HR: 17:25h
AN: T34A-06    [Abstracts]
TI: Lateral variations in mantle viscosity and the lithospheric stress field
AU: * Conrad, C P
EM: cpconrad@umich.edu
AF: University of Michigan, Department of Geological Sciences, 425 E. University Ave., Ann Arbor, MI 48109 United States
AU: Lithgow-Bertelloni, C
EM: crlb@umich.edu
AF: University of Michigan, Department of Geological Sciences, 425 E. University Ave., Ann Arbor, MI 48109 United States
AB: For wavelengths longer than $\sim$ 200-500 km, the lithospheric stress field is controlled largely by tractions exerted on the lithospheric base by viscous mantle flow. The mantle flow field, and the tractions that it exerts on the lithosphere, are governed by the mantle's heterogeneous viscosity structure, which is not well constrained everywhere. Lateral variations in mantle viscosity in particular are poorly constrained, but significantly affect the mantle flow field and its coupling to the lithospheric base. For example, viscous flow in the upper mantle will exert tractions directly on the base, or even the sides, of deeply penetrating continental roots, but may be effectively decoupled from the oceanic lithosphere that overlies a low-viscosity asthenosphere. To characterize the effects of lateral viscosity viscosity variations on the lithospheric stress field, we developed models of mantle flow driven by either tomographically-inferred mantle density heterogeneity or imposed surface plate motions using a spherical 3-D finite element code. We then used a spherical model of the elastic lithosphere to transmit basal tractions to the surface. We examined the effects of several different lateral viscosity variations expected for the mantle, including those associated with strong, deeply-penetrating continental roots, ocean-continent and age-dependent variations in lithospheric thickness and strength, as well as depth- and temperature-dependent effects. For density-driven flow, we find that stronger lithosphere that penetrates more deeply into the upper mantle couples more effectively to mantle flow, increasing the magnitude of lithospheric surface stresses. We also find that large lateral variations in basal shear tractions are induced by flow associated with plate motions. These variations tend to resist plate motions, and show strong resisting stresses ahead of, and around the periphery of, a deeply-penetrating continental root moving through the mantle. The sum of the stress fields generated by density- and plate-driven flow provides a prediction of lithospheric stresses, which, when compared to the observed lithospheric stress field, can help constrain lateral variations in mantle viscosity, and their effect on mantle flow.
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8155 Plate motions--general
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting