Olympus Mons, Mars: Detection of extensive preaureole volcanism

Olympus Mons, Mars: Detection of extensive preaureole
volcanism and implications for initial mantle plume behavior
Elizabeth R. Fuller 

James W. Head III 
Department of Geological Sciences, Brown University, Providence, Rhode Island 02906, USA
ABSTRACT
The early volcanic history and structural development of Olympus Mons, Mars, has
been obscured by the formation of the large circum-Olympus aureole deposits in the Early
Amazonian. Mars Orbiter Laser Altimeter data reveal an enormous preaureole extrusive
flow unit in Amazonis Planitia that is more than 300 km wide and extends ;1800 km
radially away from the Olympus Mons aureole. We interpret this volcanic unit to represent proto–Olympus Mons lava flows emplaced during the latest Hesperian or earliest
Amazonian. The orientation and localization of the deposits appear to be due to channeling
of the flows into a broad depression between the generally north dipping slope of Amazonis
Planitia and the southwestward-dipping slope of the Alba Patera flanks. Formation of the
aureole blocked this depression and caused subsequent summit flows to pond in the
circum-Olympus trough and flow around the aureole lobes. Emplacement of this protoOlympus flow unit formed a barrier along the northern margin of Amazonis Planitia,
causing ponding of later lava flows and outflow channel events debouching there. The
large volume and great lateral extent of the unit imply abundant magma supply and high
effusion rates during the initial stages of Olympus Mons construction; this unit may be
the result of the initial impingement of a melt-rich mantle plume head.
Keywords: volcanism, Olympus Mons, Mars, plumes, lava flows.
INTRODUCTION
Olympus Mons, the tallest known volcano
in the solar system, sits on the northeastern
flank of the Tharsis rise of Mars. It is surrounded by a several hundred kilometer wide
ring of material known as the aureole (outlined in Fig. 1), which is thought to have been
caused by massive deformation and landsliding of the edifice flanks (e.g., Morris and Tanaka, 1994; Francis and Wadge, 1983). The
early history of Olympus Mons is poorly understood because the aureole deposits have destroyed and obscured evidence of early and
flanking eruptions (Scott and Tanaka, 1986).
In their analysis of the Olympus Mons region,
Morris and Tanaka (1994) showed that the
structure of the aureole deposits is consistent
with gravitationally induced collapse, but
pointed out that such a mechanism would not
be expected to bury the entire precollapse
structure. They therefore looked for evidence
of preaureole lava flows extending beyond the
aureole, but were unable to find such structures in the Viking images (Morris and Tanaka, 1994). Previous maps, generated from Viking images, show Amazonis and Arcadia
Planitiae (Fig. 1) to be blanketed by a series
of region-wide volcanic units: the fivemember Arcadia Formation, a volcanic formation without an apparent source that persisted throughout the Amazonian (Scott and
Tanaka, 1986).
This study uses data from the Mars Orbiter
Laser Altimeter (MOLA) and images from the
Mars Orbiter Camera (MOC) to examine ev-
idence of preaureole volcanism. MOLA data,
and the derived data products, have permitted
the analysis and detection of a significant
number of geologic features that were only
hinted at in Viking image data. For example,
gradient maps, detrended topography maps,
and slope maps generated from MOLA data
(e.g., Fig. 2A) have revealed details of the geology and structure of Amazonis Planitia (Fuller and Head, 2002) and shown that it differs
from that originally mapped by Scott and Tanaka (1986). Among the new units detected
was a very large flow unit extending northwest across Amazonis Planitia and into Arcadia Planitia from the margins of the Olympus Mons aureole (Fig. 1). Here we document
the nature and detailed stratigraphic relationships of this unit and examine evidence for its
provenance and age.
OBSERVATIONS
General Characteristics
Stratigraphically, the lobate flow unit (Figs.
1 and 2) overlies the Upper Hesperian Vastitas
Borealis Formation (Scott and Tanaka, 1986),
a unit laterally correlative with the outflow
channels and thought to represent a potentially
ice-rich sublimation residue remaining from
the outflow events (e.g., Kreslavsky and Head,
2002). The Vastitas Borealis Formation is
;100 m thick and overlies Lower Hesperian
ridged plains of volcanic origin (Head et al.,
2002); the ridges on the surface of this unit
can be seen to protrude through the Vastitas
Borealis Formation, but are largely buried by
the lobate flow unit (Fig. 2). The eastern con-
tact of the flow unit is the sharp, scarp-like
western boundary of the topographically higher Early Amazonian aureole deposits, suggesting that the aureole unit was emplaced on
top of the flow unit, and is thus younger.
These general relationships place the age of
the flow unit as latest Hesperian or earliest
Amazonian, older than any mapped deposits
from Olympus Mons.
Morphology
The preaureole Olympus Mons flow unit is
extremely long (;1780 km) and wide (310–
420 km), with a maximum thickness of ;100
m; as exposed, it is the longest flow unit yet
detected on Mars, longer than any identified
in the flow catalog of Peitersen et al. (2002).
The flow unit is on a very shallow slope
(;0.00388); the substrate decreases in elevation only 120 m over the unit’s 1800 km extent. It is approximately constant in width,
suggesting preferential forward advance instead of lateral spreading. The regional topography appears to be channeling the flow, inhibiting lateral movement in favor of forward
flow. The nearly constant width along the flow
unit is common for long (.50 km) flows, according to a study of 145 martian and terrestrial lava flows (Peitersen et al., 2002).
The surface of the flow unit shows a rough,
rubbly texture (Fig. 3A), occasionally interrupted by round depressions meters to tens of
meters across (e.g., Fig. 3B). These depressions are not seen in MOC images to the north
and south, suggesting that they are unique to
this flow unit. Possible origins include impact
craters, collapse depressions, or deflation pits.
Global MOLA roughness data reveal the presence of a very young, latitudinally dependent,
fine-grained mantling deposit overlying much
of the flow unit (Kreslavsky and Head, 2000);
MOC images show extensive eolian modification of the surface. These data indicate that
the current surface has been altered in the ;3
b.y. since its emplacement, explaining why the
flow unit was not recognized in Viking images, which show only the extensively modified surface.
Flow units on Mars can be formed by lava
flows, lahars (e.g., Christiansen, 1989), or a
combination of both (e.g., Tanaka et al., 1992;
Russell and Head, 2001). Christiansen (1989),
using Viking images, mapped lahars whose
overall morphology is broadly similar to the
preaureole flow unit. Russell and Head (2001),
q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Geology; February 2003; v. 31; no. 2; p. 175–178; 4 figures.
175
Figure 1. Location of newly mapped flow unit and nearby volcanic provinces,
overlaid on topography and gradient maps generated from Mars Orbiter Laser
Altimeter. Scarp around upper portion of Olympus Mons edifice and Olympus
Mons aureole deposits are outlined. Note that scales above and below 0 m
(martian datum) are different. Lambert conformable conic projection, 90–2008W,
0–708N.
using MOLA data and MOC images, have examined the region and interpreted the broad,
lobate structures as lava flows and the narrower, higher-relief units as lahars. The broad, lobate morphology of the preaureole flow unit
therefore appears more consistent with a lavabased than a lahar-based origin. MOLA data
further suggest that the preaureole flow unit is
composed of a series of discrete lava flows.
The data reveal a few discernable flow margins on the surface of the unit (mapped in Fig.
2). Figure 3C, MOC images taken across a
crater rim, reveals laterally extensive layers in
the crater walls. These could be a result of
ejecta layering, but their cohesiveness and differential weathering patterns are more sugges-
tive of solid rock units. Their overall morphology is similar to layers seen in MOC
images elsewhere and interpreted to be lava
beds (e.g., McEwen et al., 1999). They also
resemble exposed cliffs within the Columbia
River Basalt Group. It should be noted, however, that apparent layers within the Columbia
River Basalt Group are often entablature tiers,
i.e., laterally persistent patterns of cooling
joints (e.g., McMillan et al., 1989); the layers
visible in Figure 3C may similarly represent
cooling effects instead of a series of discrete
flow events.
The southern portion of the flow unit forms
two rectilinear depressions in northeastern
Amazonis Planitia (centered at 328N, 1508W).
The western margin of these basins is approximately parallel to the wrinkle ridges that
sweep across this region (mapped in Fig. 2;
see also discussion in Head et al., 2002), suggesting that the southernmost flows abutted a
wrinkle ridge and were deflected to form the
enclosed basins. These southernmost flows
may be the oldest in the flow unit; the uppermost flows (mapped individually in the southwest corner of Fig. 2) appear to curve around
these basins.
The flow unit is surrounded by closely associated smooth plains (Fig. 2) that also overlie the Vastitas Borealis Formation. MOC images show several features interpreted to be
related to ice or water. At the margins of the
flow unit, subparallel meandering channels
carve through the substrate (e.g., Fig. 3D), and
are interpreted to represent melting and mobilization of ground ice. The meandering planform of the channels and the extensive eolian
reworking suggest that the smooth plains are
easily eroded. We interpret these deposits to
be related to the emplacement of the flow unit,
most likely resulting from melting of the underlying ice-rich sediments of the Vastitas Borealis Formation during emplacement of the
superposed flow unit, causing melting and
drainage out along the flow margins.
Provenance
The source and proximal extent of the flow
unit are buried by the Olympus Mons aureole.
There are two nearby major volcanic edifices,
Alba Patera and Olympus Mons (Fig. 1). The
flow unit extends approximately radially to
Olympus Mons. If it originated at the Olympus Mons summit, it would be on the order
of 2500 km long. If it originated from the
Alba Patera summit, it would be nearly 3500
km long.
Prior to the emplacement of this unit, the
Figure 2. A: Regional topography overlaid on gradient map, generated from Mars Orbiter Laser Altimeter data. B: Geologic sketch map
overlaid on (A). Note smooth, (low-relief) plains surrounding flow unit, sharp northern contact of flow unit and very subtle southern contact,
very smooth surface of Amazonis Planitia, and pervasive wrinkle ridges texture. AF—Acheron Fossae; OMA—Olympus Mons aureole.
Arrow points to crater shown in Figure 3D.
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GEOLOGY, February 2003
Figure 3. Mars Orbiter
Camera images. A: Surface of preaureole flow
unit. M07-03360, 148.948W,
36.778N. B: Surface of flow
unit; black arrows indicate
circular depressions. M1501063, 163.608W, 39.468N.
C: Crater rim showing layers that may represent individual lava-flow events.
M18-01140, 164.068W,
42.858N. D: Channels flowing away from flow unit.
M09-03601, 172.678W,
51.808N. Images are
available courtesy of
Malin Space Science
Systems.
regional topography was dominated by the regional slope, grading downward to the north
from the martian southern highlands toward
the northern lowlands (Fig. 1). The flanks of
the Alba Patera edifice formed a slope upward
to the northeast, and the flanks of the preaureole proto–Olympus Mons volcanic edifice
(Morris and Tanaka, 1994) would have formed
a similarly steep slope to the southeast. This
topography would have preferentially channeled flows extending north from Olympus
Mons or southwest from Alba Patera along a
relatively narrow trough to the northwest. A
flow extending south or southwest from Alba
Patera would first have banked against the
preaureole Olympus Mons volcanic edifice or
the Noachian (i.e., pre-Hesperian) Acheron
Fossae (Fig. 1). Reconstructing the likely extent of this preaureole edifice indicates that
lava flowing from Alba Patera would have had
to climb over the flanks of Olympus Mons to
flow into Amazonis Planitia. In addition, the
kilometer-scale roughness of the flow unit
GEOLOGY, February 2003
(Kreslavsky and Head, 2000) supports an origin at Olympus Mons (Fuller and Head,
2002): as discussed in previous papers in more
detail, lava flows from a single source typically have internally consistent surface roughness. There are no other preaureole Olympus
Mons lava flows to which this flow unit can
be compared, but comparison with flows from
Alba Patera and the Tharsis Montes shows
that those flow surfaces are significantly
rougher (at 2.4 km base-length roughness)
than this preaureole flow unit. On the basis of
these observations, we conclude that the most
likely provenance of this flow unit is the
preaureole Olympus Mons.
margin of the flow unit stands out sharply
from surrounding topography, while the
southern margin is less distinct (Figs. 2A and
4B). This is consistent with the interpretation
that the southern margin was repeatedly embayed and mantled by younger volcanic and
outflow events in Amazonis Planitia (Fuller
and Head, 2002). MOLA data reveal that the
flow unit mapped here crosses the contacts of
all five members of the Arcadia Formation,
mapped on the basis of Viking data by Scott
and Tanaka (1986) and previously thought to
have been emplaced throughout the Amazonian. Thus, the Arcadia Formation is in need
of redefinition in future studies.
Stratigraphic Implications
The flow unit and its surrounding smooth
plains overlie the Lower Hesperian ridged
plains and the Upper Hesperian Vastitas Borealis Formation, but are truncated by the
Lower Amazonian Olympus Mons aureole
(Fig. 4A). MOLA data show that the northern
DISCUSSION
Several researchers have proposed a
gravity-spreading origin for the Olympus
Mons aureoles (e.g., Francis and Wadge,
1983; Morris and Tanaka, 1994). Morris and
Tanaka (1994) argued that ice-lubricated gravitational collapse implies the presence of an
177
Figure 4. A: Stratigraphic column (after
Scott and Tanaka, 1986); approximate timing
of preaureole proto–Olympus Mons flow is
indicated by arrow. Amazonian is youngest
martian era; time of Hesperian-Amazonian
boundary is ca. 3.5 Ga. B: Interpretive cross
section. Surface is generated from Mars Orbiter Laser Altimeter gridded topography at
1558W; other units are shown with approximate thicknesses (to scale). Elevation values are referenced to martian datum. Hr—
Lower Hesperian ridged plains. VE—Vertical
exaggeration.
existing large volcanic edifice, evidence of
which would be lava flows still visible beyond
the margins of the aureole; they considered the
fact that they had not found any in Viking
images to be a weakness of the model. This
study therefore strengthens their interpretation
and supports their primary conclusion, the
collapse of preexisting edifice margins as an
origin for the aureole.
The very large volume (50,000–75,000
km3) of the exposed flow unit is approximately half the volume of the Columbia River Basalt Group (Tolan et al., 1989), and this is a
minimum estimate, because the flow unit likely originated closer to the Olympus Mons
summit region. The lack of such extensive
flow units later in the history of Olympus
Mons suggests that the early stages of eruptive
activity were characterized by a greater magma supply rate and potentially higher effusion
rates than later (Wilson et al., 2001). This behavior could be a function of source mantle
plume geometry, where early massive flows
were fed by a plume head and subsequent
lower flow volumes were due to the diminishing volume of the plume tail. This dichotomy
in eruptive styles between plume head and
plume tail is well documented for terrestrial
plumes (e.g., Ernst and Buchan, 2001). Of additional significance is the relationship between plume head size and initial eruption
volume. The size of the plume head is primarily a function of the depth at which the
plume originated (Campbell, 2001); the enormous volumes of lava erupted here suggest
that a source plume may have originated deep
in the interior, perhaps at the martian coremantle boundary.
178
ACKNOWLEDGMENTS
We thank Patrick McGovern, Karl Mitchell, Devon Burr,
and James Zimbelman for helpful discussions. We gratefully acknowledge the use of Mars Orbiter Laser Altimeter
data, available at http://ltpwww.gsfc.nasa.gov/tharsis/
mola.html, and Mars Orbiter Camera images, processed by
Malin Space Science Systems, available at http://
www.msss.com/mocpgallery/. We thank Patrick McGovern
and Richard Ernst for their thoughtful reviews and helpful
comments.
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Manuscript received 12 June 2002
Revised manuscript received 10 October 2002
Manuscript accepted 12 October 2002
Printed in USA
GEOLOGY, February 2003