Click
Here
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, E08004, doi:10.1029/2006JE002852, 2007
for
Full
Article
Lineated valley fill and lobate debris apron stratigraphy in Nilosyrtis
Mensae, Mars: Evidence for phases of glacial modification of the
dichotomy boundary
Joseph S. Levy,1 James W. Head,1 and David R. Marchant2
Received 31 October 2006; revised 7 March 2007; accepted 17 May 2007; published 9 August 2007.
[1] The Nilosyrtis Mensae region is important among dichotomy boundary fretted terrain
outcrops, as it provides evidence of overprinting of ancient landscapes by a suite of glacial
features, providing a composite view of the variety of midlatitude glacial modification
processes that can occur during recent Martian ice ages. On the basis of a series of criteria
developed for the identification of a glacial origin for lineated valley fill and lobate
debris aprons, we interpret stratigraphic, topographic, and textural relationships between
lineated valley fill and lobate debris apron morphological units as evidence of local
and regional glacial overprinting of the landscape during the recent Amazonian. We
document (1) stratigraphic relationships between lineated valley fill subunits, including
the presence of apparently superposed and small-scale lobate features, (2) the regional
integration and flow of lineated valley fill material, (3) lineated valley fill degradation, and
(4) the nature and stratigraphic position of lobate debris aprons. These observations
suggest multiple phases or episodes of midlatitude valley glacier activity. These
observations, together with those of surface units elsewhere in northern Arabia Terra
interpreted as glacially modified landforms, suggest the possibility of midlatitude glacial
deposits extending over broad portions of the Martian dichotomy boundary within the past
several hundred million years.
Citation: Levy, J. S., J. W. Head, and D. R. Marchant (2007), Lineated valley fill and lobate debris apron stratigraphy in Nilosyrtis
Mensae, Mars: Evidence for phases of glacial modification of the dichotomy boundary, J. Geophys. Res., 112, E08004,
doi:10.1029/2006JE002852.
1. Introduction
1.1. Previous Analyses and Interpretations of Fretted
Valleys and Lineated Valley Fill
[2] In order to more completely understand the geological
history of the Martian dichotomy boundary, it is essential to
first understand the erosional and modification processes
which have acted on the boundary, producing the present
topography and features [Frey and Schultz, 1988, 1991;
McGill, 2000; Watters, 2003; Head et al., 2006b; Irwin et
al., 2004; Kiefer, 2005; Frey, 2006; Watters and McGovern,
2006]. Lineated valley fill (LVF) and lobate debris aprons
(LDA) are some of the most common modification features
present on the dichotomy boundary. LVF and LDA form in
association with fretted terrain and fretted channels in
northern Arabia Terra [Sharp, 1973; Squyres, 1978, 1979,
1989]. Geological mapping of the region indicates that LVF
and LDA represent Amazonian-aged modification of Hesperian-aged fretted terrain [Greeley and Guest, 1987;
McGill, 2000]. Recent analyses of LVF in northern Arabia
1
Department of Geological Sciences, Brown University, Providence,
Rhode Island, USA.
2
Department of Earth Sciences, Boston University, Boston, Massachusetts, USA.
Copyright 2007 by the American Geophysical Union.
0148-0227/07/2006JE002852$09.00
Terra [Head et al., 2006a, 2006b] show local sources for
LVF in valley-wall alcoves, down-valley flow, merging of
flow into trunk valleys, and terminations in lobe-shaped
deposits: features similar to terrestrial valley glacier systems
[Eyles, 1983; Benn et al., 2003]. Likewise, analyses of LDA
features indicate creep of rocky debris material with interstitial pore ice in a manner analogous with terrestrial rock
glaciers [Mangold, 2003; Pierce and Crown, 2003; Li et al.,
2005] and debris-covered glaciers [Mangold, 2003; Pierce
and Crown, 2003; Li et al., 2005]. Finally, hemisphere-wide
surveys of dissected mantle terrain provide evidence of
widespread glacial conditions during recent Amazonian
time [Head et al., 2003; Milliken et al., 2003]. Taken
together, these lines of evidence suggest a complex glacial
history for portions of the northern hemisphere, particularly along the dichotomy boundary, and a potential link
between the stratigraphy of these terrains (desiccated
mantle, LVF, LDA) and global glacial events during the
past millions to hundreds of millions of years.
[3] Questions about LDA/LVF which can be answered by
morphological and stratigraphical analysis include: (1) the
direction, extent, and continuity of flow in LDA/LVF [Carr,
2001]; (2) the sequence of emplacement and/or stratigraphic
position of LDA/LVF [McGill, 2000]; (3) the climate
significance of LDA/LVF [Madeleine et al., 2007]; and
(4) the mode of origin of the LDA and LVF (for example,
E08004
1 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
E08004
Figure 1. Shaded relief map of the Martian dichotomy boundary region of northern Arabia Terra over
MOLA gridded topography. The Nilosyrtis Mensae region (box) is at the far eastern and southern
extremity of this portion of the dichotomy boundary.
groundwater-fed [Sharp, 1973; Carr, 1995]), ice-assisted
rock creep [Squyres, 1978; Lucchitta, 1984], ice-rich landslides [Carr, 2001], rock glaciers [Mangold, 2003; Pierce
and Crown, 2003], or debris-covered glaciers [Li et al.,
2005; Head et al., 2006a, 2006b]. The unique physical
characteristics and geographical setting of the Nilosyrtis
Mensae region (Figures 1 –3) provide information on the
confluence of multiple sources of LVF, LVF/LDA stratigraphy, and stages of LVF/LDA evolution, and thus, evidence of regional, potentially climate-driven changes at the
dichotomy boundary.
[4] The Nilosyrtis Mensae region LVF is similar in
morphology to LVF described in Deuteronilus and Protonilus Mensae by Head et al. [2006a, 2006b]. Lineated
valley fill there has been interpreted as a suite of features
analogous to terrestrial debris-covered glaciers. We use the
term debris-covered glacier to mean a lobate mass of glacial
ice, overlain by a mantle of rock-fall-derived and aeolianderived till that is generated by sublimation of the uppermost portion of the buried ice. Terrestrial debris-covered
glaciers are distinguished from ice-cemented rock glaciers
in which ice is largely present as an interstitial phase
Figure 2. Context map of the Nilosyrtis Mensae region of the dichotomy boundary, composed of
MOLA gridded topography and the THEMIS global mosaic. The 500-m contour lines highlight major
topographic features.
2 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
E08004
Figure 3. Context map of the Nilosyrtis Mensae region showing approximate locations of image
centers.
between boulders, and in lenses, rather than as the dominant
phase of the glacier substrate [White, 1976]. Outwardly
some of the features mapped in Deuteronilus and Protonilus
Mensae, as well as some of those in Nilosyrtis Mensae,
share characteristics with typical terrestrial rock glaciers and
rock-glacier deposits. However, the integrated flow patterns
in each region are most consistent with the formation,
maturation, and degradation of debris-covered glaciers,
similar to those observed in alcoves and tributary valleys
in the Antarctic Dry Valleys. In this analysis we build on
previous studies designed to test a glacial origin for LVF
and LDA [e.g., Marchant and Head, 2007]. Our new
criteria expand the observations of terrestrial glacier systems
to the Martian environment, in which ice stability is
enhanced by the generation of a mantling debris cover
[Mangold, 2003; Head et al., 2005]. In the Nilosyrtis
Mensae region, we examine spatial and temporal relations
among the following features, each numbered below and
provided with specific interpretations that are most consistent with debris-covered glacier activity. The greater the
number of criteria observed, the stronger the argument for
glacier activity: (1) alcoves, theater-shaped indentations in
valley and massif walls (that may be inherited landscape
features, and which form local snow and ice accumulation
zones and sources of rock debris cover), (2) parallel arcuate
ridges facing outward from these alcoves and extending
down slope as lobe-like features (deformed flow ridges of
debris), (3) progressive tightening and folding of parallel
arcuate ridges where abutting adjacent lobes or topographic
obstacles (constrained debris-covered glacial flow), (4) progressive opening and broadening of arcuate ridges where
there are no topographic obstacles (unobstructed flow of
debris-covered ice), (5) circular to elongate pits in lobes
(differential sublimation of surface and near-surface ice),
(6) individual LVF tributary valleys converging into larger
LVF trunk valleys (local valley debris-covered glaciers
merging into larger intermontaine glacial systems), (7) complex folds in LVF where tributaries join trunk systems
(differential flow velocities causing folding), (8) integrated
LVF flow systems extending for tens of kilometers (intermontaine glacial systems), and (9) rounded valley wall
corners where flow converges downstream, and narrow
arete-like plateau remnants between LVF valleys (both
interpreted to be due to valley glacial streamlining).
1.2. Geological Setting
[5] We examined a portion of the dichotomy boundary in
the Nilosyrtis Mensae region of northwest Arabia Terra
(30.5°N – 36°N, 66.5°E– 73.5°E) in which surficial exposures of dissected mantle, lineated valley fill, and lobate
debris aprons are present (Figures 1 – 4 and Table 1). This
portion of the dichotomy boundary is characterized by
abrupt changes in topography, evident in the transition from
south to north from relatively smooth upland plains
(800 m elevation), through a region of extensive linked
valleys (5 –10 km long, 5 km wide, and up to 1.5 km
deep), to a region of isolated mesas (10 – 20 km wide and
1.5– 2 km high), and ultimately, to the northern plains
(2000 m elevation) [Greeley and Guest, 1987]. The
alignment of mesa edges and buttes, as well as the
straightness of many valleys in the study area (Figure 2),
are interpreted as indications of the presence of a number
of large, Noachian-aged graben along this portion of the
dichotomy boundary, consistent with lithospheric flexure
associated with loading of the northern lowlands [McGill,
2000; Watters, 2003; Kiefer, 2005; Watters and McGovern,
2006]. Some graben-initiated valleys may have been
subsequently pirated by fluvial systems during the late
Noachian and early Hesperian, while other valleys in the
3 of 19
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
E08004
E08004
Figure 4. Schematic diagram of LVF and LDA landforms observed in the Nilosyrtis Mensae region.
Scale bar is 1 km in all images. Gray shading indicates plateau surfaces. Bold lines indicate landform
contacts; thin lines indicate surface lineations. (a) Regional LVF (R-LVF): see Figure 5 in section 2.1.
(b) Superposed LVF (S-LVF): see Figures 17 and 18 in section 2.2. (c) Small-scale superposed LVF
(SSS-LVF): See Figures 20 and 21 in section 2.3. Light gray lines suggest steep lobe fronts. (d) Lobate
debris aprons (LDA): See Figures 22 and 23 in section 2.4.
region are purely fluvial in origin [Carr, 1995]. Where
LVF and LDA modification is not present, fretted terrain
shows evidence of fracture-controlled erosional processes
[Irwin et al., 2004]. Phyllosilicates have been detected in this
region, indicative of weathering of ultramafic Noachian
basement [Poulet et al., 2006].
2. Description and Stratigraphy of Nilosyrtis
Mensae Region Morphological Units
2.1. Regional Lineated Valley Fill
[6] High-resolution MOC, HRSC, and THEMIS-VIS
data, coupled with MOLA topography, provide geomorphic
clues that point toward locally integrated, alcove-derived
valley glaciation in the Nilosyrtis Mensae region. LVF is
present in many of the Nilosyrtis Mensae valleys and is
commonly lineated along the axis of the valley (Figure 5).
Some cross-valley lineations are present, commonly as part
of a lobate lineation structure [Levy and Head, 2006]
(Figures 5 and 6). Tracing lineations in MOC images leads
to the conclusion that most regional LVF is sourced in
valley-head alcoves at a variety of elevations (1000–
200 m), where it embays the lower valley wall slopes. The
valley-head alcoves and lower valley wall slopes provide a
ready source for LVF debris content. The LVF has a pit-andbutte surface texture (Figure 7), suggesting modification of
Table 1. Summary of Morphological Characteristics of Glacial Modification Features Present in the Nilosyrtis Mensae Study Area
Morphological
Characteristic
Regional LVF
Superposed LVF
Small-Scale
Superposed LVF
Lobate Debris
Aprons
Distribution
widely distributed
widely distributed
concentrated distribution
widely distributed
Size
several kilometers wide,
tens of kilometers long
1 km wide, <10 km long
<1 km wide, <5 km long
1- to 2-km radial width
Length-scale of
integration
ones to tens of kilometers
not integrated
not integrated
not integrated
Marginal steepness
low steepness
moderate steepness
very steep
moderate steepness
Surface patterning
axial and lobate
lineations, pitand-butte texture
axial lineations
terminating in
expanding lobate
lineations
axial lineations,
rough surface texture
concentric lineations,
4pit-and-butte texture
4 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
E08004
[7] Documenting LVF lineations in the Nilosyrtis Mensae
study area provides insight into the origin and flow characteristics of LVF units. LVF can be traced from source
alcoves to regional sinks in which the LVF texture is lost
(Figures 8 – 10). Although Nilosyrtis LVF is sourced in
spatially and topographically isolated alcoves (Figure 8,
lower middle), flow lineations visible in high-resolution
MOC images suggest confluence of LVF and deflection of
LVF around topographic obstacles (Figure 5), implying
integrated flow of an ice-rich medium [Head et al.,
2006b; Lucchitta, 1984]. LVF forms an integrated pattern
over kilometers to tens of kilometers. Lineations from
several sources merge and respond to topography together
(Figures 8 –10), and can be traced from alcoves, through
broad trunk valleys, to local topographic lows. Flow directions are inferred from the orientation of unambiguous lobe
structures visible in MOC-scale images, where available
(e.g., Figures 5 and 6); else gridded topographic data are
also used to determine downslope direction. For clarity in
comparison with other Nilosyrtis Mensae region LVF
deposits, these broadly integrated LVF deposits that line
the floors of trunk and tributary valley systems will be
referred to as ‘‘regional’’ LVF deposits.
[8] Changes in regional LVF texture as a function of
distance downslope provide evidence of ice-rich material in
the Nilosyrtis Mensae region, and its ablation during and
following the cessation of regional LVF flow. At local
topographic minima, regional LVF pit-and-butte surface
textures with pronounced lineations grade into rough, nonlineated, softened surface textures (Figures 11 and 12).
Regional LVF lobes maintain a convex-up profile approaching these roughly patterned topographic lows (Figure 11).
This morphology is interpreted to be analogous to the
profile of a retreating terrestrial glacier toe, rather than as
Figure 5. (a) Regional LVF showing indications of
integration, confluence of flow, and deflection around
obstacles. Lineations are commonly oriented along the axis
of the valley, though transverse, lobate features also occur.
LVF is commonly sourced in valley-head alcoves, where it
embays the lower reaches of the valley walls. Images are
THEMIS-VIS image V09834018 and MOC images
E0102224, M0303672, and M0401029. (b) Sketch map of
regional LVF features in Figure 5a. R-LVF denotes regional
lineated valley fill; P denotes plateaus between valleys; W
denotes valley walls. Fine lines indicate surface lineations.
the surface by sublimation subsequent to flow [Levy and
Head, 2006; Mangold, 2003]. At some locations, the LVF
abuts a smooth, topographically lower terrain (Figure 7,
left). MOLA altimetry point data across these contacts yield
a depth, and thus, an inferred thickness of the LVF unit of
50 m.
Figure 6. Close-up view of lobate lineations present in
regional LVF from Figure 5. Regional LVF commonly has a
pit-and-butte surface texture. Lineations are commonly
parallel to the valley axis, although valley-crossing or lobate
lineations also occur.
5 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
E08004
Figure 7. Pit-and-butte surface textures of regional LVF in the Nilosyrtis Mensae region. MOLA point
profiles across the contact between the pit-and-butte texture, and the smooth, unpatterned texture at image
left suggest the regional LVF is presently up to 50 m thick. Note the large, undeformed, inverted crater
in the regional LVF, suggesting that significant sublimation has occurred following the cessation of
regional LVF flow [e.g., Mangold, 2003]. Image is MOC E1300869.
evidence of embayment of the regional LVF lobe by dust or
debris. In the latter case, the topographic profile would be
expected to be more uniform in height across the textural
contact. Moraine-like bands and ridges are present in some
topographic minima (Figure 13), further suggesting the
morphological analogy between the regional LVF textural
transition and features associated with the retreat of terrestrial glaciers. In summary, we interpret the transition from
Figure 8. Map of regional LVF in the western portion of the study area, combining THEMIS global
mosaic and MOLA gridded data. LVF is integrated, as lineations from several sources merge and respond
to topography as a package. Lineations can be traced from alcoves to local topographic lows. Flow
directions are inferred from unambiguous lobe structures where present; otherwise, current gridded
topographic data are used. Ellipsis dots following arrows indicate textural transitions between strongly
lineated pit-and-butte morphology and a rough, patchy, degraded texture.
6 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
Figure 9. Map of regional LVF in the central portion of the study area, combining THEMIS global
mosaic and MOLA gridded data. LVF is integrated, as lineations from several sources merge and respond
to topography as a package. Lineations can be traced from alcoves to local topographic lows. Flow
directions are inferred from unambiguous lobe structures where present; otherwise, current gridded
topographic data are used. Ellipsis dots following arrows indicate textural transitions between strongly
lineated pit-and-butte morphology and a rough, patchy, degraded texture.
Figure 10. Map of regional LVF in the eastern portion of the study area, combining THEMIS global
mosaic and MOLA gridded data. LVF is integrated, as lineations from several sources merge and respond
to topography as a package. Lineations can be traced from alcoves to local topographic lows. Flow
directions are inferred from unambiguous lobe structures where present; otherwise, current gridded
topographic data are used. Ellipsis dots following arrows indicate textural transitions between strongly
lineated pit-and-butte morphology and a rough, patchy, degraded texture.
7 of 19
E08004
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
Figure 11. MOLA point profile across the transition
between axially-lineated regional LVF and a local topographic minimum covered by a patchy, rough surface
texture. (top) Note the low-angle, relatively flat topography,
with roughened surface texture found in the depression
between E and E0, in contrast to the convex-up lobate form
of the regional LVF to the right of E0. (bottom) Highresolution view of the transition between axially lineated
regional LVF and a patchy, softened, surface texture in a
local topographic low. The relatively flat topography of the
local depression is consistent with the degraded surface
texture, suggesting sublimation of regional LVF lobe
termini. The convex-up profile of the still-lineated regional
LVF unit, coupled with the abrupt topographic transition at
the contact between surface textures suggests that the LVF
lobe is retreating from the topographic low, rather than
being embayed or covered by dust (or some other obscuring
mantle). The transition between strongly lineated surface
texture and a softened surface texture is interpreted as a
result of the sublimation of ice from terminal regional LVF
in the topographic depression, and the production of a
sublimation lag deposit. Images are THEMIS-VIS image
V09834018 and MOC images M0401229 and E0102224.
strongly lineated, pit-and-butte textured regional LVF to a
roughly patterned surface texture in local topographic
minima as evidence of sublimation of regional LVF associated with glacier retreat. In these local termini, a portion of
the ice component of the regional LVF was removed by
E08004
sublimation, producing a lag deposit of sublimation residue.
This process is analogous to the stagnation, sublimation,
and till thickening of debris-covered glacier tills in the
stable upland zone of the Antarctic Dry Valleys [Marchant
and Head, 2004, 2007; Marchant et al., 2002; Rignot et
al., 2002].
[9] Further evidence of regional LVF ice volume loss
subsequent to the cessation of flow includes the presence of
steep scarps in the talus deposits above the regional LVF/
valley-wall contact (Figure 14), similar to the marginal
furrows described by White [1976] for terrestrial debriscovered glaciers. Had no volume been removed from the
regional LVF since the cessation of flow, talus deposits on
the valley walls would be expected to slope toward a
conformable contact with the regional LVF. Inspection of
high-resolution MOC data reveals an abrupt scarp at the
base of the talus slopes abutting regional LVF deposits,
suggesting that meters to tens of meters of material have
been removed from the regional LVF since the deposition of
the bulk of the marginal talus deposits (Figures 14 and 15).
The lack of channels or wind-sculpted features (e.g., yardangs) in the vicinity of the regional LVF units suggest that
melting of ice and/or wind erosion have not been major
sources of regional LVF volume loss. Rather, we interpret
the apparent change in regional LVF level as deflation of the
surface by the sublimation of a portion of the ice component
of the regional LVF [Mangold, 2003]. This process may be
analogous to the sublimation of debris-covered glaciers in
the Antarctic Dry Valleys, which produces a thick, complexly patterned sublimation lag deposit which limits subsequent sublimation, protecting the remaining buried ice
[Kowalewski et al., 2006; Marchant et al., 2002; Sugden et
al., 1995].
Figure 12. Regional LVF exiting a valley and transitioning into a nonlineated, softened texture. LVF lineations can
be traced directly from the theater-shaped valley head
alcove to a topographic low, several kilometers away, where
the lineated texture grades into a softened texture (arrow
indicates contact). Contours are 100 m. Image is THEMISVIS image V11057007 over MOLA gridded data.
8 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
E08004
2006b]), coupled with the interpretation of retreat of regional LVF in local topographic lows, suggests that climate
conditions during the formation and evolution of the Nilosyrtis regional LVF were generally cold and dry. Sufficient
water vapor was present in the atmosphere to permit snow
and ice accumulation in protected alcoves in valley headwalls and to allow long-term preservation of ice under
debris cover. In exposed areas, such as along glacier toes
in local topographic minima and within broad trunk valleys,
conditions favoring sublimation prevailed, promoting the
slow sublimation of buried ice, limiting the extent of
regional LVF integration, and promoting the generation of
residual regional LVF lag deposits observed at present.
Figure 13. Transition between axially lineated regional
LVF (arrow 1) and smoother, patchy surface textures (image
upper right) in a local topographic low. At the termination
of the well-lineated regional LVF texture is a ridge feature
oriented transverse to the regional LVF lineations (arrow 2),
which is interpreted as a moraine-like structure, consistent
with the retreat of glacier-like regional LVF lobes in
response to climate conditions favoring terminal ablation.
Images are THEMIS-VIS images V11319006, V11631004,
and V11943010 and MOC image M1200021.
[10] Craters were counted on 293 km2 of Nilosyrtis
Mensae regional LVF using HRSC image data. These
counts yield ages of 0.1– 1 Ga for craters larger than
250 m in diameter [Hartmann, 2005] (Figure 16), suggesting that the regional LVF surface is of late Amazonian age.
Given the highly degraded surface texture of LVF, determining accurate surface ages with crater counts is challenging, as small craters are rapidly removed, and large craters
are heavily modified [Mangold, 2003]. The paucity of
deformed craters on the regional LVF indicates that the
majority of the craters counted on the surface were
emplaced subsequent to the cessation of LVF flow, suggesting that the crater retention age is a minimum age for the
regional LVF unit. The age derived from this count is
consistent with crater counts on LVF in Deuteronilus and
Protonilus Mensae made by Mangold [2003] that suggest a
surface age of between 0.1 and 1 Ga. Crater counts made
using HRSC images covering the full extent of regional
LVF at 12.5 m/pixel suggest that small craters (<100 m
diameter) are being removed or made unrecognizable as fast
as they are being created, while large craters provide a
relatively intact record [Hartmann, 2004]. The removal of
small craters on the regional LVF is consistent with surficial
slumping and deflation associated with sublimation of
buried ice [Mangold, 2003].
[11] The kilometers to tens of kilometers scale integration
of regional LVF in the Nilosyrtis study area (compared to
the hundreds of kilometer scale integration described in
Deuteronilus and Protonilus Mensae by Head et al. [2006a,
2.2. Superposed LVF
[12] In some locations (Figures 17 and 18), axially
lineated, terminally lobate features are present in steeply
inclined, small valleys that debouch into larger Nilosyrtis
Mensae region trunk valleys. These features are morphologically similar to regional LVF. Like regional LVF,
surface lineations on these features can be traced from
valley headwalls (Figures 17 and 18), suggesting these
features are also sourced in protected alcoves. Like regional
LVF, these features are commonly lineated parallel to the
long axis of the valley in which they are found. Like
regional LVF, lineations in these features merge and are
deflected as a group around obstacles, suggesting coalescence and flow of ice-rich materials similar to terrestrial
debris-covered glaciers [Head et al., 2006b] (Figure 18).
Unlike regional LVF, these features form in small valleys
only a few km in length and are not integrated on tens of
kilometer scales. The key difference between these features
and regional LVF is that these features terminate in
expanded, concentrically lineated lobes at the junction
with the adjoining trunk valleys. This terminally lobate
morphology is analogous to the expansion of debris-covered
and other cold-based glacier lobes in the Antarctic Dry
Valleys, where small valley glaciers debouche into larger
trunk valleys [Levy et al., 2006; Marchant and Head, 2005]
(Figure 19).
[13] Stratigraphically, these expanding-lobe LVF features
are interpreted as representing some of the youngest activity
in the system, possibly even overlying material present on
the floor of the trunk valley. In order to distinguish these
features from other LVF features in the Nilosyrtis Mensae
region, we refer to them as ‘‘superposed’’ LVF. In some
locations, (e.g., Figure 17), superposed LVF appears to
overlie regional LVF present on the trunk valley floor. In
such cases, there is a sharp contact between the regional
LVF in the trunk valley (which is lineated parallel to the
trunk valley axis) and the concentrically lineated superposed LVF lobe. Concentric lineations appear to cross-cut
trunk-valley axial lineations, suggesting that the concentrically lineated superposed LVF formed later than, and
possibly overlies, the trunk valley regional LVF. Topographically, the contact between regional LVF and superposed
LVF is determined by the abrupt transition from the relatively flat-lying, low-angle regional LVF to the steeper
(3.6°), markedly convex-up superposed LVF lobe front
(Figures 17 and 18). Other examples of superposed LVF
lobes apparently overlie patchy, softened, and stippletextured valley floor material in the adjoining trunk valley
9 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
E08004
Figure 14. (a, c) Steep scarps in the talus deposits above the regional LVF/valley-wall contact,
interpreted as evidence of volume loss from the regional LVF subsequent to the cessation of flow (see
Figure 15). Had no volume been removed from the regional LVF since the cessation of regional LVF
flow, talus deposits on the valley walls would be expected to slope toward a conformable contact with the
regional LVF. The abrupt scarp at the base of the talus slopes abutting regional LVF deposits suggests that
meters to tens of meters of material have been removed from the regional LVF since the deposition of the
bulk of the marginal talus deposits. (b) Close-up view of the talus scarp shown in Figure 14a. (d) Talus
scarp at regional LVF margin with trough separating LVF from talus slope, consistent with marginal
retreat of debris-covered glaciers from valley walls [White, 1976]. Arrows indicate illumination direction
in all frames. All images are excerpted from THEMIS-VIS image V09834018; MOC images E0102224
and E0202120.
(Figure 18). Such valley floor material is texturally similar
to material interpreted as residual regional LVF found
predominantly in local topographic minima elsewhere in
the Nilosyrtis Mensae region (e.g., Figure 11). The textural
contact between the patchy, softened trunk-valley floor
material and the concentrically lineated, expanding superposed LVF lobe is sharp. Slope illumination suggests
significant relief of superposed LVF lobes, supporting the
interpretation of superposition over the softened, trunkvalley floor material. This relationship is supported by
MOLA point profiles (Figure 18), which indicate an abrupt
transition between a gentle (0.04°) slope for the trunk-valley
floor material, and a steep (up to 8°), strongly convex-up
profile of the superposed LVF lobe. Alternatively, if the
contact between the superposed LVF lobe and the regional
LVF is conformable, the relative positions of the two LVF
subtypes still suggest a sequence of emplacement in which
the formation of the superposed LVF occurs in the very
latest stage of the formation of the regional LVF, implying
different phases of glacial activity in the Nilosyrtis Mensae
region within the past <100 Ma.
[14] Taken together, abrupt changes in surface texture and
topography between trunk-valley floor material and superposed LVF features are interpreted as evidence of later stage
development or the superposition of the latter group of LVF
features. Clearly, there is morphological evidence for earlier,
integrated regional LVF flow, followed by later, less integrated superposed LVF flow. In particular, this apparent
superposition relationship suggests that superposed LVF
formed subsequent to the formation of the Nilosyrtis Mensae regional LVF. Had the superposed LVF been present in
smaller, tributary valleys, formed contemporaneously with
the regional LVF, contacts between the two (e.g., Figure 17)
would likely show more evidence of coalescence and
integration of the two LVF streams. Despite regional LVF
integration on tens of kilometer length scales and super-
10 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
Figure 15. Schematic diagram of current relationships
between the plateaus; talus deposits; and valley-floor,
regional LVF in the Nilosyrtis Mensae region. The abrupt
scarp present in many high-resolution MOC images of the
contact between talus deposits and regional LVF suggests
that regional LVF levels may have been higher in the past.
Volume loss can be accounted for by deflation of ice-rich
regional LVF by sublimation.
posed LVF integration on kilometer length scales, there is
no evidence of integration between the two LVF subtypes.
Rather, the contact between the two suggests a distinct
change in phase from early integrated flow to later, less
E08004
integrated flow, and a late-stage superposition relationship
between the superposed LVF and the regional LVF. Abundant evidence has been presented for waxing, waning, and
multiple phases of glaciation in the case of the Arsia Mons
tropical mountain glacial deposits [Shean et al., 2005];
similar late-stage, superposed, smaller debris-covered glacial deposits are seen on the western flanks of Olympus
Mons [e.g., Head et al., 2005]. We interpret the stratigraphic
relationship between regional LVF and superposed LVF as
an indication of changing conditions promoting valley
glacier activity in the Nilosyrtis Mensae region, characterized by the differential preservation and mobilization of
debris-rich ice in sheltered valley microenvironments.
2.3. Small-Scale, Superposed LVF
[15] A subset of superposed LVF features are interpreted
as evidence of a period of valley-glacier activity in the
Nilosyrtis Mensae region characterized by enhanced debris
content (Figure 20). Like other examples of superposed
LVF, these features (1) are present in smaller tributary
valleys, (2) are axially lineated and expand into adjacent
trunk valleys (commonly characterized by a textural transi-
Figure 16. Crater size-frequency distribution plot of regional LVF in the Nilosyrtis Mensae study area
made using HRSC image 1391_0000. Only fresh craters, clearly superposed on the regional LVF were
counted. The Hartmann [2005] isochrons suggest deposition of the regional LVF 100 Ma to 1 Ga ago,
with subsequent loss of small craters. The downturn in the number of craters per bin, per unit area at
smaller crater sizes is likely due to crater removal at these sizes, suggesting the regional LVF surface is
being actively modified, perhaps through sublimation and deflation.
11 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
tion to concentric lineation), and (3) have lineations that can
be traced from valley headwall alcoves, downslope, to
terminal, convex-up lobate fronts. The primary difference
between these features and superposed LVF is that these
features are shorter, narrower, and terminate at more abrupt
lobe heads than other superposed LVF features. In order to
distinguish these features from other LVF features in the
Nilosyrtis Mensae region, we refer to these features as
‘‘small-scale, superposed’’ LVF. Small-scale, superposed
LVF is more compact than superposed LVF and regional
LVF, and is only present in very small valleys, commonly
<1 km wide and <5 km long. Unlike regional LVF and
E08004
superposed LVF, small-scale, superposed LVF is not composed of integrated flows from multiple alcoves. Rather,
small-scale, superposed LVF is only composed of single,
lobate structures. In image data, small-scale, superposed
LVF is strongly positive in relief and supports steep and
abrupt marginal slopes; however, the small size of smallscale, superposed LVF features makes absolute height and
slope measurements with MOLA point data impossible.
Inspection of THEMIS-VIS imagery suggests that smallscale, superposed LVF overlies valley-wall talus deposits,
which in turn overlie patchy, rough valley-floor material in
valleys which do not contain regional LVF (Figure 20).
[16] One small-scale, superposed LVF feature in the
Nilosyrtis Mensae region provides numerous lines of evidence for glacial processes involved in small-scale, superposed LVF evolution. This small-scale, superposed LVF
lobe (Figure 21) is approximately 3 km long and fills the
headward portion of a small tributary valley 5 km in
length. The small-scale, superposed LVF lobe is axially
lineated and has a pit-and-butte surface texture. It terminates
in a convex-up lobe front oriented down-valley. Between
the lobe front and the valley mouth, the valley floor has a
subdued pit-and-butte surface texture with faint axial lineations. The tributary valley containing the small-scale,
superposed LVF lobe debouches into a larger trunk valley,
at which point a curving ridge spans the tributary valley
mouth, oriented convexly into the trunk valley. We interpret
this ridge form as a possible terminal moraine, marking the
farthest advance of the small-scale, superposed LVF lobe.
The muted LVF-like surface texture between the ridge and
the lobe front is interpreted as small-scale, superposed LVF
residue. The LVF-residue-like texture, coupled with the lack
of ridge-breaching channels or morphological features associated with aeolian erosion strongly suggest that the lobe
was truncated by removal of ice through sublimation. The
sharp contact between the residual small-scale, superposed
LVF material and the present relief of the lobe front is
interpreted as an indication of the retreat of the small-scale,
Figure 17. (a) MOLA point profile of an apparently
superposed LVF lobe. Between A and A0, the convex-up
morphology of the superposed LVF lobe front is clearly
visible. The abrupt increase in slope and unit thickness at
A and to the right, coupled with the sharp textural contact,
suggests the superposed LVF lobe may be overlying the
relatively flat-lying, trunk valley regional LVF. The
apparently superposed LVF lobe continues up the steep
slope of the small valley in which it is sourced. (b) Highresolution MOC and THEMIS-VIS image data of the contact
between a superposed LVF lobe and regional LVF. Note the
transition between axial lineations and concentric lineations
as the superposed LVF exits its source valley. Also, note that
the concentric lineations of the superposed LVF lobe appear
to cross-cut the axial lineations of the regional LVF. Images
are THEMIS-VIS images V11319006, V11631004, and
V11943010 and MOC image M1200021. (c) Sketch map of
the superposed LVF/regional LVF contact in Figure 17b.
Superposed LVF and regional LVF are labeled S-LVF and
R-LVF, respectively; P denotes the surrounding Nilosyrtis
Mensae region plateau. Lineations in both LVF subtypes are
denoted with fine lines.
12 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
superposed LVF lobe in a manner analogous to the retreat of
terrestrial cold-based, debris-covered glaciers [Benn et al.,
2003; Head et al., 2006b; Marchant and Head, 2005;
White, 1976].
[17] The positive relief of small-scale, superposed LVF
features, the presence of muted small-scale, superposed
LVF textures on valley floors, and the presence of pronounced terminal-moraine-like features suggest that the
conditions under which the small-scale, superposed LVF
evolved promoted the incorporation of enhanced debris
E08004
content in the small-scale, superposed LVF features. Enhanced debris content would engender stronger, more rigid
ice under present Martian conditions which could account
for the steep lobe margins of the small-scale, superposed
LVF [Arenson and Springman, 2005a, 2005b]. In response
to sublimation, enhanced debris content could form a
thicker sublimation till, which could more readily support
pit-and-butte textures, preserving small-scale, superposed
LVF textures as the retreating lobe was lowered to the valley
floor [Mangold, 2003]. Finally, enhanced debris content
could account for the pronounced terminal ridge observed at
one small-scale, superposed LVF valley mouth, a feature not
observed at the termini of superposed LVF lobes in the
Nilosyrtis Mensae region. Enhanced debris content is constitutively identical to reduced ice content. The small spatial
extent and concentrated distribution of small-scale, superposed LVF features, compared to superposed LVF and
regional LVF, coupled with the lack of integration of
small-scale, superposed LVF features, strongly suggests that
small-scale, superposed LVF formed under climate conditions which permitted the accumulation of relatively small
volumes of ice, only in very specific, protected, valley-head
alcove microenvironments. This interpretation suggests that
the small-scale, superposed LVF phase of alcove-sourced
valley glacier activity involved smaller volumes of ice
accumulation and flow than earlier periods of LVF activity.
The greater surface-area-to-volume ratio of small-scale,
superposed LVF compared to regional or superposed LVF
suggests that sublimation of buried ice may have been a
more significant process for limiting small-scale, superposed LVF activity [Mangold, 2003]. If this is the case,
small-scale, superposed LVF may be a very sensitive
indicator of microclimate conditions permitting glacial
modification of the dichotomy boundary.
2.4. Valley Wall LDA
[18] Lobate debris aprons surrounding isolated mesas and
forming on valley walls in the Nilosyrtis Mensae region are
morphologically similar to LDA described in many locations along the dichotomy boundary (Figures 22 and 23)
[Chuang and Crown, 2007; Lucchitta, 1984; Mangold,
2003; Squyres, 1978, 1979]. LDA units in the Nilosyrtis
Figure 18. (a) MOLA point profile across the contact
between an superposed LVF lobe and a region of patchy,
softened surface texture. Between D and D0, the convex-up
morphology of the S-LVF lobe front is clearly visible. The
increase in slope at the textural contact is interpreted as an
indication that the superposed LVF is overlying the
relatively flat, softened material. The superposed LVF lobe
embays the steep slope of the small valley in which it is
sourced. (b) THEMIS-VIS image data of the contact
between superposed LVF debouching from a small valley
and a unit of softened, patchy material covering the trunk
valley floor. Image is THEMIS-VIS image V11918012.
(c) Sketch map of the superposed LVF contact with softened
material in the trunk valley floor in Figure 18b. S-LVF
denotes the superposed LVF lobe; P denotes the surrounding plateau; W denotes the valley walls; M denotes an
isolated mesa; and PSF denotes patchy, softened floor
material.
13 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
E08004
Figure 19. Canada and Commonwealth glaciers in Taylor Valley, Antarctica. Terrestrial, cold-based
glaciers flow through internal deformation from ice sheets and accumulation zones in protected alcoves in
the Antarctic Dry Valleys, debouche into large trunk valleys, and expand into lobate fronts. These
Antarctic Dry Valley morphologies are broadly similar to superposed LVF lobe morphologies in the
Nilosyrtis Mensae region (see Figures 17 and 18), although these specific terrestrial examples are not
debris-covered, as numerous other Dry Valley glaciers are (e.g., Mullins and Friedman debris-covered
glaciers in Beacon Valley). Such glaciers highlight the expansion of glacial lobes over valley floor
materials in hyper-arid, cold desert climates. Image is a Landsat 7 subframe.
Mensae region are commonly lineated concentrically with
mesa or valley wall topography. LDA units in the Nilosyrtis
Mensae region have convex-up topographic profiles and a
pit-and-butte surface texture, similar to that described elsewhere along the dichotomy boundary (Figure 23) [Li et al.,
2005; Mangold, 2003]. The presence of LDA surrounding
isolated mesas has been interpreted as evidence of the
generation of LDA by means of atmospheric water-vapor
diffusion [Squyres, 1978], rather than groundwater seepage,
consistent with interpretations of proximal LVF units as
valley-glacier-like features.
[19] Stratigraphically, LDA units in the Nilosyrtis Mensae
region are interpreted as overlying regional LVF units on
the basis of the following observations. First, the contact
between concentrically lineated LDA units and axially
lineated regional LVF units is commonly sharp, with
concentric lineations cross-cutting axial lineations
(Figures 22 and 23). Secondly, topographically, the transition from regional LVF to LDA along MOLA point profiles
is marked by an abrupt increase in thickness and slope, from
the relatively gentle (1° – 1.5°) slopes of the regional LVF
to strongly convex-up LDA slopes (up to 5°) on the LDA
units (Figure 23). These topographic and textural relationships are identical to those that suggest the superposition of
superposed LVF lobes over regional LVF and patchy,
softened, valley-floor material elsewhere in the Nilosyrtis
Mensae region, suggesting that similar processes have been
involved in the generation of this suite of morphological
contacts. The abrupt changes in morphology suggest a
record of multiple styles of glacial activity in the Nilosyrtis
Mensae region of the dichotomy boundary.
[20] The relatively small spatial extent of LDA units
compared to LVF units in the Nilosyrtis Mensae region,
coupled with concentric LDA lineations, and LDA occurrence only on steep slopes is consistent with an interpretation of LDA as an ice-assisted flow feature sensitive to
formation in protected environments [Head et al., 2005; Li
et al., 2005]. On the basis of the relationships between the
Nilosyrtis regional LVF and LDA (the presence of LDA
along valley walls and isolated buttes; the sharp textural
contact between regional LVF and LDA units, suggesting
overriding of valley-floor regional LVF by LDA; and the
enhanced relief and slope of the LDA compared to the
largely flat and low-angle regional LVF) we suggest that
the LDA may have formed in a subsequent period of
glacial activity following the period of extensive regional
LVF activity. This later stage of glacial activity may have
been characterized by reduced ice supply to the alcoves,
resulting in more debris-rich ice, which resulted in a
better-preserved ice core, as suggested by the strongly
convex-up morphology of the LDA [Squyres, 1978,
1979; Mangold, 2003; Li et al., 2005] and the more
pronounced relief of the LDA. The lack of definitively
illustrative contacts between LDA units and superposed
LVF and small-scale, superposed LVF in the Nilosyrtis
Mensae region makes placing the LDA units in the
broader LVF stratigraphy impossible.
3. Discussion and Conclusions
[21] Taken together, the presence of alcove-derived regional lineated valley fill; superposed lineated valley fill;
14 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
Figure 20. (a) Three small-scale, superposed LVF lobes
overlying valley-wall talus deposits and a patchy, softened
unit on the trunk valley floor. The presently steep lobe
fronts and small size may indicate a higher debris content in
the small-scale, superposed LVF lobes than in superposed
LVF and regional LVF in the Nilosyrtis Mensae region.
Images are THEMIS-VIS images V11943010 and
V14102005 and MOC image R1402174. (b) Sketch map
of small-scale, superposed LVF lobes in Figure 20a. Smallscale, superposed LVF lobes commonly cross-cut valley
wall units and talus deposits present on the valley walls.
SSS-LVF denotes small-scale, superposed LVF lobes; P
denotes surrounding plateaus; W denotes valley walls; T
denotes talus deposits; and PSF denotes patchy, softened
floor material.
small-scale, superposed lineated valley fill; and lobate
debris aprons; provide evidence of different styles of glacier
activity in the Nilosyrtis Mensae region within the past
0.1 –1 Ga. Although all four landforms are interpreted as
E08004
Figure 21. (a) A small-scale, superposed LVF lobe filling
the headward portion of a small tributary valley. A subdued
pit-and-butte surface texture with faint axial lineations is
present between the midvalley lobe front and the tributary
valley mouth. A curving ridge spans the tributary valley
mouth, oriented convexly into the trunk valley, which is
interpreted as a possible terminal moraine, marking the
farthest advance of the small-scale, superposed LVF lobe.
The muted LVF-like surface texture between the ridge and
the midvalley lobe front is interpreted as small-scale,
superposed LVF residue. The sharp contact between the
residual small-scale, superposed LVF material and the
present relief of the lobe front is interpreted as an indication
of the retreat of the small-scale, superposed LVF lobe in a
manner analogous to the retreat of terrestrial glaciers. Image
is portion of MOC R1402174. (b) Sketch map of the smallscale, superposed LVF lobe in Figure 21a. SSS-LVF
denotes the current extent of the small-scale, superposed
LVF lobe; TX denotes the subdued pit-and-butte surface
texture interpreted as small-scale, superposed LVF residue;
R denotes the terminal ridge; P denotes the surrounding
plateau; and W denotes the valley walls.
15 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
Figure 22. (a) Lobate debris aprons (LDA) surrounding
isolated mesas and forming on valley walls. The LDAs are
similar in morphology to those observed on isolated mesas
elsewhere along the dichotomy boundary. The LDAs are
lineated concentrically to the mesa surfaces and valley walls
on which they form. The concentrically lineated LDA
apparently cross-cuts axially lineated regional LVF, suggesting that the LDA is superposed on the regional LVF.
Images are THEMIS-VIS images V13216005 and
V09834018. (b) Sketch map of the contact between LDA
and regional LVF in Figure 22a. Concentric LDA lineations
apparently cross-cut axial regional LVF lineations, suggesting that the LDA is superposed on the regional LVF.
Regional LVF (R-LVF) and LDA are denoted accordingly;
M denotes the mesas on which the LDA is present.
evidence of debris-covered-glacier activity, the landforms
can be distinguished by their distribution, size, degree of
integration, marginal steepness, surface patterning (Figure 4
and Table 1). Different morphologies are interpreted as
E08004
evidence of variability in the ice and debris content of the
landforms, which is in turn tied to the climatic conditions
under which the deposits formed and were modified
[Mangold, 2003; Arenson and Springman, 2005a; Li et
al., 2005; Madeleine et al., 2007]. Considering the stratigraphic, topographic, and morphological relationships,
between the three LVF landforms and LDA, the most
straightforward interpretation of the glacial deposits in the
Nilosyrtis Mensae region indicates an initial phase of
extensive, integrated valley glaciation (regional LVF) followed by one or more phases of glacial activity involving
diminished ice volume and/or local glaciation (superposed
LVF; small-scale, superposed LVF; and LDA).
[22] We present the following conceptual model to account
for the observed relationships between LVF morphology,
topography, and stratigraphy. Tracing regional LVF from
accumulation zone to terminus, during periods of regional
LVF-formation, debris-covered, valley glacier-like, ice-rich
regional LVF originated in protected alcoves, present in
previously carved valley heads, in the Nilosyrtis Mensae
region. These alcoves provided a sheltered environment for
ice accumulation and a source of debris for the Nilosyrtis
lineated valley fill [Head et al., 2005, 2006b]. Regional
LVF, composed of debris-covered-glacier-like material,
flowed along local gradients as part of an integrated
drainage system, and terminated in local topographic lows.
The pit-and-butte surface texture of the regional LVF, the
presence of depressed areas with softened textures within
the regional LVF lobes, retreat of regional LVF from valleywall talus deposits, the presence of moraine-like and glaciertoe-like features in local topographic sinks, as well as the
presence of largely undeformed craters on the LVF, are all
consistent with a period of reduced glacial activity and
sublimation following the peak flow activity of the Nilosyrtis Mensae region regional LVF [Mangold, 2003; Levy
and Head, 2006]. The presence of apparently superposed
LVF features (superposed LVF and small-scale, superposed
LVF) suggests that the generation of LVF in the Nilosyrtis
Mensae region may have occurred in phases characterized
by different glacier styles. It is difficult to ascertain precisely
how many glacial phases or cycles are recorded in the
Nilosyrtis Mensae region, particularly because glaciation
events overprint the substrate, obscuring evidence of previous glacial deposits. The low crater densities present on
Nilosyrtis Mensae region glacial features, particularly superposed LVF and small-scale, superposed LVF, preclude the
generation of reliable crater retention ages that could be
used to relatively date the features; however, the presence of
a variety of LVF morphologies with a variety of integration
and topographic characteristics suggests that several phases
or possibly periods of alcove-sourced, valley-glacier activity
have occurred in the Nilosyrtis Mensae region, within the
past 0.1– 1 Ga, leading to extensive modification of the
valleys of the dichotomy boundary.
[23] Advanced GCM and mesoscale climate models will
enhance our ability to interpret the geomorphic features
observed in the Nilosyrtis Mensae region, as well as
improve the precision to which glacial landforms can be
matched to climate fluctuations driven by orbital parameters
[Milkovitch and Head, 2005; Madeleine et al., 2007].
Recent models suggest that as Martian obliquity is reduced
from high values (45°) to values approaching 35°, it may
16 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
be possible to accumulate 100 – 1000 m of ice in the
northern midlatitudes over a 50 ka obliquity cycle: ice
accumulations consistent with observations of a variety of
glacial landforms [Madeleine et al., 2007]. Effective new
models must address two scales of ice deposition: regionalscale, latitude-dependent deposition [Levrard et al., 2004];
and local-scale deposition dominated by kilometer-scale
topography and microclimate conditions [Forget et al.,
2006]. Any new models which attempt to constrain orbital
parameters which produce ice accumulation in the Nilosyrtis
Mensae region must account for both regional-scale ice
deposition responsible for producing regional LVF and
widely distributed LDA, as well as for local-scale ice
deposition occurring only in protected alcoves, valley walls,
butte faces, and crater rims. Of particular interest will be
improved understanding of the effects of annually averaged
versus seasonal conditions. Annual conditions permitting
accumulating of water ice may produce regional glacial
E08004
features, while annually ablating conditions with only brief
seasonal periods permitting deposition may limit glacial
activity to alcove-sourced valley glaciers, potentially accounting for the various styles of glacial modification
observed in the Nilosyrtis Mensae study area [Madeleine
et al., 2007]. Better understanding the microclimatic effects
of topography and relief on ice stability [Aharonson and
Schorghofer, 2006] will be key in understanding the limits
of, and feedbacks to, conditions predicted by even the
highest-resolution climate models, and may have the potential to significantly refine our understanding of the climate
signal present in the stratigraphic relationships between
LVF, LDA, and other glacial features along the dichotomy
boundary [Marchant and Head, 2004].
[24] In summary, morphological, stratigraphic, and textural relationships between lineated valley fill (LVF) and
lobate debris apron (LDA) units in the Nilosyrtis Mensae
region of the Martian dichotomy boundary were analyzed in
order to better understand (1) the nature and composition of
LVF, (2) the relationship between LVF and LDA, and (3) the
climatic significance of LVF and LDA units. Three distinct
subtypes of LVF were described in the area, including
regional LVF, which suggests periods of integrated LVF
flow on tens of kilometer length scales; superposed LVF,
which apparently superposes regional LVF units, and is
integrated on kilometer scales or less; and small-scale,
superposed LVF, which forms only in very small (<5 km)
tributary valleys, supports steep margins, and is not composed of integrated lobes. Morphological analysis of LVF
subtypes suggests that analogies between LVF units and
terrestrial debris-covered glaciers are appropriate and readily account for many observed lobe and contact morphologies. In the Nilosyrtis Mensae region, LVF and LDA units
exist in a stratigraphic relationship with LDA apparently
superposing regional LVF units. Superposed LVF units are
Figure 23. (a) MOLA point profile across the contact
between LDA on a valley wall and regional LVF. Between
B and B0, the LDA is considerably thicker than the regional
LVF, and supports a steeper surface slope (5°, versus <1°).
The abrupt change in topography along the contact is
interpreted to support a superposition relationship between
the regional LVF and the LDA, suggesting that the LDA
was emplaced after the regional LVF. (b) High-resolution
MOC and THEMIS-VIS image data of the contact between
regional LVF and LDA. Arrows indicate the sharp contact
between the LDA and the regional LVF units. Note the pitand-butte surface texture common between the two
morphological units, as well as the presence of both fresh
and inverted craters on the regional LVF surface. Images are
THEMIS-VIS images V13216005 and V09834018 and
MOC images E1300869 and E0202120. (c) Sketch map of
the contact between regional LVF and LDA in Figure 23b.
Note that the concentric lineations of the LDA unit
apparently cross-cut the valley-axial lineations of the
regional LVF. Together with the abrupt increase in slope
and topography at the contact between the two units, this is
interpreted to mean that the LDA is superposed on the
regional LVF, suggesting that the LDA formed subsequent
to the regional LVF.
17 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
found to superpose regional LVF units, and small-scale,
superposed LVF units are interpreted to superpose valleywall talus deposits; however, stratigraphic relationships
between LDA and superposed LVF are not discernable in
the study area. Both LDA and superposed LVF appear to
overlie regional LVF units, suggesting that the two features
may form under similar climate conditions in different
geographical settings (i.e., valley headwall alcoves for
LVF subtypes, valley wall and isolated mesa environments
for LDA). Morphological relationships between LVF subtypes suggest a record of multiple phases and styles of
valley glacier activity in the Nilosyrtis Mensae region,
characterized by sequential reductions in LVF ice volume
and flow intensity. This may indicate a climatic process of
increasing desiccation with time, leading to increasingly
limited ice accumulation and enhanced sublimation of
buried ice. Climate conditions during more recent history
have resulted in sublimation and ice removal from these
glacial terrains, although it is possible that ice remains at
depth at the present, buried under protective sublimation
till.
[25] Acknowledgments. The authors would like to thank Caleb
Fassett for assistance in crater count interpretation, analysis, and data
processing. Thanks go to Malin Space Science Systems for use of the
MOC data library, to Philip Christensen and the THEMIS team for use of
THEMIS-VIS data, and to Gerhard Neukum and the HRSC team for use of
HRSC image data for crater counting. This research was supported by the
Rhode Island Space Grant Program, a part of the National Space Grant
College and Fellowship Program, and by the NASA Mars Data Analysis
Program (grants NNG04GJ99G and NNG05GQ46G) to J. W. H.
References
Aharonson, O., and N. Schorghofer (2006), Subsurface ice on Mars with
rough topography, J. Geophys. Res., 111, E11007, doi:10.1029/
2005JE002636.
Arenson, L. U., and S. M. Springman (2005a), Mathematical descriptions
for the behavior of ice-rich frozen soils at temperatures close to 0 degrees
C, Can. Geotech. J., 42, 431 – 442.
Arenson, L. U., and S. M. Springman (2005b), Triaxial constant stress and
constant strain rate tests on ice-rich permafrost samples, Can. Geotech. J.,
42, 412 – 430.
Benn, D. I., M. P. Kirkbride, L. A. Owen, and V. Brazier (2003), Glaciated
valley landsystems, in Glacial Landsystems, edited by D. J. A. Evans,
p. 372 – 406, Edward Arnold, London.
Carr, M. H. (1995), The Martian drainage system and the origin of networks
and fretted channels, J. Geophys. Res., 100, 7479 – 7507.
Carr, M. H. (2001), Mars Global Surveyor observations of Martian fretted
terrain, J. Geophys. Res., 106, 23,571 – 23,593.
Chuang, F. C., and D. A. Crown (2007), Modification of the ancient highland plateau along the dichotomy boundary, Deuteronilus Mensae, Mars,
Lunar Planet. Sci., XXXVIII, abstract 1455.
Eyles, N. (1983), The glaciated valley system, in Glacial Geology: An
Introduction for Engineers and Earth Scientists, edited by N. Eyles,
p. 91 – 107, Pergamon, New York.
Forget, F., R. M. Haberle, F. Montmessin, B. Levrard, and J. W. Head
(2006), Formation of glaciers on Mars by atmospheric precipitation at
high obliquity, Science, 311, 368 – 371.
Frey, H. V. (2006), Impact constraints on the age and origin of the lowlands
of Mars, Geophys. Res. Lett., 33, L08S02, doi:10.1029/2005GL024484.
Frey, H., and R. A. Schultz (1988), Large impact basins and the megaimpact origin for the crustal dichotomy on Mars, Geophys. Res. Lett., 15,
229 – 232.
Frey, H., and R. A. Schultz (1991), Geological and topographic constraints
on the origin and development of the Martian crustal dichotomy: What
they do and don’t require, Lunar Planet. Sci., XXII, 417 – 418.
Greeley, R., and J. E. Guest (1987), Geologic map of the eastern equatorial
region of Mars, Misc. Invest. Ser. Map I-1802-B, U.S. Geol. Surv.,
Boulder, Colo.
Hartmann, W. K. (2004), Updating the crater count chronology system for
Mars, Lunar Planet. Sci. Conf., XXXV, abstract 1374.
Hartmann, W. K. (2005), Martian cratering 8: Isochron refinement and the
chronology of Mars, Icarus, 174, 294 – 320.
E08004
Head, J. W., J. F. Mustard, M. A. Kreslavsky, R. E. Milliken, and D. R.
Marchant (2003), Recent ice ages on Mars, Nature, 426, 797 – 802.
Head, J. W., et al. (2005), Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars, Nature, 434, 346 – 351.
Head, J. W., D. R. Marchant, M. C. Agnew, C. I. Fassett, and M. A.
Kreslavsky (2006a), Extensive valley glacier deposits in the northern
mid-latitudes of Mars: Evidence for late Amazonian obliquity-driven
climate change, Earth Planet. Sci. Lett., 241, 663 – 671.
Head, J. W., A. L. Nahm, D. R. Marchant, and G. Neukum (2006b),
Modification of the dichotomy boundary on Mars by Amazonian midlatitude regional glaciation, Geophys. Res. Lett., 33, L08S03,
doi:10.1029/2005GL024360.
Irwin, R. P., T. R. Watters, A. D. Howard, and J. R. Zimbelman (2004),
Sedimentary resurfacing and fretted terrain development along the crustal
dichotomy boundary, Aeolis Mensae, Mars, J. Geophys. Res., 109,
E09011, doi:10.1029/2004JE002248.
Kiefer, W. S. (2005), Buried mass anomalies along the hemispheric
dichotomy in eastern Mars: Implications for the origin and evolution of
the dichotomy, Geophys. Res. Lett., 32, L22201, doi:10.1029/
2005GL024260.
Kowalewski, D. E., D. R. Marchant, J. S. Levy, and J. W. Head (2006),
Quantifying summertime sublimation rates for buried glacier ice in Beacon Valley, Antarctica, Antarct. Sci., 18, 421 – 428.
Levrard, B., F. Forget, F. Montmessin, and J. Laskar (2004), Recent ice-rich
deposits formed at high latitudes on Mars by sublimation of unstable
equatorial ice during low obliquity, Nature, 431, 1072 – 1075.
Levy, J. S., and J. W. Head (2006), Lineated valley fill surface textures,
Nilosyrtis Mensae, Mars: Comparison with analagous glacier surface
textures in the Antarctic Dry Valleys, Lunar Planet. Sci., XXXVII, abstract
1245.
Levy, J. S., D. R. Marchant, and J. W. Head (2006), Distribution and origin
of patterned ground on Mullins Valley debris-covered glacier, Antarctica:
The roles of ice flow and sublimation, Antarct. Sci., 18, 385 – 397.
Li, H., M. S. Robinson, and D. M. Jurdy (2005), Origin of Martian northern
hemisphere mid-latitude lobate debris aprons, Icarus, 176, 382 – 394.
Lucchitta, B. (1984), Ice and debris in the fretted terrain, Mars, J. Geophys.
Res., 89, 409 – 418.
Madeleine, J. B., F. Forget, J. W. Head, B. Levrard, and F. Montmessin
(2007), Mars: A proposed climatic scenario for northern mid-latitude
glaciation, Lunar Planet. Sci., XXXVIII, abstract 1778.
Mangold, N. (2003), Geomorphic analysis of lobate debris aprons on Mars
at Mars Orbiter Camera scale: Evidence for ice sublimation initiated by
fractures, J. Geophys. Res., 108(E4), 8021, doi:10.1029/2002JE001885.
Marchant, D. R., and J. W. Head (2004), Microclimate zones in the Dry
Valleys of Antarctica: Implications for landscape evolution and climate
change on Mars, Lunar Planet. Sci., XXXV, abstract 1405.
Marchant, D. R., and J. W. Head (2005), Equilibrium landforms in the Dry
Valleys of Antarctica: Implications for landscape evolution and climate
change on Mars, 36th Annual Lunar Planet. Sci., XXXVI, abstract 1421.
Marchant, D. R., and J. W. Head (2007), Antarctic Dry Valleys: Microclimate zonation, variable geomorphic processes, and implications for
assessing climate change on Mars, Icarus, in press.
Marchant, D. R., A. R. Lewis, W. M. Philips, E. J. Moore, R. A. Souchez,
G. H. Denton, D. E. Sugden, N. Potter, and G. P. Landis (2002), Formation of
patterned ground and sublimation till over Miocene glacier ice in Beacon
Valley, southern Victoria Land, Antarctica, GSA Bull., 114, 718 – 730.
McGill, G. E. (2000), Crustal history of north-central Arabia Terra, Mars,
J. Geophys. Res., 105, 6945 – 6959.
Milkovitch, S., and J. W. Head III (2005), North polar cap of Mars: Polar
layered deposit characterization and identification of a fundamental climate
signal, J. Geophys. Res., 110, E01005, doi:10.1029/2004JE002349.
Milliken, R. E., J. F. Mustard, and D. L. Goldsby (2003), Viscous flow
features on the surface of Mars: Observations from high-resolution Mars
Orbiter Camera (MOC) images, J. Geophys. Res., 108(E6), 5057,
doi:10.1029/2002JE002005.
Pierce, T. L., and D. A. Crown (2003), Morphologic and topographic
analyses of debris aprons in the eastern Hellas region, Mars, Icarus,
163, 46 – 65.
Poulet, F., et al. (2006), The distribution of phyllosilicates on Mars from
the OMEGA-MEX imaging spectrometer, Lunar Planet. Sci., XXXVII,
abstract 1698.
Rignot, E., B. Hallet, and F. Fountain (2002), Rock glacier surface motion in
Beacon Valley, Antarctica, from synthetic-aperture radar interferometry,
Geophys. Res. Lett., 29(12), 1607, doi:10.1029/2001GL013494.
Sharp, R. P. (1973), Mars: Fretted and chaotic terrains, J. Geophys. Res., 78,
4073 – 4083.
Shean, D. E., J. W. Head, and D. R. Marchant (2005), Origin and evolution
of a cold-based tropical mountain glacier on Mars: The Pavonis Mons
fan-shaped deposit, J. Geophys. Res., 110, E05001, doi:10.1029/
2004JE002360.
18 of 19
E08004
LEVY ET AL.: NILOSYRTIS MENSAE LVF/LDA STRATIGRAPHY
Squyres, S. W. (1978), Martian fretted terrain: Flow of erosional debris,
Icarus, 34, 600 – 613.
Squyres, S. W. (1979), The distribution of lobate debris aprons and similar
flows on Mars, J. Geophys. Res., 84, 8087 – 8096.
Squyres, S. W. (1989), Water on Mars, Icarus, 79, 229 – 288.
Sugden, D. E., D. R. Marchant, N. Potter, R. A. Souchez, G. H. Denton,
C. C. Swisher, and J.-L. Tison (1995), Preservation of Miocene glacier
ice in East Antarctica, Nature, 376, 412 – 415.
Watters, T. R. (2003), Lithospheric flexure and the origin of the dichotomy
boundary on Mars, Geology, 31, 271 – 274.
Watters, T. R., and P. J. McGovern (2006), Lithospheric flexure and the
evolution of the dichotomy boundary on Mars, Geophys. Res. Lett., 33,
L08S05, doi:10.1029/2005GL024325.
E08004
White, S. E. (1976), Rock glaciers and block fields, review and new data,
Quat. Res., 6, 77 – 97.
J. W. Head and J. S. Levy, Department of Geological Sciences, Brown
University, Box 1846, Providence, RI 02912, USA. (james_head@brown.
edu)
D. R. Marchant, Department of Earth Sciences, Boston University,
Boston, MA 02215, USA.
19 of 19