AN: T31A-1264 [Abstracts]
TI: Marginal Stability of Thick Continental Lithosphere
AU: *
Cottrell,
E EM: liz@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, RT 9W , Palisades, NY 10964 United States
AU: Jaupart, C
EM: jaupart@ipgp.jussieu.fr
AF: Institut de Physique du Globe, 4 pl. Jussieu, Paris, 75252 France
AU: Molnar, P
EM: molnar@cires.colorado.edu
AF: Dept of Geological Sciences and Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Benson Earth Sciences Building, Campus Box 399, Boulder, CO 80309 United States
AB: Xenoliths record a relationship between lithospheric mantle density, age, and thickness such that Archean-aged lithosphere is significantly thicker and chemically more buoyant than Proterozoic-aged lithosphere, and Proterozoic lithosphere in turn is thicker and more buoyant that Phanerozoic-aged lithosphere. The alteration of ancient lithosphere within plate interiors, independent of spatial proximity to continental margins, suggests that lithosphere may not be perfectly stable. These observations pose the question: what controls the thickness and stability of cratonic lithosphere? We argue that subcontinental mantle lithosphere, not just on its margins, but also within its interior, has persisted in a thermal and mechanical state near the threshold of stability, such that the local Rayleigh number for the lithosphere is close to a critical value, $Ra_c$. Recognizing that both chemical and temperature differences affect stability, we carried out laboratory and numerical experiments to examine how $Ra_c$ depends on chemical differences in density. We find that when a chemically buoyant layer overlies a hotter but otherwise denser layer, analogous to continental mantle lithosphere over asthenosphere, convective stability depends strongly on both the critical Rayleigh number and the buoyancy number, $B$, of the lithosphere-like layer. Sufficient cooling at low buoyancy number results in an oscillatory convective instability whereby the colder, more viscous, but chemically lighter layer is drawn into zones of downwelling flow adjacent to laterally extensive zones of upwelling. The critical Rayleigh number for instability increases with the buoyancy number from as little as $\approx 30$ for $B=0$ to $\approx 1000$ for $B\approx 0.5$. Applied to continental lithosphere in a thermal and mechanical state near the instability threshold, this relationship implies that the lithospheric thickness \emph{must} decrease as the mean density of the lithospheric mantle increases, consistent with the geological record. The experiments emphasize that stability must be assessed not only as a function of buoyancy, but also as a function of Rayleigh number. Insofar as continental lithosphere is in a state close to the threshold of instability, more depleted, less dense lithosphere corresponding to large $B$, requires a greater local Rayleigh number to become unstable. The dependence of $Ra_c$ on $B$ near the threshold of instability can therefore account for the existence of multiple stable states and the relationship between thickness and intrinsic density.
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8125 Evolution of the Earth
DE: 8159 Rheology--crust and lithosphere
DE: 8162 Rheology--mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting