Roger,
A couple of points. The heat budget of the earth has certainly varied
through time and, over the long haul, the rate of plate motion should slow
(all other things being equal). However, if you talk to the Geodynamicists
and get them to speak candidly, they will tell you that the best paper on
plate driving forces ever written was by Forsyth and Uyeda (1975) and there
is still much we don't know. Basically (for those who have not read F&U,
1975), they observed the rates of motion during the present day along with
lengths of subduction zones attached to the plates, length of ridges,
continental versus oceanic area and concluded that oceanic plates will move
faster than contintal plates (a very brief summary to be sure). As I
previously mentioned, India moved at over 20 cm/year in its northward motion
toward Asia but was attached to a large piece of old Tethyan ocean. Looking
back into the past, a number of authors have looked at plate motion on the
basis of paleomagnetic data. We can, with pmag determine only minimum rates
of latitudinal motion. There are documented cases (and argued of course) of
large plates (Gondwana and Laurentia) moving at >25 cm/year in the latest
Neoproterozoic. There are any number of controversial explanations for that
motion including Inertial interchange true polar wander (where the entire
mantle and lithosphere tumbles through 90 degrees in 15 million years= 66
cm/year), true polar wander + rapid drift and a few other explanations. My
personal preferred explanation follows the work of Gurnis (1988) who
demonstrated that supercontinents can act as a thermal lid on the mantle.
The supercontinental lid will lead to a long wavelength geoid high and
eventually rip the supercontinent apart. Gurnis and Torsvik (1994) expanded
on this idea to try and provide a 'speed limit', and concluded that the
speed of continental plates (with a significant root) could be enhanced
providing the driving force is deep seated (plumes). Meert (1999) has a
cartoon of this HOG hypothesis (where the plume acts as the 'Hand O God' to
lift the continent off the geoid high and 'toss' it toward the geoid low.
Such bursts in plate motion are likely to be short-lived and consistent with
paleomagnetic observations.
As to your question about viscosity it is clear that plate motions can
be 'varied' by quite a bit depending on the assumed viscosity. This is
exactly what John Baumgardner did with his Noachian flood model of plate
tectonics.
Cheers
Joe Meert
1. Meert, J.G., A Paleomagnetic analysis of Cambrian true polar wander,
Earth Planet. Sci. Lett., 168 (1-2), 131-144, 1999.
-----Original Message-----
From: Tectonics & structural geology discussion list
[mailto:[log in to unmask]]On Behalf Of Musson, Roger MW
Sent: Tuesday, March 05, 2002 4:51 AM
To: [log in to unmask]
Subject: Re: Maximum rate of continental drift
This is not my subject, but would it be possible to estimate a theoretical
maximum velocity from constraints due to mantle viscosity? Also, one would
assume that the primum mobile (the heat supply) is not greatly varying over
time, which would argue for a system behaving in a fairly steady way with
respect to rates of movement.
Roger Musson
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