MUSINGS ON TECTONIC ROTATIONS

 I wasted an annoyingly large amount of time searching for a suitable diagram illustrating geological block rotations (in the horizontal plane, natch.)  Finally I gave up and resorted to this picture of an honest paleomagician plying his trade - no doubt uncovering more evidence of Cordilleran clockwise rotation.  This almost could be me - except for the color of the notebook and the abundance of hair peeking out from under the cap.

Must I always repeat myself?  These are the musings of a guy who has been out of the game for ~30 years, so if you want to take them seriously, fine.  But be warned.

The topic is tectonic rotations; specifically, rotations with respect to the North American craton.  Having just moved to a nice but smallish retirement apartment, I seem to have lost my access to raw data, so everything I say here will be highly generalized and unsupported by citable fact.  Nevertheless, as one of the people who first attempted to apply paleomagnetism to regional tectonic analysis, I think I am entitled to sound off.  So here goes:

Considering late Cretaceous – early Tertiary rock units currently lodged upon the western margin of North America, rotations (as defined above) are ubiquitous and overwhelmingly clockwise.  Documented rotations range in magnitude from just a few degrees to nearly 90 degrees.  In a vague – very vague – sort of way, rotation magnitudes increase with age and with proximity to the present continental margin.  Some rotated blocks (from paleomagnetic evidence) are associated with large northward displacements, whereas others are not.  So, the question:  How come?

Well, I can think of three - maybe four* – “driving mechanisms”.  The first involves northward displacement (relative to interior, stable, North America, always remember).  As first conceived, the Baja BC story involved northward displacement along the continental margin.  At present that continental margin very roughly approximates a small circle** about the pole of relative rotation between North America and the Pacific plate.  Any crustal block travelling along that path would pick up a small degree of clockwise relative rotation; the further the latitudinal displacement, the greater the rotation.  This mechanism probably is a minor contributing factor. 

A more important element in the rotation story concerns large scale, coherent block rotation.  For instance, many years ago Jim Magill and Allan Cox noted that extension in the Basin and Range province is not consistent throughout but rather increases toward the south.  This would have the effect of imparting a clockwise rotation to the entire terrain located west of the B & R.  Note that the rotation so constructed would be the same everywhere for rocks older than the B & R activity.

Finally, we come to my baby, clockwise rotation of small blocks caught up in a wide dextral shear zone – caused by right-oblique interaction between NA and neighboring oceanic plates to the west.  This has been identified as the “ball bearing” model, made famous (once upon a time) by some clever remarks attributed to Allan Cox.  If this idea is dominant there would be little consistency between neighboring rotation values, although of course there might exist a vague correlation between rotation amounts and either age of the rotated block, its northward displacement, or how close it lies to the continental margin.

In other words, the Beck’s Ball Bearings model predicts near total chaos.  That’s what I seem to remember.  Wish I still had my tables!

So, that probably muddies the waters enough for now.  Thank goodness there are young bucks like Ray Wells to  to sort it all this stuff out!***

* Some rotations may be attributed to the path followed by an exotic terrane before it docked.  Probably older rocks, no prediction possible at present.

** This is your chance to put your Wikipedia skills to use.

***But what’s this I hear?  Ray Wells, retired?  Oh Lord almighty!