True polar wander: An analysis of Cenozoic and Mesozoic paleomagnetic poles

Abstract

A true polar wander path is presented for the past 180 Ma tracing the relative motion between the hotspot framework and the geomagnetic poles deduced from worldwide paleomagnetic data. A relative displacement of 22°±10° is observed. The motion between the two reference frames has not been smooth. Episodes of rapid motion, on the order of 80–100 mm/yr, include the present, 65–50 Ma, 115–85 Ma, and 180–160 Ma. Periods of slow or little polar motion, less than 20 mm/yr, are observed at 50–5 Ma and 160–115 Ma. The effect of plate motions is removed using Morgan's plate motion model A19. This model is tied to a hotspot reference frame assumed to be fixed in the mantle. The rotated paleopoles are grouped by ages and then averaged. They are found to cluster about points distinct from the north pole as defined by the hotspots. These points trace out a path of motion between the rotation axis and the hotspots or mantle. This is interpreted as a shifting of the entire earth in response to a change in the principal axes of the moment of inertia of the mantle. Paleomagnetic poles of the past 7 m.y. are examined independently. This eliminates a dependence on the fixed hotspot hypothesis, since plate motions are essentially negligible for this time span. There is a deviation of 5°±2° between the present geographic north pole and the average of the paleopoles at 5 Ma. This is too great to attribute to plate motion. Interpreted as true polar wander, the rate of this polar motion is about 1°/m.y. Astronomical measurements for the past 80 years have recorded actual motion of the earth's rotational axis up to 1°/m.y. in the direction of eastern Canada. This agrees in rate with paleomagnetic polar motion of the past 7 m.y. and in both rate and direction for the past 2 m.y.