major terranes of nw north america
This will be presented by Nick Zantner as part of one of his UTube geology lectures - next Friday, I believe.
I am going
to record what I remember about the early days of the terrane concept. My memories are 40 or so years old, thus
subject to a high degree of mental erosion.
Also, at the time I was concerned almost exclusively with the North
American Cordillera, and with rocks that were younger than early Mesozoic;
similar thinking may have been going on elsewhere, and with older rocks, but I
would have been largely unaware of it. I
am sure that Dutch geologists – Zijderveld and van der Voo for instance – were
thinking along similar lines with regard to the tectonics if the Mediterranean
region.
Anyway, the
undisputed godfather of Cordilleran “microplate tectonics”, as we called it
then, was David (Davey) Jones. Davey
worked for the USGS, and his specialty was radiolarian paleontology. Clearly he went far beyond the little
siliceous bugs, though, because he came up with the terrane idea while doing
geologic field work in Alaska. As I
remember the story, Davey and his crew were mapping – probably central Alaska –
and they were frustrated by the exposure problem. Their field area apparently consisted of a
bunch of elevated ridges, separated by deep valleys filled with dense vegetation. When it was possible to see rock in the
valleys it consisted of dark colored, organic-rich shale and mudstone – over
beer, Davey referred to it as the “black crap”.
Pretty clearly, the valleys housed faults.
The most
puzzling aspect of the geology, however. was that neighboring ridges suggested
completely different geological histories; their rock suites were dissimilar in
lithology and/or age, as were their structural histories (faulting, folding,
etc.). You couldn’t correlate across the
black crap! So Davey began to refer to
these isolated ridges as “terranes”, probably to reserve the alternate spelling
“terrain” for more descriptive uses – volcanic terrain, basin and range
terrain, etc. It would have been obvious
to Davey and others that these terranes had been juxtaposed by movement along
the bounding faults. Fortunately, at
about that time paleomagnetism began to show that big blocks of crust had been
displaced by hundreds or even thousands of kilometers. That’s how I got into the game.
