10 July 2023

Tectonic revolutions and the weirdness of Western North America

About 40 years ago a good friend introduced me to the concept that the Sierra foothills and Coast Range in California were accreted to North America as "island arcs" comparable to Japan, and I found that fascinating, but I admit I pictured Honshu smashing California, one and done. I probably would've been flabbergasted to realize that it wasn't just one island arc, but a whole series of "exotic terranes," and it wasn't just California. All or nearly all of British Columbia, Washington, Oregon, and most of California are made up of exotic slivers and slices, some having moved along faults or caught up in moving plates that originated far away. Some less far. So a big island system, actually probably closer to Kamchatka than Japan, collided and slid along (now North, then mostly east along the now-western, then equatorial and E-W oriented margin of Laurentia (North America). That superterrane, now called Intermontane, consisted of two detached pieces of old North America with a slice of coastal material from Gondwana... now Indonesia... slivered in between. It collided about 170 million years ago. I won't try to account for all of this in a paragraph, but another big one, the Insular superterrane (docked with N. Amer. about 100 million years ago), forms a lot of British Columbia and Alaska. It consisted of Wrangellia, which originated in what is now coastal Siberia (it's controversial) docked way out in the ocean, which was probably the Kula ocean, not the modern Pacific (it's really complicated), with a slice of continental crust, the Alexander Terrane, that made its way along the Uralian ocean then east and north of North America from a point near Europe. A long way, but this was before the modern Atlantic opened up. And that's how the Yellow Aster formation in North Cascades National Park is actually a little piece of the red sandstones that the founder of modern Geology, James Hutton (1726-1797) referred to as the Great Unconformity (coining that word). In Wales. Rocks identical to rocks in Wales are found in the Pacific Northwest, not because they're similar, but because they are the same rocks. So if that doesn't boggle your mind just a bit, you aren't really thinking about it. 

I said this was weird, but actually there are a lot of places in the world that are similarly complex, with bits and pieces rifting, translating, docking... it's extremely complex. There have been at least two, probably three sequences where all the landmasses have come together in a supercontinent, most recently about 400 million years ago as Pangaea. Most people have heard of Pangaea, but the details of how the surface of the Earth has evolved over time are truly amazing, and virtually all of this science has arisen in my lifetime. 

No comments:

Post a Comment

Gyromantic Informicon. Comments are not moderated. If you encounter a problem, please go to home page and follow directions to send me an e-mail.