Francis Nimmo - PhD Thesis

I completed a Shell-sponsored PhD studentship at Cambridge University, entitled Volcanism and tectonics on Venus and supervised by Dan McKenzie, in October 1996. The abstract of this dissertation is as follows:

Venus is the most similar planet to Earth in terms of size, mass and composition, and one of the best studied. The aim of this dissertation is to investigate volcanism and tectonics on Venus using a combination of geophysical techniques and terrestrial analogues.

Devana Chasma, a rift valley on Venus, has similar fault lengths and vertical fault offsets to the East African Rift on Earth but has a wider (150 km) graben. The two areas have similar elastic thicknesses of about 30 km. Because of the wider rifts and absence of sedimentation, faults on Venus must be capable of withstanding stresses of about 80 MPa, compared with 10 MPa on Earth. Faults are probably weak on Earth because pore fluid pressure reduces the effective normal stress on a fault and hence its frictional resistance to motion; this effect is absent on Venus.

An axisymmetric isoviscous convection model is used to reproduce three probable mantle plume features, using topographic uplift, gravity and melting as constraints. The melt constraint requires the mechanical boundary layer thickness to be 150 km or less. Compared to Earth, the internal mantle potential temperature is similar (about 1300 C) and the mantle viscosity an order of magnitude higher (about 3x10**20 Pas). The surface heat flux is about 20 mWm**-2 , roughly half the heat generation rate, whereas on Earth it is about double.

A parameterized thermal convection model is used to investigate the effect that a cessation of plate tectonics would have on a planet's mantle temperature. For a two-layered isoviscous mantle, the upper mantle temperature changes by 100-700 C over 1 Ga, whereas the temperature change is less than 60 C for a single-layered mantle. A 200 C temperature change will have major effects on mantle and lithosphere properties, and on melt generation.

The high fault strength and ~30 km thick, buoyant basaltic crust on Venus make the initiation of subduction difficult. The only mechanism capable of doing so is drag by downwelling mantle, though subduction would be aided by the transition of basalt to eclogite. If the upper mantle temperature were increased, fault friction would be reduced and the basalt-eclogite transition speeded up, possibly making subduction easier. A period during which plate tectonics occurred can account for the inferred rapid (<100 Ma) global resurfacing event and mantle cooling, though it is not clear what could have caused the plate tectonics to cease.

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This page was last modified on 13 March 2003.