The gravitational potential of the Earth varies due to the mass anomalies associated with plate tectonics, and the equipotential surface, or geoid, has been shown to be sensitive to plate rheology at the surface as well as relative strength variations within the planet. To match observed longest-wavelength geoid features (λ > 4000 km), numerical models of global flow require weak subducting plates (less than 103 Pa-s variation), in agreement with laboratory models that investigate relative strengths in subduction zones. In contrast, based on experimentally determined rheology for plate and mantle minerals, much larger strength variations (108 Pa-s) are expected to occur in subduction zones due to the temperature and stress dependence of viscosity, and regional models using viscosity derived from deformation experiments predict strong slabs that weaken plastically in regions of high stress. Furthermore, recent numerical models suggest that the geoid is sensitive to upper mantle slab strength at shorter spatial scales than previously studied, but the computational ability to investigate this sensitivity with realistic rheology did not exist until now. Here we show that while the longest-wavelength geoid is mainly sensitive to weak plate margins, the long-to-short wavelength geoid is sensitive to slab and mantle rheology because these control surface deformation near convergent margins. We find that when realistic rheology allows plates to weaken locally, the predicted geoid is nearly indistinguishable from models in which the slab is uniformly weak. Therefore, the longest-wavelength geoid is a constraint only on the minimum strength of the subducting plate, and can not constrain the maximum strength of the slab away from the bending region. However, the long-to-short wavelength geoid may be useful to constrain certain aspects of subduction zone rheology in high resolution models of individual convergent margins.
|Advisor:||Billen, Magali I.|
|Commitee:||Kellogg, Louise H., McClain, James S.|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||MAI 50/05M, Masters Abstracts International|
|Keywords:||Dynamic topography, Geoid, Mantle viscosity, Nonlinear rheology, Slab strength, Subduction zone|
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