ANCIENT REFLECTIONS

 

As read by Nick last night

  For what they may be worth, here are some of my reflections on the state of geotectonic thinking about the Western Cordillera during the 1970s and 80s.  Note that they are MY reflections and recollections: other players such as Ted Irving and Davey Jones may have had somewhat different views; too bad they’re not around to comment.

Well, first and foremost, I didn’t regard the bulk of displaced crustal blocks as in any way “exotic”.  Without much thought, I casually assumed that they had originated as part of North America itself, and had been transported relatively north by the north-oblique nature of subduction of the Farallon plate.  In fact, I spent inordinate amounts of time and energy pondering the circumstances under which this would occur.  It turns out that, theoretically, what is needed are:  a shallow subduction angle, a high angle of obliquity – and a mysterious factor measuring  the “stickiness” between the interacting plates, which I never figured out how to measure.  I wrote several papers on the subject which you can find very easily; the most cited one appeared in the 1991 volume of Physics of the Earth and Planetary  Interiors. 

Another player in the northward-transport game was, of course, the Kula plate – but we didn’t know how far south it had ever extended.  Clearly, detach a chunk of crust and attach it to the Kula – and northward displacement would be very fast.  But we didn’t know how, when, or even if this (might have) occurred.

Also, in those days I thought of northward displacement as occurring piecemeal, along a series of moderate-sized strike-slip faults and shear zones.  Then Ted introduced the concept of Baja BC, which seemed to imply existence of a San Andreas-type super through-going break.  That really got the geologists stirred up!

I said earlier that I thought of these allochthonous terranes as originally part of North America:  not “exotic” by today’s terminology.  The one universally acknowledged, indubitably exotic terrane was of course Wrangellia, which contained rocks that must have originated in the ocean.  Davey Jones, being a paleontologist, may have thought more along “exotic” lines, but I don’t recall him talking about it.

The next exotic terrane to be discussed in those early days was, I think, Stikinia, but by that time I had departed for the relative simplicity (and excellent cuisine) of the Central Andes.