Planetary mapping
Digital structural
J M Dohm, R C Anderson and K L Tanaka reveal how digital fault mapping has helped to unravel
the tectonic and volcanic history of Valles Marineris and Warrego Valles, Mars.
F
or thousands of years, Mars has captivated the human imagination. However, the
Martian surface remained a mystery until
the mid 1960s, when the Mariner probes began
to map the surface. Later, during the 1970s, the
two Viking Orbiters captured over 50 000
images of the Martian surface, opening a new
era in Mars exploration by permitting close
examination of landscapes and active surficial
processes. It is now generally accepted that
Mars has had an active and varied geological
history similar to Earth’s, but with two major
exceptions: the Martian surface has been much
less affected by large-scale erosional processes
(Scott and Dohm 1997) and plate tectonics has
not been observed (Pruis and Tanaka 1995).
Thus, a substantial archive of Mars’ palaeotectonic information (structural development
through time) has been recorded in the crustal
rocks, particularly in the western equatorial
region (90°N to 90°S latitude and 0° to 180°W
longitude; see Scott and Tanaka 1986).
The western equatorial region has been the
site of numerous scientific investigations,
because it contains many notable geological
features, including the enormous volcanic rise
of Tharsis (some 5000 km across, rising 10 km
above average elevation) and Valles Marineris,
a series of canyons that together stretch over
4000 km. Tharsis structurally dominates the
western equatorial region for several thousand
kilometres; it is the central source region of
Noachian and Hesperian volcanic- and intrusive-related tectonic activity (see table 1). Tectonic activity produced narrow (<5 km across)
and wide (>5 km) extensional and contractional structures found primarily in older rocks of
Tharsis and surrounding provinces. Associated
with the rise is an enormous radiating system of
agmatic and tectonic activity have
both contributed significantly to
the surface geology of Mars. Digital
structural mapping techniques have
now been used to classify and date
centres of tectonic activity in the
western equatorial region. For example,
our results show a centre of tectonic
M
3.20
grabens that extends outward for thousands of
kilometres. For example, Claritas Fossae (the
southern arm of the Tharsis axial trend of
faults) is a significant recorder of Tharsis activity because it forms one of the densest, longlived system of faults and grabens in the
region. Utilizing Geographic Information Systems (GIS), Dohm et al. (1997) determined that
Tharsis-centred faulting at Claritas commenced
during the Early to Middle Noachian, declined
during Late Noachian and Early Hesperian,
and substantially diminished during Late Hesperian and Early Amazonian (billions of years
of activity: see table 1). The most conspicuous
radial structures are the huge rift systems associated with Valles Marineris (figure 1), which
extend several thousand kilometres east from
Tharsis. Valles Marineris apparently developed,
in part, along fault systems associated with the
early development of the Tharsis rise.
Previous investigations of Tharsis-dominated
structural features in the western equatorial
region have identified specific centres of tectonic activity based on orientations of radial
grabens and concentric wrinkle ridges. In our
study, we analysed the largest, most comprehensive palaeotectonic dataset of the western
equatorial region ever assembled, to unravel its
complex geologic history (more than 25 000
tectonic structures were mapped, characterized, and relative-age dated). These data permitted identifying and time-constraining
regional and local centres of tectonic activity
(see below). Our work has revealed numerous
centres of tectonic activity through time, which
has significant implications for the historical
development of the Tharsis region as well as
other important Martian features such as
Valles Marineris, nearby outflow channels, and
activity at Valles Marineris, which may
be associated with uplift caused by
intrusion. Such evidence may help
explain, in part, the development of the
large troughs and associated outflow
channels and chaotic terrain. We also
find a local centre of tectonic activity
near the source region of Warrego
small valley networks such as Warrego Valles.
The following overview cites examples of significant centres that were identified from
detailed mapping and analysis of our palaeotectonic dataset (Anderson et al. 1998).
Our technique
For this investigation, 25 413 features of the
western equatorial region were mapped, digitized, characterized with respect to type, and
relative-age dated using Viking 1:2 000 000scale photomosaic base maps; the data were
then compiled into spreadsheets. In this digital
format, the tremendous amount of data was
easily manipulated. For simplification, sinuous
features were subdivided into multiple linear
segments along natural breaks in trend, and
each structure was carefully checked and verified. For regional age correlation, stratigraphic
and cross-cutting relations among the rock
units (e.g. Scott and Dohm 1995) permitted us
to construct a map of the faults and grabens as
they prevailed during six successive stages
(Anderson et al. 1998). The six stages are
defined by major periods of geologic activity
largely correlated with those at Thaumasia
(Dohm and Tanaka 1996), which are recorded
in sets of rock-stratigraphic units of the western equatorial region. Each unit was assigned a
stage based on crater densities and stratigraphic and structural relations, and their stratigraphic positions are correlative with the Martian stratigraphic scheme (Tanaka 1986). This
paper addresses Stage 2 (Late Noachian to
Early Hesperian; Dohm and Tanaka 1996) features, which includes Valles Marineris and the
source region of Warrego Valles.
We utilized a computer program developed
by Anderson and Peer (1995) to identify con-
Valles. Here, we suggest that the valley
system may have resulted largely from
intrusive-related hydrothermal activity.
We hope that this work, together with
the current Mars Global Surveyor
mission, will lead to a better understanding of the geological processes
that shaped the Martian surface.
June 1998 Vol 39
Planetary mapping
mapping of Mars
80°
75°
70°
65°
–5°
–10°
–15°
1 Merged colour with Mars Digital Image
Mosaic (MDIC) of Valles Marineris. The 5°
grid squares are roughly 298 km wide.
Table 1: Model chronologies for the Moon and Mars
Martian epochs
model 2
2.30
0.70
n
nia
zo
ma
te A
La
0.25
n
onia
maz
le A
Midd
0.70
nian
mazo
A
Early
4.40
3.55 3.70 3.80
ian
sper
He
Late
1.80
rian
Ear
3.10
h
ac
h
ac
spe
e
ly H
ian
ian
t
o
eN
La
d
dle
Mi
No
n
hia
ac
o
yN
rl
Ea
3.50
model 1
Eratosthenian
Copernican
1.10
0
1
Imbrian
3.20
lunar periods
3
2
Nectarian
pre-Nectarian
3.85 3.92
4
4.6
age (billions of years)
Absolute ages for lunar periods from Wilhelms (1980, 1987). Inferred ages for Martian Hesperian and Amazonian epochs are based on crater densities of series
and correlation to model chronologies of Neukum and Wise (1976) for model 2 and Hartmann et al. (1981) for model 1. Because of the obliteration of smaller
Noachian craters, these models could not be used directly for Noachian boundaries. Neukum and Wise (1976) assigned a 4.4 billion-year age to cratered
terrain material (Middle Noachian rocks), but gave no age range; thus the boundaries of this epoch are dashed. Noachian ages derived in this paper are
compatible with the Hartmann model and are combined with the Hesperian and Amazonian ages of that model (after Tanaka 1986).
June 1998 Vol 39
3.21
Planetary mapping
Implications of our work
Key pieces of evidence (centres of tectonic
activity expressed via fault intersection contour
anomalies; figure 2) combined with other geological information help to explain the development of some of the more notable features
of the western hemisphere, including Valles
Marineris and Warrego Valles. Until recently,
assessing the roles of individual tectonic events
in terms of cause and effect has been difficult
because the tectonic activity of Mars is so
closely related in space and time to the planet’s
volcanic evolution; a close association exists
between radiating fault systems and volcanoes
in the western equatorial region. Furthermore,
deep-seated igneous intrusive bodies have been
noted in the western equatorial region; their
locations have been previously inferred where
fault swarms have been deflected around their
central cores (Scott and Dohm 1990). Many of
the putative intrusives have no observable
topographic or other surface expression and
are not associated with lava flows. On the
other hand, several sites of tectonic, possibly
intrusive-related, activity are near topographic
highs and associated with fault and rift systems, such as a regional Stage 2 centre of our
study near the south-central part of Valles
Marineris (figure 2; close proximity to that
defined by Scott and Dohm 1990) and a Stage
2 local centre near the source region of Warrego Valles in the Thaumasia region of Mars
(figure 2; also see Dohm et al. 1998). Curiously, during a recent pass by Mars Global Surveyor, the Mars Orbiter Laser Altimeter
(MOLA) identified a possible topographic rise
in the central part of Valles Marineris (Herb
Frey, oral commun. 1998). Although interesting, with only one pass it is too early to tell
how far the rise extends and whether it represents tectonic uplift (Witbeck et al. 1991).
3.22
2 Frequency plot of
projected Stage 2
features for the
western equatorial
region of Mars.
Contour interval
equals 50 projected
radial features.
Tectonic centres
(approximately 300
radial features) for
Valles Marineris
(VM) and for
Warrego Valles
Source Region
(WVSR) are
shown here.
60
latitude
20
100
40
100
centrated regions (centres) of tectonic activity.
In this technique, all mapped features on the
surface are projected as vectors; this method is
very similar to plotting linear data on a beta
diagram (for example, see Golombek 1989)
except that the data can be projected onto a
two-dimensional surface for comparison with
pre-existing maps. Lengths and orientations of
each feature were then calculated relative to a
central line of longitude. The study area was
then divided into a grid system (for this study
1–5° increments were used) and examined for
possible centres of tectonic activity. Spherical
trigonometry was used to determine which features projected radially to each grid point. The
number of lineaments radial to each possible
centre area was then determined and contoured. The area which contained most radial
lineaments was chosen as the primary centre of
tectonic activity for the region, although secondary centres can also be identified.
0
VM
–20
100
–40
WVSR
–60
–150
–100
longitude
Valles Marineris appears to have developed,
in large part, along fault systems associated
with the early development of the Tharsis rise
(Plescia and Saunders 1982), which began to
form during Stage 2 (Dohm and Tanaka 1996).
Additionally, rifting, magma withdrawal, and
tension fracturing have been proposed as possible processes involved in the initiation and
development of the canyons. Lucchitta et al.
(1992) noted that the depth of the large
troughs may have been caused by (1) collapse
of near-surface materials due to withdrawal of
underlying material or opening of tension fractures at depth; (2) development of keystone
grabens at the crest of a bulge; or (3) failure
and subsequent drifting of plates. Lucchitta
(1987) also recognized that many of the valley
faults associated with Valles Marineris may
have been associated with volcanic activity.
The tectonic centre at Valles Marineris identified by our work, which we interpret as a possible site of uplift related to intrusive activity
(Stage 2 through possibly Stage 4 activity of
Dohm and Tanaka 1996), may help explain, in
part, the development of the large troughs and
the associated outflow channels and chaotic terrain. At this time, it is difficult to determine the
size of the intrusive body or bodies that may
have contributed to the surface expression of
the Valles Marineris region. For example,
although we have generated a comprehensive
palaeotectonic database, our techniques are still
limited because a substantial portion of the evidence (fault intersections) may have been
destroyed during the development of the canyon
systems. However, we believe that our structural evidence along with Mars Orbital Camera
(MOC) images and MOLA data of the current
Mars Global Surveyor (MGS) mission will help
better constrain such enigmatic features.
Finally, we have identified a local centre of
tectonic activity near the source region of
Warrego Valles, which is at the southern limit of
the Thaumasia Plateau. Although precipitation
as a resurfacing agent has been suggested to
–50
0
explain the formation of Warrego Valles, the
geological evidence indicates that Warrego may
have developed as a result of Stage 2 intrusiverelated hydrothermal activity. In addition to
identifying a local centre of tectonic activity
near the source region of Warrego (figure 2),
geological and geographical arguments are as
follows: (1) Warrego Valles formed concurrently with nearby Stage 2 fault and rift systems and
collapse pits and depressions, and (2) Stage 2
faults appear deflected about and absent within
the source region such as at other proposed sites
of intrusive activity on Mars. Such proposed
sites of possible intrusive- and tectonic-related
hydrothermal activity may have produced
zones of mineral alteration that can be searched
for in MGS data. Additionally, these sites may
be optimum for future hydrological, mineralogical, and exobiological investigations. ●
J M Dohm and K L Tanaka work at the US Geological Survey, Flagstaff, AZ 86001; R C Anderson
works at the Jet Propulsion Laboratory, Pasadena,
CA 91011, USA. [email protected]
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