Saturday, April 13, 2013

A new story for the West Coast of North America


You know what? The West Coast of North America is a mess.

Well, maybe not from whatever perspective you’re thinking…my apologies to everyone I just (quasi-deliberately) offended, let’s be a little more specific. From a geologic perspective, the West Coast of North America is a mess.

If you look at the Andes Mountains, they are something of a clean story. For hundreds of millions of years, there has been subduction of an oceanic plate beneath South America, building the mountain range we see today. Oh, there are a few plateaus, there are a few volcanoes, but overall, it’s a thin mountain range that doesn’t intrude much into the continent.

Compared to that, the Rocky Mountains in North America are a nightmare. The Andes are about 500 kilometers wide, while the Rocky Mountains in places stretch nearly 2000 kilometers into the continent.

There are a number of causes for this huge damage zone in North America, but a big part of the puzzle is stuff that wasn’t always part of North America. Docked to the west coast of North America, there are a series of what are called “accreted terranes”. They have names like Stikina, Wrangellia, etc. These accreted terranes can be understood as ocean island chains that rode in on the subducting oceanic plate, slammed into the continent, but couldn’t make it down the subduction zone and wound up stuck where they arrived.

The image you see here is a beautiful shot from the National Park Service of a place in Wrangell-St. Elias national park (see the name?). The rocks that make up this shot are part of one of those accreted terranes. They rode in on a plate that was being subducted and stuck in place.

So, all of these terranes have been accreted in North America, but South America doesn’t have them. Obviously, that brings up the question “Why”?

New research published in the journal Nature tries to answer that question with a new model for subduction along the North American coast.

Once a plate is subducted, most of the evidence of that plate disappears. Piecing together a plate that has been subducted is an arduous challenge. Prior to this paper, the basic model for the Pacific Ocean involved 3 plates: the Pacific plate, the Kula plate, and the Farallon plate. The Kula plate is basically gone today, while there are a few fragments of the Farallon plate left, such as the Juan de Fuca plate currently subducting beneath Washington, Oregon, and British Columbia.

Although tracking down a subducted plate is difficult, there is one tool that might help called seismic tomography. Seismic tomography involves measuring the speed of earthquake waves very precisely as they travel through the Earth and using those speeds to understand the material the waves travel through. Subducted plates have a very distinct signature, so they can be fairly easily detected using this technique.

These authors performed seismic tomography underneath the Western U.S. and North Pacific Ocean and found something that seems a bit startling; extra plates! Underneath the North Pacific there are what appear to these authors to be multiple old, subducted plates, and there are other fragments of plates deep beneath North America.

If the authors’ identification of these plates is correct…that means there was more subduction happening in what is today the Pacific Ocean than anyone thought. Subduction within oceanic plates tends to create island arcs; the same type of material seen off the coast of Asia today in the Philippines or Japan, and the same type of material that was repeatedly accreted to North America.

This work might give an answer for why there is so much stuff stuck to North America. If the simple “Pacific-Kula-Farallon” model was much more complicated, if the Farallon plate was broken into many pieces with subduction zones in-between, that could create the island arcs that slammed into North America and became the accreted terranes. The authors actually give names to some of these subducted plate fragments: the Angayucham and Mezcalera plates. They could have even played a very important tectonic role; if they were attached to North America while they were being subducted, they could have applied a force to the continent that helped open up the Atlantic Ocean 200+ million years ago.

It’s an interesting story and one that will be worked on, particularly now that things have names (that always helps!). If the story is right, it’s one that can only be told using information trapped a thousand kilometers deep in the Earth’s mantle. From the surface, all we can see is that North America is a mess, but maybe for once the mantle has made things simpler and told a story we’d otherwise never see.


Photo Source, public domain photos provided by the National Park Service:
http://www.nps.gov/common/uploads/photogallery/akr/park/wrst/3BF3A378-1DD8-B71C-07A66530C4E13429/3BF3A378-1DD8-B71C-07A66530C4E13429-large.jpg

Full paper:
http://www.nature.com/nature/journal/v496/n7443/full/nature12019.html

News & Views summary:
http://www.nature.com/nature/journal/v496/n7443/full/496035a.html

Sciencedaily article:
http://www.sciencedaily.com/releases/2013/04/130403141402.htm

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