Geology Reference
In-Depth Information
Figure 7.16. A small shield volcano on the plains south of Pavonis
Mons, showing multiple thin lava
flows erupted from an elongate
central vent (ESA HRSC frame 0891).
Figure 7.17. The Amenthes Rupes lobate scarp stands higher than
1 km and is thought to be a low-angle thrust fault; the area shown is
about 500 km by 500 km (NASA Viking Orbiter mosaic).
periglacial, and possibly
fluvial and lacustrine processes,
the bedrock is considered to be volcanic because
ow
lobes are seen in some areas and remote sensing data
are indicative of basaltic compositions. The
Extensional structures include joints, grabens that are as
large as several kilometers wide, hundreds of meters deep,
and tens of kilometers long, and troughs (chasmata)of
Valles Marineris that are 1,000 km long and as wide as
100 km. The smaller extensional features are thought to
involve high-angle faults that, when projected to the subsur-
face, might converge at depths as great as several kilometers.
The larger troughs of Valles Marineris could re
ect fractur-
ing completely through the lithosphere to greater depths. On
the other hand, some extensional features could reflect more
shallow deformation resulting from intrusion by dikes, or
swarms of dikes, including parts of the Valles Marineris rift
system.
Compressional features include ridges and lobate
scarps. The ridges are comparable to mare ridges on the
Moon and are linear-to-arcuate features as large as 100 m
high, several hundred kilometers long, and more than
10 km wide. As on the Moon, they often consist of a
broad, gentle
“
question-
able
category of volcanic material
(Table 7.2)
refers to
deposits found around some of the large shield volcanoes
and mapped as aureole material. These deposits are inter-
preted as being debris mass-wasted from the
flanks of the
volcanoes, perhaps comparable to the lahars seen on
Earth which consist of volcanic mud
ows triggered by
eruptions.
The presence of water and ice on and near the surface, at
least in the past, suggests that magma
-
water interactions
were common. Such interactions would explain the forma-
tion of small cones in many areas, as well as the explosive
eruptions to form the ash shields in the Circum-Hellas
Volcanic Province. In addition, the Medusae Fossae
Formation in the equatorial region has been suggested to
be a vast volcanic ash deposit that could have involved
volatile-rich explosions.
”
surmounted by a narrow crenu-
lated ridge and are considered to reflect folded layers of
rocks, such as lava
flows, underlain by thrust faults. The
Hesperian ridged plains in the southern hemisphere and
Lunae Planum just north of the equator are characterized
by ridges and are inferred to be thick basalt
ows that were
subjected to regional compression.
Lobate scarps on Mars
(Fig. 7.17)
are comparable to
those on Mercury and can reach heights of several
“
swelling
”
7.5.4 Tectonic features
Surface features resulting from tectonic deformation on
Mars include both compressional and extensional structures.
These features are found in association with volcanoes,
impact structures, local deformation, and regional-scale
deep-seated deformation of the lithosphere.
