166 Encyclopedia of the Solar System
FIGURE 17 Radar image of a
ridge belt in Atalanta/Vinmara
Planitia, approximately 85× 110
km in dimension and centered at
41 ◦N, 196◦E. The belt comprises a
series of northeast-trending ridges.
There are both very narrow ridges,
down to the resolution of the data,
with ridges several kilometers wide.individual fractures and the long, narrow topographic highs
that comprise individual ridges, ridge belts are interpreted
to be a result of compressional stresses (see Fig. 17). Indi-
vidual ridges are typically less than 0.5 km high, 10–20 km
wide, and 100–200 km long with a spacing of∼25 km. The
two largest concentrations of ridge belts occur in Atalanta/
Vinmara Planitiae and Lavinia Plantia. The belts in Atalanta/
Vinmara Planitiae are roughly an order of magnitude larger
than those elsewhere. Belts in Livinia are unusual in that
they have extensional fractures roughly parallel to com-
pressional features within the same belt, possibly due to
topographic uplift along the ridge. Larger belts are be-
lieved to result from mantle downwelling, similarly to the
proposed downwelling origin for crustal plateaus, but with
lower strain. Smaller belts may be associated with more
local scale tectonics.
Wrinkle ridges are extremely common features on Venus
and are also interpreted as simple compressional folds and
or faults but are much narrower (∼1 km or less in width)
than ridges. They have positive relief, based on the fact
that lava flows can be seen to pond against some wrinkle
ridges, but that relief is too small to be seen inMagellan
altimetry. Most ridges occur in evenly spaced set, 20–40 km
apart. These sets of wrinkle ridges can be local in nature,