Previous to the Magellan mission, the only well-constrained
estimate of the flexural rigidity of Venus' lithosphere was of an
area north of Freyja Montes, interpreted as a flexural bulge due
to underthrusting of lowland plains under the mountain belt
[ Solomon and Head, 1990]. Using Venera 15/16
topography data, these workers found best-fitting elastic
thicknesses of approximately 11--18 km and thermal gradients near
the expected global mean (14--23 K km
). The spatial
resolution and incomplete coverage of Venera 15/16 data generally
did not allow similar measurements to be carried out elsewhere.
Magellan altimetry permits similar analysis to be carried out at
many suitable locations on Venus; thus far, most are in the
topographic annuli of coronae. Magellan observations of over 300
coronae have revealed a great deal of morphologic diversity
[ Stofan et al., 1992]. Within that diverse group
are a number of relatively large coronae (typically >300 km in
diameter) that are characterized by a deep topographic trough (or
chasma) with an outer rise. Noting the similarity between the
outer rise/trough topography and examples of flexural topography
on Earth, Sandwell and Schubert [1992] modeled
the trench-outer rise topography as a thin-plate flexure due to a
line load and bending moment. They chose four large coronae
(Eithinoha, Heng-o, Artemis, and Latona) and found best-fitting
elastic thicknesses of 15, 40, 37, and 35 km, respectively. The
larger elastic thicknesses require extremely low thermal gradients
(<10 K km
) and heat flows of less than half the expected
planetary average. For comparison, the elastic thickness at Hawaii
is estimated to be 
km [ Watts and Cochran,
1974] and at subduction zones may reach values as large as 50 km
[ Caldwell and Turcotte, 1979].
Brown and Grimm [1993] have examined an inelastic
model (i.e., one allowing for the finite strength of the
lithosphere) for Artemis Corona, and suggest that thermal
gradients of <5 K km
and the addition of unrealistically
high in-plane stresses are required to reproduce the observed
topography. It is worth noting that none of these studies consider
the effect of finite (as opposed to infinitesimal) amounts of
flexure of the lithosphere, which might be expected to relax the
constraints cited by Brown and Grimm [1993]. A
subduction hypothesis for chasmata (i.e., deep topographic
troughs) in the vicinity of Latona and Artemis Corona has also
been criticized as failing to explain aspects of the deformational
features observed in Magellan images [ Hansen and
Phillips, 1993], although the detailed morphologic analysis did
not match the regions examined by Sandwell and
Schubert [1992].
Johnson and Sandwell [1994] identified an
additional 12 possible sites of flexure, and found somewhat
smaller values of elastic thickness (12--34 km), corresponding to
thermal gradients in the range of 8--14 K km
. Possible
implications of the low thermal gradients thus obtained are
discussed below.
In addition to requiring extremely strong lithospheres and low thermal gradients, Sandwell and Schubert's [1992] flexural model requires the presence of a load sufficient to create a large bending moment. The author's favored explanation was that the outer rise of these large coronae was a manifestation of retrograde subduction of the lithosphere beneath the corona. More recent analyses of Magellan gravity data (see below) appear broadly consistent with such an hypothesis.