The crust of Venus is widely believed to be composed primarily of
basaltic material (or the intrusive equivalent, diabase). One
major source of uncertainty in models for deformation of Venus
crustal rocks has been the rheology of diabase. Previously, the
best available experimental data suggested that under the high
temperature conditions of Venus, diabase would undergo ductile
deformation at a depth of a few km and would be considerably
weaker than mantle rocks. Thus, if the crust were more than
km thick, a zone of weak lower crust sandwiched between a
stronger, brittle upper crust, and a strong uppermost mantle would
be present on Venus. Such a rheological structure has numerous
implications for Venus tectonics, including the response of the
crust and lithosphere to mantle convective stresses
[ Bindschadler and Parmentier, 1990], and an upper
bound of 
--30 km for crustal thickness on Venus
[ Zuber, 1987; Grimm and Solomon,
1988].
Previous experiments were done using samples that contained far more water than is likely to be present in the arid, high-temperature conditions on Venus. In a recent set of experiments [ Mackwell and Kohlstedt, 1993; Mackwell et al., 1994], diabase samples were thoroughly dried before deformation. The result was a large increase in strength, suggesting that Venus' crustal materials are not significantly weaker than mantle rocks. The new data do not rule out the presence of a ductile lower crustal layer on Venus, but they do suggest that higher temperature gradients and/or lower strain rates are required for crustal rocks to deform in a ductile manner. Moreover, the data indicate that if the lower crust is weak compared to upper crustal rocks, it is not significantly weaker than mantle rocks under the same conditions, and may even be stronger under certain conditions [ Mackwell et al., 1994]. Thus, the thickness of Venus' crust remains quite uncertain.
A related issue is raised by the presence of extreme topographic
slopes in some parts of Venus, most notably in Western Ishtar
Terra [ Ford and Pettengill, 1992;
Kaula et al., 1992]. Western Ishtar Terra is a high plateau,
rising 
km above the nearby lowlands and almost completely
surrounded by mountain belts which rise an additional 2--8 km
above the plateau. The highest part of the region is Maxwell
Montes, which is dominated by compressional tectonic features and
all but devoid of extensional features [ Kaula et
al., 1992]. Topographic slopes commonly exceed 2--3
in the
region and reach values in excess of 20
.
Smrekar and Solomon [1992] used a finite-element approach to
model viscoelastic relaxation of a high plateau such as W. Ishtar
Terra. Results obtained using a now-obsolete (and relatively weak)
flow law for diabase suggested that a region like Maxwell Montes
is either less than 10 m.y. old, or is currently supported by
tectonic stresses. Although these results are not simply
extrapolated to include the new flow law, additional calculations
performed using a relatively stiff websterite flow law (i.e., more
similar to the dry diabase results) [ Smrekar and
Solomon, 1992] suggest that the steep topography of the region
is unlikely to have been supported by the strength of the
lithosphere for 
m.y.
Notwithstanding the new data on the strength of crustal rocks,
observations of tectonic features on Venus indicate a strong
tendency toward spacings of 
--20 km [ e.g.,
Solomon et al., 1992], which are best explained
by tectonic models that include significant weakening of the
lithosphere below depths of a few km. Since a relatively weak
Venus lithosphere is at odds with interpretations of the
present-day gravity and topography, it may be that such features
are a remnant of a past epoch in which thermal gradients were
higher and lithospheric strengths lower [ Zuber,
1994].