Regional Structure and Tectonics of Venus Inferred from Admittance Analysis of Gravity and Topography.
The Pioneer Venus orbiter provides us with two important data sets pertaining directly to the tectonics on Venus. These are the altimetry and the line-of-sight (LOS) gravity, which holds information on the subsurface structure. Unfortunately, the LOS gravity is not amenable to standard techniques of gravity interpretation. I have divided this problem into three steps. The first step consists of transforming the LOS gravity to an estimated vertical gravity field at a constant altitude for a number of regions on Venus. This was done by inverting the LOS gravity to solve for an equivalent surface mass distribution, using the surface topography as an a priori constraint on the smoothness and amplitude of the solution. The vertical gravity was then calculated from this equivalent mass distribution. The second step is an admittance analysis of the resulting vertical gravity and the topography. This analysis compares the gravity and topography as a function of wavelength. From the relationship between gravity and topography, and the variation of this relationship with wavelength, various parameters can be estimated, especially the effective depth of compensation and the flexural rigidity. The third step consists of an analysis of these results, and comparison with known regions on Earth and with models of various tectonic processes. The results indicate a number of intriguing possibilities. The simplest models of loading, flexural rigidity, and compensation do not seem to be capable of providing an explanation for any of the regions. Models of surface loading positively correlated with deep support can give a good fit to the data. This model indicates the possibility of topographic support originating in the mantle, correlated with magmatism occurring over convective upwellings. Another possibility is dynamic support of topography with no association to surface or near surface magmatism. Simple dynamic models do not fit the data well, but inclusion of internal density interfaces, viscosity variations, and non-linear interactions with the thin but horizontally immobile lithosphere could explain the descrepancies. This appears to be the direction which future study should take.
- Pub Date:
- December 1987
- Geophysics, Physics: Astronomy and Astrophysics