HR: 08:45h
AN: GP21A-02    [Abstracts]
TI: Theoretical Constraints on True Polar Wander
AU: * Tsai, V C
EM: vtsai@fas.harvard.edu
AF: Dept. Earth and Planetary Sciences, Harvard, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Stevenson, D J
EM: djs@gps.caltech.edu
AF: Caltech, MC 150-21, Pasadena, CA 91125 United States
AB: For the present geologic epoch, long term true polar wander (TPW) is relatively small, but simple theoretical considerations suggest that it could have been larger in other epochs and could have been responsible for 90 degree inertial interchange events. In this work, we use scaling arguments to assess the qualitative behavior of TPW and a simple Maxwell model to analytically describe how changes in mass anomalies translate into TPW. Unlike previous work, our goal is to derive simple analytical estimates of TPW based on the characteristic amplitudes and timescales for changes in the moment of inertia. We find estimates for both the amplitude and speed of TPW as a function of various Earth properties. Our analysis shows that there are four main factors that strongly influence how large the maximum TPW can be: the (geological) timescale of the forcing τ_force, the viscosity structure of the mantle which yields a weighted average viscosity η, the characteristic amplitude of the non-hydrostatic changes in moment of inertia ΔC_dyn, and the added moment of inertia due to the equatorial bulge (C-A). The maximum TPW speed is not sensitive to the timescale of forcing although the total TPW reorientation angle and the TPW acceleration do depend on this timescale. The maximum TPW speed is (7 degrees per Million years) * (ΔC_dyn/0.003(C-A)) * (10²² Pa*s/η). For a model where τ_force = 10⁸ years, an η of 10²² Pa*s (higher than standard upper mantle estimates), and ΔC_dyn/(C-A) = 0.003, we obtain a maximum TPW angle of 88 degrees. With the same values except τ_force = 10⁷ years, the maximum TPW angle is 23 degrees. The same analysis provides the dependence of maximum TPW angle on the 4 factors. TPW is shown to act as a low pass filter: rapid changes in the moment of inertia produce smaller and delayed TPW. Thus, TPW may have been an important contributor to plate motions over relatively long timescales but not over shorter timescales. Our simple approach allows us to assess whether multiple TPW events are possible but the major uncertainty continues to be the mantle viscosity structure.
DE: 1527 Paleomagnetism applied to geologic processes
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8125 Evolution of the Earth
DE: 8157 Plate motions--past (3040)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly