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What supports the Rocky Mountains in North America? Or put another way, Why are they there? and Why are they still there?

It might be tempting to think of compression, especially in an east-west direction. However at the south end, New Mexico is experiencing east-west extension (grabens including Rio Grande Rift Valley, plus associated volcanism).

Hot spot and/or mantle plume uplift doesn't appear to explain them either. Yes there's the Yellowstone Hotspot under NW Wyoming. Regardless of whether this is really caused by a mantle plume or not, the related volcanism does not correlate with the mountains.

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  • $\begingroup$ This is also part of "What created them?" $\endgroup$
    – Richard
    May 1, 2014 at 19:12

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Part of the current topography might be due to a hot upper mantle below the Rocky mountains. This can be determined by seismic tomography. The hot mantle creates isostatic uplift.

Just one of many pictures I found:

http://srl.geoscienceworld.org/content/83/1/23/F2.large.jpg

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  • $\begingroup$ Interesting pictures and I see this ties in with recent work on the Farrallon Plate. It explains the regional uplift, but I don't see it explaining the sharp eastern boundary of the Rockies that we see in Colorado? $\endgroup$
    – winwaed
    Apr 16, 2014 at 17:20
  • $\begingroup$ I'm going to "Accept" this answer. It is the best explanation that I've seen - it does a good job of explaining the regional uplift (e.g. AZ&UT) and possibly even the activity in NM. However, it doesn't explain why the Rockies themselves are still higher and why their Eastern boundary is so sharp. I guess it is an ongoing area of research! $\endgroup$
    – winwaed
    Apr 22, 2014 at 19:28
  • $\begingroup$ @windwaed: Hopefully somebody with in-depth knowledge will see this soon. Feel free to change your accept-vote then. It is kind of difficult to research that topic. In Europe we have the advantage that the topo-europe.eu project has greatly advanced the knowledge about the reasons behind topography. Is there something similar for North America? $\endgroup$
    – tobias47n9e
    Apr 22, 2014 at 20:41
  • $\begingroup$ USArray! It's a 'machine producing Nature and Science papers.' $\endgroup$
    – user2821
    May 4, 2015 at 22:40
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A way to understand Rocky Mountains is (as mentioned) to look at the destiny of the Farrallon Plate. In the collision with North America, it didn't subduct but rather slide under the lithosphere until it finally get a steeper angle and subducts at the eastern rim of the Rockies. This long journey could also lead to processes causing destabilization and delamination of the lithosphere under the Colorado Plateau, and causing its rapid uplift during Cenozoic. Recent data from the USArray have improved the models (and generates new questions).

The orogeny is somehow still ongoing as the Farrallon Plate still affect and weakens the North America Plate so that the Rio Grande rifting can take place. The Front Range Uplift is also the reason for the Denver Basin that fills with sediments from the orogeny.

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    $\begingroup$ If it subducts on the eastern rim - okay that explains the sudden 'front', but the andesite volcanism is in Cascades, not eastern Colorado. I think there's a simplification or two. Someone needs to produce a big summary book of the USArray results! $\endgroup$
    – winwaed
    May 5, 2015 at 12:58
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    $\begingroup$ Yes, an extreme simplification and still many unsolved questions. I'll try to expand the answer later. The Cascades are, however, related to the compression and subduction of Juan de Fuca Plate, not Farrallon. $\endgroup$
    – user2821
    May 5, 2015 at 13:24
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This may not answer your question to your liking but there is fairly relevant research on the topic.

I think there is still a debate about the causes of the recent (Late Cenozoic) uplift, however. The current dominant theory relates the shallow subduction of the Farallon Plate to subcontinental delamination and increased bouyancy all the way to the Front Range (see @Tbb's answer).

This theory essentially states that tectonic forcing is mimicked and in a sense bolstered by incisional unloading and subsequent depositional loading between the two landforms (montane vs. high plains). There has been a lot of research on erosion rates in the Eastern Rockies to test parts of Anderson's theories. It seems that most of the research I remember from learning about this problem in college is not freely available, but the papers in the links above have good summaries and discussions of the tectonic forcing and incision-based hypothesis.

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