At 09:23 PM 2001/06/01 +0100, John Milsom wrote:
>...
>There IS an asymmetry. East facing subduction zones (western Pacific,
>Caribbean, Scotia) seem to have a tendency to migrate outwards
>('roll-back'). West facing zones (Pacific coasts of the Americas) seem to
>be locked to their continental margins. First working hypothesis would be
>that the asymmetry in the Earth's rotation (it goes one way, not the other)
>has something to do with it.
At 09:22 PM 2001/06/02 +0100, John Milsom wrote:
>... I can't believe that no-one has looked at this,
>perhaps back as far as when they were rubbishing Wegener. ...
Sir Harold Jeffreys, in the several editions of his influential textbook
"The Earth", was the chief rubbisher of Wegener's idea of "Westwanderung" -
westward drift of continents due to tidal drag by the moon and sun.
Jeffreys argued that the force of tidal drag (calculated from the rate of
slowing of the earth's rotation) was much too small to cause shearing
strain within the upper mantle. But his value for mantle viscosity was much
too large - it was based on a false premise that the
larger-than-hydrostatic part of the earth's measured ellipticity was due to
a lag time (~10 million years) in readjusting its shape to the slowing of
its rate of rotation (and consequent lessening of centripetal force).
In the 1970's Robert Bostrom, George Moore and others revived the idea of
tidal drag as an important driving force of plate tectonics. This time Tom
Jordan was the chief rubbisher - he calculated that in order for a rigid
lithosphere to be tidally dragged westward at typical plate-tectonic
velocity the asthenosphere would have to be no more viscous than honey.
Moore and others countered that the asthenosphere does not shear at
constant velocity - instead the plates wriggle westward like caterpillars
as the tidal bulges propagate through them twice every day from east to
west. But leading geodynamicists were not persuaded, and the prevailing
geodynamic consensus does not explain why so-called "slab roll-back" is
most prevalent on west-dipping slabs, nor why earthquakes are virtually
absent in polar regions, nor why deep-focus earthquakes are concentrated at
low latitude, nor why the Antarctic Plate is bounded by so many
left-lateral transform faults, nor why east-dipping slabs dip so shallowly
under North and South America.
Dugald M Carmichael Phone/V-mail: 613-533-6182
Dept of Geological Sciences and Geological Engineering
Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]
