HR: 1340h
AN: U33A-0025    [Abstracts]
TI: Theoretical Constraints on True Polar Wander
AU: * Tsai, V C
EM: vtsai@fas.harvard.edu
AF: Dept. Earth & Planetary Sciences, Harvard, Cambridge, MA 02138
AU: Stevenson, D J
EM: djs@gps.caltech.edu
AF: Caltech, 150-21, Pasadena, CA 91125
AB: It is now standard to attribute most of the apparent polar motion to continental drift caused by plate tectonics. However, an important component of this apparent motion is the true motion of the pole relative to geographic axes, i.e. true polar wander (TPW). For the present geological epoch, long term TPW is small compared to APW, but simple theoretical considerations suggest that it could have been large(r) in other epochs and could have been responsible for 90$\deg$ inertial interchange events. In this work, we use 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 variables of primary importance: the (geological) timescale of the forcing $\tau$$_{force}$, the viscosity structure of the Earth which yields a combined viscous relaxation time $\tau$$_{relax}$, the characteristic amplitude of the non-hydrostatic changes in moment of inertia $\Delta$C$_{dyn}$, and the added moment of inertia due to the equatorial bulge $\Delta$C$_{rot}$. Although the amplitude of the forcing is relatively small, substantial TPW arises because the viscous relaxation time is small compared to geological time scales. However, the maximum velocity of TPW is not sensitive to the geologic timescale although the total reorientation and the TPW acceleration do depend on this timescale. We find that the maximum TPW speed is (9$\deg$ per million years). ($\Delta$C$_{dyn}$/0.003$\Delta$C$_{rot}$).(1000 yrs/$\tau$$_{relax}$). For a model where $\tau$$_{force}$=10$^{8}$ years, and an average mantle viscosity of 10$^{22}$ Pa.s (higher than standard upper mantle estimates), and $\Delta$C$_{dyn}$/ $\Delta$C$_{rot}$ = 0.003, we obtain a maximum TPW angle of 89.6 degrees. This simple approach allows us to assess whether multiple TPW events are possible but the major uncertainty continues to be the mantle viscosity structure and layering.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8125 Evolution of the Earth
DE: 8157 Plate motions--past (3040)
DE: 1527 Paleomagnetism applied to geologic processes
SC: Union [U]
MN: 2004 AGU Fall Meeting