THE J2 CUSP

                                                   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!