EDGE EFFECT
Main forces that move plates
Here is my
choice for “paper I wrote that’s most difficult to understand”:
Beck, M. E., and B. Housen, Absolute
velocity of North America during the Mesozoic from paleomagnetic data,
Tectonophysics, v. 377. pp 33-54. 2003.
I say this
because I picked it up lately and, after 17 years, could barely get through it
myself! However, it may have some relevance
to Nick Zentner’s discussion of exotic terranes, so here goes:
First we
need some basic assumptions:
1) A paleomagnetic study of rocks of a particular
age and geographic location gives an accurate approximation of the location of
that area at that time with respect to the spin axis of the earth – the geographic
pole.
2) As a continent moves with respect to
the spin axis it will, in effect, paint a path of apparent polar wander in the
form of a string of paleomagnetic poles that reflects that motion. These are called curves of apparent polar
wander (APW).
3) Continents, like everything else,
move in response to the sum of forces applied to them. If the sum of forces remains constant so does
the motion, in direction and speed.
4) North American APW can be described as
a series of intersecting paths.
Each path should represent a period of constant applied external
forces; intersections of these paths accordingly should represent episodes at
which the sum of applied forces changed.
This has been discussed previously by Ted Irving, Richard Garden, and
Alan Cox.
5) External forces that may influence
plate motions include subduction zones, rifts and strike-slip faults. It follows that events such as initiating or
choking off of large subduction zone might be expected to alter the velocity
of plate motion.
AND NOW,
finally, I get to the point. The
incomprehensible paper cited earlier recognizes a half-dozen or so “cusps”,
defined as times when North America abruptly changed its direction of
motion with respect to the spin axis. One
of the better defined of such cusps occurs at about 160 Ma, the J2 cusp. Could that represent the effect of cramming
the Intermontane Superterrane into a west-dipping subduction zone? I suggest an alternative in the 2003 paper –
but who knows? I need to study the
damned thing again!
That's Richard Gordon, obviously. Damn spell check, anyway!
ReplyDeleteTed Irving must have had something like this in mind When he wrote a paper titled "Hairpins and Super intervals" some time in the early 1976s. Ted frequently was way out in front of the rest of us.
ReplyDelete