Relaxation of compensated topography and the evolution of crustal plateaus on Venus

Abstract

[1] Crustal plateaus, dominant physiographic features on Venus, likely originate through dynamic mantle processes, although a debate exists on whether they formed by mantle upwellings or downwellings. Regardless of the mode of formation, several observations led to the hypothesis that viscous relaxation may be the driving force behind the apparent evolutionary sequence from a high-standing plateau to a low-standing plateau with elevated margins. We apply analytic and finite element models to test this hypothesis for isostatically compensated topography, as modeling of gravity data suggests that crustal plateaus are presently supported by crustal roots. Geotherm values 5 K km À1 combined with a surface temperature of 740 K preclude relaxation within 10 9 years, while geotherm values !20 K km À1 can yield relaxation times of 10 8 years or less. Hence significant relaxation requires hot conditions in order to occur within the appropriate 1-Gyr timescale set by crustal plateau ages. We also show that a compensated plateau can either retain its shape as it relaxes or become more domical in appearance. Mantle temperatures <1400 K allow strong crust-mantle coupling that hinders flow of crustal material from the center of the plateau and produces domical relaxed profiles. Higher temperatures lead to a relatively inviscid mantle and relaxation that is largely insensitive to topographic wavelength, thus preserving the original topographic shape during relaxation. In either case, relaxation of compensated plateaus does not yield elevated rims. We propose that the state of compensation must be considered variable.

Pulsating continents on Venus: An explanation for crustal plateaus and tessera terrains

“…Since we propose that tessera terrain represent low density differentiated crust, the diabase rheology is not a good approach for our model. Although we do not know the real composition of tessera, two facts indicate that we should use viscosities significantly lower that those of the models of Nunes et al (2004) and Nunes and Phillips (2007): (1) The differentiated continental-like composition that we propose for tessera terrains will be softer than the diabase, and (2) the strong relation of continental crust origin and water content on Earth suggests that Venusian continents should not be fully dehydrated.…”

“…The very high viscosity values (10 24 _10 26 Pa s) used by Nunes et al (2004) and Nunes and Phillips (2007), in their modelling of crustal plateaus collapse are not appropriate here, as they used the dry diabase rheology of Mackwell et al (1998) for the plateau crust. Since we propose that tessera terrain represent low density differentiated crust, the diabase rheology is not a good approach for our model.…”

“…But they cannot be too much older than the regional volcanic plains considering that they have approximately the same crater density. The crater density of crustal plateaus indicates that they were probably formed around 500 ma, although, considering that their area is not statistically representative, they could be as much as 1000 ma old (Nunes et al, 2004). If crustal plateaus were formed and deformed diachronously since the early history of Venus they should record different crater densities always higher than the average planetary crater density, which does not occur.…”

“…Hansen and Willis (1996) distinguished between tessera inliers dominated by graben, and tessera inliers dominated by fractures, proposing that the former are remnants of collapsed crustal plateaus and the latter flooded ancient corona-chasmata chains. Nevertheless, the analytical and finite element models performed by Nunes et al (2004) indicate that the range of preserved crustal plateau morphol ogies and tessera inliers is not possible to achieve through lower crustal viscous flow during collapse of a compensated plateau. The analytical and finite element models for the evolution of uncompen sated plateaus of Nunes and Phillips (2007) better address the morphologies of crustal plateaux and tessera terrains although these models do not explain all the extensional structures observed.…”

“…Tessera inliers dominated by extensional tectonics outcropping across the Venusian regional volcanic plains are generally considered remnants of collapsed crustal plateaus (e.g., Bindschadler et al, 1992b;Bindschadler, 1995;Ivanov and Head, 1996;Hansen, 2006;Willis 1996, 1998;Nunes et al, 2004;Nunes and Phillips, 2007). Hansen and Willis (1996) distinguished between tessera inliers dominated by graben, and tessera inliers dominated by fractures, proposing that the former are remnants of collapsed crustal plateaus and the latter flooded ancient corona-chasmata chains.…”