surface area; includes both exposed
and buried deposits). These new estimates would more than double
the area covered by Hesperian ridged plains.
[87] If the southern upland plains are typically 500 m thick
and those in the northern lowlands are 900 m thick (on the basis
of the average thickness of the flooding models), then the total
volume of Hr would be of the order of 3.3 107 km3. These
estimates represent an increase in the importance of Hesperianaged volcanic plains from previous estimates [e.g., Tanaka et al.,
1992a; Greeley and Schneid, 1991] of more than twice the area and
almost twice the volume. Greeley [1987] estimated gas exsolution
from volcanic effusion and input of volatiles into the atmosphere as
a function of time during Mars history and showed that the peak
input was during the Early Hesperian. These new data suggest that
this peak volatile input into the atmosphere was significantly more
important than previously recognized.
[88] These results further emphasize the importance of events
occurring during the Early Hesperian. On the basis of a variety of
data, it appears that significant volcanic activity, perhaps the peak
global flux [Greeley and Schneid, 1991; Tanaka et al., 1992a],
occurred during this period. This was accompanied and followed
closely by regional and global contraction to produce the circumTharsis and other wrinkle-ridge systems [e.g., Chicarro et al.,
1985; Watters, 1993]. On the basis of our interpretations here, the
northern lowlands played a very important role in the Early
Hesperian volcanic activity and in the subsequent period of
regional and global-scale contraction. In addition, flooding of the
northern lowlands by extensive Hesperian-aged lavas could easily
obscure previous structures, rendering ancient impact basins or
tectonic structures more difficult to detect.
3.7. Implications for the Oceans Hypotheses
[89] Several researchers have proposed that the northern lowlands were once the site of an extensive standing body of water or
ocean [e.g., Parker et al., 1989, 1993; Baker et al., 1991; Scott et
al., 1992, 1995; Clifford and Parker, 2001]. Estimates of the ages
of the proposed standing bodies of water range from Noachian to
Amazonian. Tests of the hypotheses of Parker et al. [1989, 1993]
have found that several of the predictions of the hypothesis are
consistent with the new data but that others are not [Head et al.,
1999, 2001b]. The new data and interpretations presented here do
3 - 27
not directly address the presence or absence of large standing
bodies of water in the northern lowlands. They do, however,
provide additional possible insight into the nature and evolution
of the region and the origin of some of its unusual properties.
[90] For example, one of the most unusual characteristics of the
northern lowlands is its extreme smoothness at several scale
lengths, comparable to abyssal plains on Earth [e.g., Smith et al.,
1998; Aharonson et al., 1998]. The presence of Hesperian-aged
ridged volcanic plains mantled by the Vastitas Borealis Formation
(Figure 15) provides an alternate interpretation for the timing and
mode of formation of this smoothing. Instead of long-term sedimentation in a standing body of water, the general regional
smoothing could have taken place as part of the Early Hesperian
volcanic resurfacing, and the later fine-scale smoothing could have
occurred as a result of the emplacement of outflow channel
sediments and their subsequent modification.
[91] Although providing an alternative explanation for some of
the smoothing, the emplacement of these volcanic plains does not
preclude the presence of oceans. They could have existed prior to
this time (Noachian [e.g., Clifford and Parker, 2001]), during this
time (in which case the volcanic plains might have been emplaced
subaqueously), or following the emplacement of the ridged plains
as a result of outflow channel basin filling [e.g., Baker et al., 1991].
In addition, some of the channel-like mass deficits observed in
Mars Global Surveyor (MGS) gravity data in the northern lowlands
[e.g. Zuber et al., 2000, Figure 5] suggest that the history of the
northern lowlands prior to the emplacement of the Hesperian-aged
volcanic plains may have been quite different, with substantial
amounts of water and sediment being transported to the northern
lowlands before the cessation of resurfacing [Zuber et al., 2000].
The presence of water in the northern lowlands could be responsible for the removal of stealth crater rims before or during the
emplacement of the Vastitas Borealis Formation (Hv).
[92] Carr [1996] pointed out that substanital amounts of the
floodwater that was emplaced in the northern lowlands during the
Late Hesperain outflow channel events could have frozen and then
reached thermal and diffusive stability owing to a thin cover of
sediment or lavas. Clifford and Parker [1999, 2001] concluded that
the presence of such massive ice deposits beneath the northern
plains was consistent with a variety of observations, including their
smoothness and geomorphology of landforms. The results and
interpretations reported here do not favor the hypotheses of
massive ice deposits beneath the northern lowlands. First, if thick
frozen residues of the outflow channel floodwaters existed in the
northern lowlands, they should largely bury underlying topography. However, the detected presence of the underlying Early
Hesperian plains, stealth craters, and wrinkle ridges suggests that
any layer overlying these plains must be thin, less than a few
hundred meters in thickness. A more likely explanation is that the
overlying material (the Vastitas Borealis Formation) is composed
of the sedimentary residue of the sediment/water effluent of the
outflow channels [e.g., Kreslavsky and Head, 2001b]. In this case,
a significant part of the smoothness of the northern lowlands is due
to an Early Hesperian volcanic infilling of a Noachian-aged surface
(Figures 16, 17, and 18). Buried massive ice deposits below the
Hesperian ridged plains cannot be ruled out, but the large number
of quasi-circular depressions dating from the Noachian [Frey et al.,
2001] would seem to argue against the presence of such a deposit.
4. Conclusions
[93] Detrended MOLA altimetry data have provided a new
picture of the Martian northern lowland basin topography, morphology, evolution, and relation to the history of Mars. We
interpret our results to indicate that the northern lowlands are
underlain by a regional unit containing a basin-wide system of
subparallel wrinkle ridges and arches. This unit is laterally contiguous with Hesperian-aged ridged plains in the southern uplands
3 - 28
HEAD ET AL.: MARS NORTHERN LOWLAND EVOLUTION
and contains highly modified craters, the number of which suggests an Early Hesperian age. The orientation and location of the
wrinkle ridges in the North Polar Basin completes a global circumTharsis ridge system forming a band approximately 7000 km wide
and extending over the whole circum-Tharsis region. Several
subareas of the northern lowlands show individual patterns (e.g.,
basin-like areas of Isidis and Utopia). The present spacing and
height of wrinkle ridges and geometry of buried craters in the
northern lowlands suggest that the Late Hesperian Vastitas Borealis
Formation is a sedimentary unit superposed on Hr (regional
plains). Hesperian-aged channels entering Chryse Planitia are
controlled by the orientation and topography of wrinkle ridges
deep into the basin, indicating that wrinkle ridges had largely
formed by Late Hesperian. These channels are among the strongest
candidates for providing the material of the Vastitas Borealis
Formation. Amazonian-aged smooth plains units of volcanic
origin, particularly in Amazonis Planitia, further bury and obscure
the underlying wrinkle ridges and the Vastitas Borealis Formation.
[94] Recognition of these units and their stratigraphic relationships provides a new perspective on the history of the northern
lowlands (Figure 17). In this scenario, in the Early Hesperian the
majority of the northern lowlands was filled with volcanic plains
similar to those presently exposed in the southern uplands (Hr). As
with those deposits in the southern uplands, evidence for volcanic
vents was scant or subdued in topography. The Hesperian-aged
plains in the northern lowlands were pervasively deformed soon
thereafter by Tharsis-circumferential and basin-related wrinkle
ridges. Circum-Chryse outflow channels formed in the Late Hesperian following courses largely controlled by wrinkle ridge
orientation and height and deposited material in the basin to form
a major contribution to the Vastitas Borealis Formation.
[95] Widespread emplacement of the Hesperian-aged ridged
plains of apparent volcanic origin is interpreted to mean that the
volcanic phase represented by this unit was global in nature and
resurfaced the northern lowlands, in addition to the 10% of the
planet previously known, for a total resurfacing of about 30% of
Mars. This remarkable event increases by a factor of 2 the amount
of volatiles that might have been degassed into the atmosphere
during this time period of peak volcanic flux.
[96] Acknowledgments. We gratefully acknowledge the MOLA
instrument team and the MGS spacecraft and operations teams at the Jet
Propulsion Laboratory and Lockheed-Martin Astronautics for providing the
engineering foundation that enabled this analysis. We particularly thank
Greg Neumann for superior professional performance in data reduction and
preparation and presentation. We also thank the MOLA science team and
Co-PI’s David Smith and Maria Zuber for productive scientific discussions.
Detailed reviews by Michael Carr and an anonymous reviewer improved
the clarity of the manuscript and are greatly appreciated. Nadine Barlow
kindly provided a copy of her impact crater catalog. Anne Côté rendered the
final sketch maps, and Peter Neivert assisted in figure preparation. This
work was partially funded by a grant from the National Aeronautics and
Space Administration.
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. E1, 10.1029/2000JE001445, 2002
Figure 18. Lava flooding simulation of a portion of the southern uplands of Mars. Noachis Terra, west of the Hellas
Basin, composed predominantly of Npl1, the widespread cratered unit of Early to Middle Noachian age [Scott and
Tanaka, 1986; Greeley and Guest, 1987]. The first step (upper left) begins at 1000 m above mean planetary radius
(MPR). The 1000-m equipotential level is raised by 500 m in each succeeding step, progressively flooding the terrain
until only a few crater remnants remain.
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. E1, 10.1029/2000JE001445, 2002
Figure 19. (opposite) Lava flooding simulation of a portion of the southern uplands of Mars. Northern Terra Tyrrhena, west of Hesperia
Planum, composed predominantly of Npld, the extensive Early to Middle Noachian age dissected unit [Scott and Tanaka, 1986; Greeley and
Guest, 1987] which is similar to the widespread cratered unit (Npl1) but more dissected by small valleys. The first step (upper left) begins at
1500 m above mean planetary radius (MPR). The 1500-m equipotential level is raised by 500 m in each succeeding step, progressively
flooding the terrain until only a few crater remnants and small plateaus remain.
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