There is an
important aspect of Apparent Polar Wander and Euler Poles that I have not yet
addressed, and I will attack it right now.
You might call this essay Euler APW, part 2.
Let us
assume that continents (or tectonic terranes and other scraps of crust) move in response to the
sum of all forces applied to them, and that such sum of forces remains
reasonably constant for geologically significant blocks of time. Knowing as we do that APW is the result of
the motion of crustal blocks rather than movement of the pole itself, it
follows that APW paths should trace arcs of circles centered upon an appropriate
Euler pole. It also follows that abrupt
changes in APW should be evidence of major changes in the sum of driving
forces*, which ought to be evident in the geological record. That is the reasoning upon which a paper by
Bernie Housen and myself is based**.
I should
emphasize that Bernie and I did not first come up with this rather clever idea;
Richard Gordon and Allan Cox (both Stanford at the time, I think) get that
credit. Bernie and I simply tried to
ride it to its logical geological conclusion.
Much to my
consternation, when Bernie and I first ran this idea up the flagpole, damned
few geologists saluted it! I have been
out of the field for a long time, so I can’t do an adequate job of even speculating
about why this should be so. Perhaps the
sum of applied external forces acting on your typical crustal fragment
undergoes very frequent and very important changes in some random manner. This would throw a large monkey wrench into
the works, at least insofar as the correlation between APW and important
geotectonic events are concerned.
*Driving forces? Well,
for instance “ridge push” and “slab pull”, occurring at spreading centers and
some subduction zones, respectfully, act to move a plate in what we may call a
positive direction, whereas friction along transform plate boundaries retard
such motion. When I last studied this
stuff, the effect of the mantle underlying the plate was under debate.
** Beck and Housen, Absolute velocity of North America during
the Mesozoic from paleomagnetic data, Tectonophysics, v. 203, pp. 33-54, 2003.
So, anyway –
I thought that this line of reasoning is basic enough to be worth expounding – even if
it turns out to be incorrect!
Next time, triple junctions.
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