TPW was first suggested in the 1920s as a mechanism for aparent polar wander observed over geological time before continental drift was discoevered. The idea was held in high regard until the mid ocean ridges were mapped and magnetic polarity reversals could be used to infer movement of continents. More recently TPW has been suggested to account for some unexplained changes in pole location and is fiercely defended by Sager and Koppers
Position a shows mass offset from the equator creating an instability.
Position b shows the model after some redistribution in a more stable state
The mechanism (an example of supercontinent breakup and formation)
True Polar Wander is in effect a shift of the Earth's spin axis, usually attributed to mass redistribution by centrifugal forces. Such mass excesses or deficits can take many forms, notable including rapid LIP emplacement or supercontinent formation away from the equatorial spin axis and others.
(Evans 2001) proposed a simple model of TPW based on some simple mantle convection models. The cycles begins by formation of a supercontinent at some latittue away from the equator. Such masses of uninterrupted continental crust tend to block downwellings usually driven by subduction, this tends to establish a 'ring' downwelling around the supercontinent and increases the likelihood of establishing a major upwelling underneath the continental crust. This upwelling is driven towards the equator by TPW and is eventually broken apart. the rotationally unstable upwelling axis can then move rapidly over a few million years until the new continents age sufficiently.
TPW supercontinent cycle
a. Formation of supercontinent off-axis driving the intraplate upwelling by TPW
b. Onset of breakup as the continental mass ____
c. Possible rapid TPW following continental rifting over a few million years
d. Reformation of supercontinent accompanied by rapid TPW of the old upwelling
(From Evans 2001)
This model is based on observations of palaeomagnetic data of the Rhodinia/Laurentia breakup with possible rapid oscillation of a previous upwelling from the Nona supercontinent.
Sager and Koppers' TPW case study
In 2000 Sager and Koppers studied apparent polar wandering paths based on Pacific plate measurements using the accurate Ar/Ar dating technique. Their results suggested rapid anomalous polar wandering events which could not explained solely by plate motion and was unlikely to be due to short term variation in the global magnetic field. Some suggestions for these anomalies include microplate rotation some time in the late Cretaceous quiet zone, but Sager and Koppers argue this to be highly unlikely as a mechanism and suggest TWP as the most likely mechanism for movement. Their modelling and fitting of the data to the theory is fairly good considering the limitations of the data, but this does not mean a conclusive answer nonetheless.
and confidence regions are shown; the eastern and western 84-Ma poles are denoted 84E and 84W. Filled circles show the predicted polar
wander path, with points at 5-My intervals, derived from a plate versus hot spot motion model. (Sager and Koppers, 2000)
Although S&K's model seems to make a lot of sense TPW has been questioned frequently. this particular paper was criticised for being too selective of the data is used and for not having enough consideration for errors and other effects on the anomalous readings.
It certainly seems TPW has played some role in polar wandering in the record, but whether this can solely account for somer of the rapid polar movements recorded by the Pacific plumes (see the Movement page) is debateable.