Polar wandering and the forced responses of a rotating, multilayered, viscoelastic planet

Abstract
Changes in the earth's rotation produced by the waxing and waning of large mobile ice sheets can be used to infer the viscosity of the mantle from a comparison with the recent secular polar drift as revealed in the International Latitude Service data and also in the estimates of the nontidal acceleration of the length of the day. We have developed by using an analytical approach a physical model of a rotating earth consisting of an elastic lithosphere, a viscoelastic mantle, and an inviscid core. Forcings from the two major ice sheets, Laurentide and Fennoscandia, have been considered. Sensitivity analysis of the parameters governing the external forcings, such as the length and the time of termination of the deglaciation phase, show in general that there exist two families of mean mantle viscosities, 0(1022 P) and 0(1023 P), which can match the data satisfactorily. However, we find it an improvement to incorporate some amount of shrinkage from the Antarctic ice sheet, obtaining simultaneously a better fit with the two rotational data sets to the lower viscosity solutions. Hence forcings from the southern hemisphere may be able to remove the ambiguity associated with the extraction of mantle viscosity from rotational data. We have also solved the initial value problem of the components of instantaneous angular velocity by means of the Laplace transform for applied loads characterizing the continental ice sheets, which since the mid‐Pliocene have periodically expanded and contracted over the northern hemisphere. In consequence of this continual forcing, polar wander of around 5–10 degrees can take place for a mean mantle viscosity 0 (1022 P). Recent reanalysis of paleomagnetic data in conjunction with hot spot tracks have revealed that several degrees of true polar wander could have occurred in the last 5 m.y. These calculations thus provide a physical explanation for the observed movement of the rotation axis. Membrane stresses of 0 (10) bars can develop from this secular polar motion.

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