Venus: Surface and Interior 163
the heavily cratered southern highlands. On Earth, plate
tectonics is clearly dominant. Tectonic features on Venus are
highly variable and enigmatic. Tessera terrains are unique
to Venus and are defined as having multiple intersecting de-
formation directions. One possible factor in creating these
highly deformed regions is that Venus experiences very lit-
tle surface erosion. In contrast, most continental regions
have experienced multiple episodes of deformation, but
surface structures are often eroded between events, leaving
evidence of only the most recent occurrence. Many of the
tectonic features on Venus are continuous for thousands of
kilometers and likely reflect underlying mantle processes
including upwelling, downwelling, and horizontal flow. We
describe next the characteristics and likely origins of the key
types of tectonic features on Venus.
7.1 Tessera and Crustal Plateaus
Tessera terrains are highly deformed and thus stand out as
very bright in radar images (Fig. 13). They are made up of
both extensional and compressional deformational features.
Each set of lineations may represent a separate deforma-
tion event, or two sets may form simultaneously if shear
deformation is involved. In some cases, the sequence of
events can be determined, but more often it is ambiguous.
Tesserae occur both as isolated fragments embayed by later
plains material and in major plateaus. There are 6 major
crustal plateaus: Alpha, Ovda, Pheobe, Thetis, and Tellus
Regiones plus Ishtar Terra. Figure 14 shows Alpha Regio,
one of the smaller highland plateaus. Western Ovda Re-
gio may be a relaxed crustal plateau. These plateaus are
1000–3000 km in diameter and 0.5–4 km higher than the
surrounding plains. Their gravity signature indicates that
they are supported by crustal roots rather than active man-
tle processes.
Ishtar Terra is unique among the highland plateaus. It
is the largest of the crustal plateaus, and is surrounded by
significant mountain belts on 3 sides, with large areas of
tesserae occurring on their exterior flanks. They are Venus’
only real mountain belts. Lakshmi Planum makes up the
interior of Ishtar Terra. This smooth plateau is elevated
3–4 km above the surrounding plains and is covered by vol-
canic flows. The Maxwell Montes to the east of Lakshmi
Planum contain the highest point on Venus, at approxi-
mately 11 km above the mean planetary radius (see Fig. 6).
Although other crustal plateaus tend to have relatively flat
interiors and rims of higher topography, no other crustal
plateau is as extreme as Ishtar Terra in terms of its diame-
ter, elevation, and circumferential deformation features.
Crustal plateaus have been proposed to form over man-
tle upwellings and over mantle downwellings. In the man-
tle upwelling scenario, a plume creates a crustal plateau
though decompression melting above the plume head, anal-
ogously to plateaus formed on the terrestrial seafloor. De-
formation occurs as the topography viscously relaxes. The
FIGURE 13 This radar image of a portion of Tellus Regio is
centered at 36◦N, 79.4◦E and is approximately 340×420 km.
The area is deformed by a northeast and a northwest set of
lineations. Locally, each set contains both narrow, linear
fractures resembling extensional graben and areas where the
fractures coalesce into ridges and appear to be compressional
ridges. In the northern section of the image, a third set of very
narrow north northwest-trending fractures cross cuts the other
sets. The dark regions are volcanically flooded valleys, with two
small vents visible in the southwest corner of the image.alternative model forms the plateaus above a cold, sink-
ing mantle downwelling. On Earth, both subduction zones
and local sites of downwelling form below cold mountain
roots. Venusian crustal plateaus are proposed to form as a
downwelling causes sinking of the lower lithosphere and
accumulation and compression of the crust at the surface.
The mechanism for forming small, local regions of tessera
is not clear. In many cases, these regions are embayed and
thus appear to be old and possibly inactive. There are few
clues as to original processes that cause deformation. One
possibility is that these areas represent sections of tessera
plateaus that were once elevated but have topographically
relaxed. If plateaus formed in an earlier, hotter time pe-
riod, relaxation may have proceeded more rapidly, allowing
for complete relaxation of plateaus. The semicircular rim
of Western Ovda Terra could be the remnant of a relaxed
plateau. Alternatively, small tessera terrains may be a result
of an entirely different type of tectonic event, such as ridge
belt formation.