HR: 1340h
AN: V43B-1373    [Abstracts]
TI: Physical Controls on the Thickness of Stable Continental Lithosphere
AU: * Fourel, L
EM: fourel@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252, France
AU: Farnetani, C G
EM: cinzia@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252, France
AU: Jaupart, C
EM: cj@ccr.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252, France
AB: Formation of cratonic mantle lithosphere is achieved through melt depletion generating a residue that is less dense and more viscous than the underlying convecting mantle. These characteristics act to stabilize cratonic roots against convective overturn. Stability, however, also depends on thickness, which provides independent constraints on the sequence of events that lead to stable roots. The stability problem of intrinsically buoyant lithosphere which is cooled from above has been studied with both laboratory experiments and numerical simulations in 2-D and 3-D involving materials with either constant or temperature-dependent viscosity. Stability of thick lithosphere depends weakly on the viscosity contrast and strongly on the density contrast between root material and the underlying mantle. The threshold thickness for a stable root increases with intrinsic buoyancy, which may account for the differences that are observed between Archean and Phanerozoic continental roots. Stable thickness values may a priori be achieved by thickening an initially thin root or thinning of an initially thick root. Both processes may involve melting, however, which generates root material with a different density. Thus, stablilization involves changes of both thickness and density and is best discussed in (thickness, buoyancy) space. Instability of a root leads to both downwellings and upwellings, and hence to partial melting and depletion in the latter. This acts to increase simultaneously buoyancy and thickness. Mixing processes are also significant. At low viscosity contrast, instability leads to mixing with the underlying fluid and to a thinner layer. At large viscosity contrast, material underlying the root gets entrained into the root, leading to a thicker layer. Different scenarii for the stabilization of a continental root that are consistent with dynamical constraints will be presented. They can be separated by the sequence of events (i.e. thickening or thinning, increasing or decreasing density contrast) as well as by the characteristic times and dimensions of successive instabilities.
DE: 8103 Continental cratons
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8130 Heat generation and transport
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting