The discovery of regions of complex ridged terrain (or tessera) was among the important findings of the Venera 15/16 mission [ Barsukov et al., 1986; Basilevsky et al., 1986]. These regions, defined by the presence of two or more distinct sets of intersecting tectonic features, were found to dominate much of the highland terrain at the high northern latitudes covered by the Venera data. Magellan data showed that complex ridged terrain (CRT) dominates several other highlands and made it clear that a class of highlands distinct from volcanic rises exists on the surface of Venus. These plateau highlands are most distinctly characterized by a plateau shape in topographic cross-section, surfaces predominately consisting of complex ridged terrain [ Bindschadler et al., 1992a], and relatively low ratios of geoid anomaly to topography [ Smrekar and Phillips, 1991]. The high resolution of Magellan data also confirmed the idea that most regions of CRT are best described by a history of extensive and intensive shortening followed by relatively minor extensional deformation [ Bindschadler et al., 1992a,b; Solomon et al., 1992]. Although plateau highlands and complex ridged terrain are stratigraphically older than surrounding geologic units nearly everywhere on Venus, there is disagreement as to whether that relative age difference is significant and quantifiable on the basis of the cratering record.
Two distinct kinds of models have been proposed to explain the origin of plateau highlands. In the coldspot model, downwelling mantle flow is transmitted via shear and normal stresses into the lithosphere, causing failure of lithospheric rocks, convergence, and thickening of crustal material. The result of such a process is an intensively deformed crustal plateau [ Bindschadler and Parmentier, 1990]. A second kind of model proposes that plateau highlands are the result of a mantle plume-related magmatic event, which extrudes and intrudes sufficient partial melt to create a large region of thickened crust, perhaps analogous to oceanic plateaus such as Ontong-Java. Originally, deformation in this hotspot model was proposed to result from cooling and subsidence of such a plateau [ Herrick and Phillips, 1990] but such mechanisms are inconsistent with the observed deformational characteristics of plateau highlands. Another possibility is that regions of thick crust absorb inordinate amounts of strain because they are inherently weak [ Solomon et al., 1992], although this model has yet to be tested quantitatively. Under such a view, the formation of a crustal plateau and its deformation (due to regional stress fields of unspecified origin) are two distinct, unrelated events.
Both the coldspot and hotspot models have been criticized for a
number of reasons; a detailed and complete discussion is beyond
the scope of this review. Hotspot models have generally been
criticized for their inability to explain the observed
deformational features, and because of a lack of highlands that
are transitional in geological or geophysical characteristics
between volcanic rises and plateau highlands. Coldspot models have
been criticized because the time required to thicken crust via
downwelling is long, particularly if thermal gradients and crustal
temperatures are relatively low, and because of a lack of features
transitional between plateau highlands and lowlands (both in terms
of geology and admittance of gravity and topography). The former
concern may be mitigated if highlands formation occurred primarily
during an epoch of high heat flow (i.e., prior to 300 Ma). The
implications for either model (or for the above criticisms) of a
global resurfacing event at 
Ma have yet to be fully
addressed. The most distinct difference between the two is the
unlinking of deformation responsible for CRT from the formation of
a region of thickened crust as espoused by the hotspot model; the
coldspot model explicitly connects the formation of these two
aspects of plateau highlands. This author suggests that the
correlation between topographic and structural trends in plateau
highlands and the remarkable similarity in morphology and relative
ages of structures in such widely separated highlands as Thetis
and Alpha Regio tend to favor a coldspot origin of plateau
highlands.