HR: 12:05h
AN: T52B-08    [Abstracts]
TI: The Role of the Gravitational Instabilities, the Density Structure and the Extension Rate in the Evolution of Slow Continental Margins and Rifts
AU: * Burov, E
EM: evgenii.burov@lgs.jussieu.fr
AF: Lab. of Tectonics, Case 129, T46-00, E2, University of Paris VI, Paris, 75252 France
AB: Formation of rifted continental margins is preceded by localized thinning and break-up of the continental lithosphere and by upraise of the lithosphere-asthenosphere boundary. Thinning creates steep density and viscosity contrasts between the deformed mantle/crustal lithosphere and the upwelling asthenosphere. This provides conditions for development of positive and negative Rayleigh-Taylor instabilities. The evolution of many continental margins (e.g., Newfoundland/Iberian Margin) is characterized by slow spreading rates. This allows the RT instabilities to grow at time scale of rifting. The impact of positive instabilities (asthenospheric upwelling) is well-studied. The negative RT instabilities, associated with lithosphere mantle downwelling is an overlooked factor. These instabilities may control rift evolution, in particular, mantle thinning or thickening below the rift flanks. Our numerical experiments show that the ratio of the RT-growth rate to the extension rate controls rift geometry and evolution. The effect of negative RT instabilities is most important for slow extension rates (2-5mm/y, Deborah number, De < 1) but it is still significant for 2-3 times higher extension rates (2-15 mm/yr, De < 10). The numerical experiments for extension rates of 2-15 mm/yr and mantle-asthenosphere density contrasts of 10-20 kg/m3 demonstrate a number of structural similarities with slow continental margins (e.g., Flemish Cap and Galicia margin). In particular, rift asymmetry results from interplay between the RT instabilities and differential stretching at De < 1. Formation of interior basins occurs at De ~ 1-3. The best correspondence with the observed geometry of rifted margins is obtained for mantle-asthenosphere density contrast of 20 kg/m3 and extension rate of 2-15 mm/yr, which is 2 times higher than the averaged values inferred from observations. This suggests that slow margins could be initiated at higher extension rates; these rates decrease at later stages of rift evolution. The influence of RT instabilities is strongly controlled by extension rate, density, rheology and thermal structure of the lithosphere. This appeals for finding better constraints on these parameters from observations.
DE: 8100 TECTONOPHYSICS
DE: 8109 Continental tectonics: extensional (0905)
DE: 8160 Rheology: general (1236, 8032)
SC: Tectonophysics [T]
MN: Fall Meeting 2005