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Séminaire général du Département TAO

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Mantle dynamics and mountain building : a case for convective instability of mantle lithosphere
Peter Molnar (University of Colorado at Boulder)

23 novembre 2004

I ask three questions. (1) Under what conditions is mantle lithosphere sufficiently negatively buoyant that if perturbed, blobs of it sink into the asthenosphere? (2) Where it is unstable, what properties of the earth does the growth of such instabilities constrain? (3) What consequences of this "deblobbing" can we recognize? Tomographic images of the mantle beneath mountain belts and high plateaux do show simple manifestations of subduction of thickening of mantle lithosphere, but instead call for complicated distributions of wave speeds that suggest convective instability. Numerical experiments on Rayleigh-Taylor instability show that non-Newtonian viscosity can retard initial growth of perturbations, but then growth becomes catastrophic, as if unstable growth can be delayed until long after perturbations have been introduced, tens of millions of years after continental collisions. The crust can perturb flow so that descent occurs not beneath the zone of crustal shortening, but beneath its surroundings; hence lithospheric thinning and volcanism are possible where convergence occurs. If blobs of dense material detach and sink, the added potential energy to the lithosphere above can perturb deformation outside the region.

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Peter Molnar Peter Molnar (University of Colorado at Boulder)
Professor of Geological Sciences, University of Colorado at Boulder.