Tectonics of one-plate planets.

Abstract

The Voyager 2 encounter with Neptune and its moons in August of 1989 completed the discovery phase of planetary exploration. In the 25 years since Mariner 4 returned the first images of another planet, geophysical models for such basic processes as mantle convection and loading which were developed for the Earth have been strained beyond their limits by features such as the Tharsis rise on Mars and the coronae of Miranda which cover as much as a quarter of their planetary circumference. In this work I develop a general planetary shell model in spherical coordinates that is capable of treating shells of arbitrary thickness and driving forces of arbitrary breadth. I then present a methodology for finding the forces exerted on the shell from two processes. I first develop a treatment for mantle convection driven by a density anomaly within a viscous mantle. This model is applied to the small moon of Uranus, Miranda, to study the three large coronae which dominate its surface and for which several competing hypotheses were offered, two of which invoked mantle convection driven by density anomalies of opposite sign. I then develop a general model for loading of the lithosphere and examine the effects of a range of load breadths and lithosphere thicknesses. I map out the combinations of these two variables where classical approximations such as the flat-plate and thin-shell models are applicable as well as determine the nature and extent of the transition between these two regimes. Finally, I employ finite element modeling to investigate the coronae on Venus, showing that morphological aspects of these features reported in the literature can be produced by flexure of the lithosphere beneath a volcanic load and gravitational sliding of a cooled crust off these volcanic mounds. I then, however, produce independent characteristic topographic profiles for three of the more regular coronae which question how typical the reported morphologies are in the coronae in general.