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Author Topic: Too many rifts. Where's the effects?  (Read 2497 times)

Offline OokieWonderslug

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Too many rifts. Where's the effects?
« on: 17/02/2014 03:52:32 »
Here is something that has been bugging me for a while. I hope someone can help me understand. In the west coast of California there is a strike slip tectonic fault. The plates slide by each other.

In Nevada there is a rift zone. The ground is pulling apart. That is why we have Death Valley.

In the midwest we have another rift zone that has been moving at about a millimeter a year for like a billion years.  (The New Madrid)

And in the middle of the ocean we have the mid Atlantic ridge. Also a rift zone.

Where is the subduction? Shouldn't there be a subduction zone somewhere in there? Why is it all moving apart and nothing is moving under?

What if these east coast earthquakes are the very beginning of a new subduction zone? It seems like there should be one in that area. The Appalachians are not growing appreciably. They erode as fast as they go up which is caused by the lesser weight on the crust. Like an ice cube melting.

If no subduction, why not mountain building?  We do not have that either. I do not understand how such a large amount of compression is not having any effects.

Any help as to why this is?


 

Offline evan_au

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Re: Too many rifts. Where's the effects?
« Reply #1 on: 17/02/2014 11:20:08 »
See a map of subduction zones here.

The mid-Atlantic ridge is a rift zone. There is not a corresponding subduction zone at the edges of the Atlantic because the Americas and Europe/Africa are being pushed apart.

The Pacific ocean is surrounded by subduction zones. The subduction zone on South America's Pacific coast has resulted in mountain building in the Andes. On the western side, there are the subduction-generated volcanoes of New Zealand, Indonesia, Japan and the Aleutian Islands.

Converging plates of similar density produce maintain chains like the Himalayan mountains.
« Last Edit: 17/02/2014 11:30:56 by evan_au »
 

Offline OokieWonderslug

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Re: Too many rifts. Where's the effects?
« Reply #2 on: 17/02/2014 13:20:12 »
The map shows my point. There are 3 rift zones in a row and no mountain building or subduction zones. I know about the ring of fire. The subduction is going on in the east pacific. That is on the other side of the planet from the US.

But you have two rift zones from California to Carolina pushing back against the mid atlantic ridge. No subduction in California or the east coast. No new mountains either. How can you have the continental US moving west at about an inch a year while you have the New Madrid moving at a millimeter a year and the Nevada rift moving at .23 mm a year?

Something does not seem right about that.
 

Offline Ophiolite

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Re: Too many rifts. Where's the effects?
« Reply #3 on: 17/02/2014 17:31:37 »
I presume you meant to write west Pacific, not east Pacific, since the US abuts the east Pacific. In addition, California is a complex area, containing several terranes - crustal fragments with diverse histories, now juxtaposed. Also, the western part of the US has, simply put, overridden a spreading zone, adding to the complexity.

Although classic plate tectonic theory requires rigid plates, in practice a great deal of strain can be taken up within a plate before movement becomes necessary. Hence the massive consequences of the Japanese and Indonesian corrections this century.
 

Offline OokieWonderslug

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Re: Too many rifts. Where's the effects?
« Reply #4 on: 18/02/2014 14:11:16 »
So you're saying the entire 4 feet of westward movement of the us continent since I was born has been completely absorbed by the subduction zone in Japan? I did not think the crust that rigid.

I knew about the spreading ridge that has been over run, but I thought it was no longer spreading since it is entirely under the continent. Where is it's subduction zone? Why is there not a chain of volcanoes somewhere in the middle of the US? I know there is a line of them from New Mexico to Nebraska, but they end and are no longer active. So doesn't that mean that the rift is dead and it is no longer spreading? Shouldn't it have locked itself to the continent over it and be one plate now?

I am trying to  get a complete picture of just what is going on. I still think there should be a subduction zone on the east coast. There isn't and that is the mystery here. That is one hell of a strong plate to be spreading from the midatlantic and subducting in Japan. And wouldn't that mean both sides of the subduction zone are moving over each other? The spreading from Europe and then the spreading on the North American plate. Both converging on Japan. No wonder it is so active, but should it not be more active if that is the case? 8ft of movement in 45 years is really fast for a tectonic plate. Is it not? You'd think there would Krakatoa size eruptions fairly regular there and yet Mt Fuji Hasn't erupted in 200 years. Why is that?
 

Offline Bass

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Re: Too many rifts. Where's the effects?
« Reply #5 on: 19/02/2014 00:41:26 »
I think the point that Ophiolite was trying to make is that we do have subduction on the west coast of North America.  The Jaun de Fuca plate, which is actively subducting beneath parts of California, Oregon, Washington and British Columbia, is the remnant of the Farallon plate that existed off the coast of California only a few million years ago. The missing part of the Farallon plate was completely subducted beneath N.A. and the remaining plate motion was transferred to the transform motion of the San Andreas Fault.  The existence of the Rocky Mountains (including volcanic rocks) so far inland suggests that the Farallon plate may have subducted at much shallower angles than normal subduction zones.
 

Offline Bass

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Re: Too many rifts. Where's the effects?
« Reply #6 on: 19/02/2014 00:56:14 »
Also, a new study, published in Geology in June 2013, suggests that subduction may have been initiated off the coast of Portugal. http://geology.gsapubs.org/content/early/2013/06/05/G34100.1.abstract
 

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Re: Too many rifts. Where's the effects?
« Reply #6 on: 19/02/2014 00:56:14 »

 

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