Planetary Geophysics and Tectonics

2 Pages·1997·0.3 MB·English

by  Parmentier, E. M

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Planetary Geophysics and Tectonics
Final Report for grant NAGW-1928
Principal Investigator: E.M. Parmentier
Period covered by report: 11/1/89-10/31/97
Institution: Brown University, Providence RI, 02912
Grant number: NAGW-1928
Research supported by grant NAGW-1928 has addressed a variety of problems
related to planetary evolution. One important focus has been on questions related to the
role of chemical buoyancy in planetary evolution with application to both Venus and the
Moon. We have developed a model for the evolution of the Moon (Hess and Parmentier,
1995) in which dense, highly radioactive, late stage magma ocean cumulates sink forming
a core. This core heats the overlying, chemically layered mantle giving rise to a heated,
chemically well-mixed layer that thickens with time. This mixed layer eventually becomes
hot enough and thick enough that its top begins to melt at a pressure low enough that melt
is buoyant, thus creating mare basalts from a high pressure source of the correct
composition and at an appropriate time in lunar evolution. In work completed during the
last year, numerical experiments on convection in a chemically stably stratified fluid layer
heated from below have been completed. These results show us how to calculate the
evolution of a mixed layer in the Moon, depending on the heat prOduction in the ilmenite-
cumulate core and the chemical stratification of the overlying mantle. Chemical
stratification of the mantle after its initial differentiation is would trap heat in the deep
interior and prevent the rapid rise of plumes with accompanying volcanism. This trapping
of heat in the interior can explain the thickness of the lunar lithosphere as a function of time
as well as the magmatic evolution. We show that heat transported to the base of the
lithosphere at a rate determined by current estimates of radioactivity in the Moon would not
satisfy constraints on elastic lithosphere thickness from tectonic feature associated with
basin loading. Trapping heat at depth by a chemically stratified mantle may also explain the
absence of global compressional features on the surface that previous models predict for an
initially hot lunar interior.
For Venus, we developed a model in which the chemical buoyancy of crust and a
depleted mantle layer stabilizes the lithosphere for long periods of time and provides a
mechanism of episodic planetary evolution (Parmentier and Hess, 1992). Continued
thickening of a residual depleted mantle layer eventually suppresses pressure release
melting and the creation of depleted mantle. Continued cooling then allows the lithosphere
to become heavier than the underlying hotter, undepleted mantle. This repeated instability
can occur on time scales appropriate for episodic global resurfacing on Venus. We have
also examined the role of the gabbro-eclogite phase transformation on crust and lithosphere
stability and as a mechanism of crustal recycling in the absence of plate tectonics. Our work
thus far concentrates on the scale of instability that would occur due to cooling or crustal
thickening associated with horizontal shortening. Whether repeated overturn can explain
the evolution of Venus depends in part on whether sufficient heat transfer can occur
between overturns and on constraints provided by understanding observed surface features
and evolution.
The following publications represent completed work supported by grant NAGW-1928.
M. T. Zuber and E. M. Parmentier, Lithospheric necking: a dynamic model for rift
morphology, Earth Planet. Sci. Lett. 77, 373-383 (1986).
E.M. Parmentier and G.Schubert, Rift propagation, Geophys. Res. Lett. 16, 183-186 (1989).
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M.T. Zuber and E.M. Parmentier, On the relationship between isostatic elevation and the
wavelengths of tectonic surface features on Venus, Icarus 85, 290-30.8 (1990).
C. Sotin and E.M. Parmentier, On the stability of a fluid layer containing a univariant phase
transition: application to planetary interiors, Phys. Earth Planet. Int. 55, 10-25 (1989).
D.L. Bindschadler and E.M. Parmentier, Mantle flow tectonics: the influence of a ductile
lower crust and the formation of topographic uplands on Venus, Jour. Geophys. Res.
95, 21329-21344 (1990).
M.T. Zuber and E.M. Parmentier, On the relationship between isostatic elevation and the
wavelengths of tectonic surface features on Venus, Icarus 85, 290-308 (1990).
E.R. Stofan, D.L. Bindschadler, J.W. Head, and E.M. Parmentier, Corona structures on
Venus" models of origin, J. Geophys. Res. 96, 20933-946 (1991).
E.M. Parmentier and P.C. Hess, Chemical differentiation of a convecting planetary
interior: consequences for a one plate planet such as Venus, Geophys. Res. Lett. 19,
2015-2018 (1992).
D.L. Herrick and E.M. Parmentier, Episodic Large-Scale Overturn of Compositionally
Stratified Mantles in Terrestrial Planets, J. Geophys. Res. 99, 2053-2062 (1994).
J.W. Head, E.M. Parmentier, and P.C. Hess, Venus: vertical accretion of crust and depleted
mantle and implications for geological history and processes, Planet. Space Sci.42, 803-
811 (1994).
P.C. Hess and E.M. Parmentier, A model for the thermal and chemical evolution of the
Moon's interior: implications for the onset of mare volcanism, Earth Planet. Sci. Lett. 134,
501-514 (1995).
M.T. Zuber and E.M. Parmentier, Formation of fold and thrust belts on Venus by thick-
skinned deformation, Nature 377, 704-707 (1995).
S.E. Smrekar and E.M. Parmentier, The interaction of mantle plumes with surface thermal and
chemical boundary layers: application to hotspots on Venus, J. Geophys. Res. 101, 5397-
5410 (1996).
M.T. Zuber and E.M. Parmentier, The role of continuous viscosity stratification in folding
of the lithosphere, J. Geophys. Res. 101, 5489-5498 (1996).