Re: Why do the Rocky Mountains occur so far from a plate boundary?

There are several factors that explain this, but first it's important to know that there are several different times when the "Rocky Mountains" were mountains. The Rocky Mts were first deformed in the late Paleozoic era, about 250 to 300 million years ago, then a second major deformation occurred in late Cretaceous to Tertiary periods, about 100-50 million years ago. The current topography of the Rocky Mountains, especially along the "Front Range" in Colorado appears to be a result of recent erosion happening faster in the weak rocks of the Plains than in the hard crystalling rocks of the ranges. In essence, the second set of "Rocky" mountains were covered by sediment and are now being exposed again.

Now, let's get to how the mountains formed. Plate interactions can, in fact, spread fairly far from plate boundaries under the right conditions. As a modern example, the collision of India and Asia is not only raising up the Himalayas, it is also pushing up a big slab of Asia to form the Tibetan Plateau and the northward push of India may be driving fault motions as far north as Lake Baikal in the former Soviet Union. Elizabeth Miller and I proposed (in a paper in the journal Geology in 1987) that the collision of South America into North America 300 million years ago to form the earliest Rocky Mountains might have affected much of what is now Utah and Nevada in much the same way.

In the case of the Cretaceous to Tertiary age Rockies, the plate boundary was along the western edge of North America, about where the coast of California is now. Deformation did occur along the plate boundary, but it also spread inland with time, usually as magmatism spread inland as well. As this plate margin evolved, the plate that was subducting under North America became younger and more bouyant. As a result, it did not sink as easily into the mantle and it sloped less steeply. It appears that by the end of the Cretaceous, this plate was almost flat from the subduction zone in California, all the way to the middle of Utah. Plates do not slide easily past each other when their boundary is mostly horizontal and so much of western North America became locked to the subducting plate and the compression produced by subduction was transferred inland to Utah, Wyoming, Montana, Colorado, New Mexico, and even South Dakota, where it gave rise to the faulting that uplifted the rocks in the present day Rockies. As the movements of plates changed, the subduction rate decreased dramatically in the Tertiary and the subducting plate had time to sink into the mantle before travelling so far eastward. This led to the end of Rocky Mountain tectonics and very shortly thereafter to the onset of rifting in the Basin and Range province and the Rio Grande Rift, which have continued to the present. This rifting is also another part of the answer to your question. This rifting has stretched western North America by approximately 100 km east-west. In Cretaceous time, the Rockies were 100 km closer to the subduction zone than they are today.

With all of these factors taken together, geologists are very comfortable that the presence of the Rockies where they are fits in with the expectations of plate tectonic theory. A very good modern analogy of the Rockies can be see in southern Argentina, where a range of mountains is forming well to the east of the Andes.

I hope this answers your question. If it is unclear, please post more questions to the network or to me directly.

David Smith
La Salle University
dsmith@lasalle.edu