Figure adjustment and tectonics during polar wander: the flowing-apart of Gondwana

Abstract

The effects to be expected if the northward motion of Gondwana represented wander of the Earth relative to its space-fixed axis of rotation (as against motion of Gondwana over a fixed Earth) are reviewed. To render the model plausible, this has been based on the Goldreich and Toomre (1969) development of the Liouville equations. Like the spin axis, the orientation of the Equatorial bulge is space-fixed. In passing from polar latitudes northward, a lithospheric region must accommodate itself to the Equatorial bulge and decrease in gravity. The pressure on material initially in P T equilbrium decreases under the influence of the axifugal forces responsible for bulge formation. The figure adjustment is likely to be accomplished through re-patterning of the convection in the mantle. If wander is as fast as suggested in recent paleomagnetic studies, up to 1°/Ma, during fast intervals bulge-motion may have pre-empted a major fraction of the convection. During polar-wander, the Earth's surface is divided into four quadrants. In the two moving towards the Equator, upwelling convection is favored; but in the two moving poleward, figure-adjustment favors subduction. Gondwana “flowed apart” during its residence in Quadrant 1, affected by motion towards the Equatorial bulge. Quadrant II, in which the mantle is experiencing motion towards the North Pole, is the site of the Mesozoic-Tertiary concentration of subduction represented by the Tethys tectonic belt. Owing to rapid longitudinal motion unrecorded in paleomagnetism, events in the Pacific hemisphere cannot be re-constructed.