Today I’m tacking the Laramide Orogeny of western United States. It’ll be a long one, in part because I’ve done work on it myself, both for general academic understanding and for oil exploration, and because it's a complex system of mountain building.
You recall that ‘orogeny’ is just Greek for mountain building, and over the course of these podcasts I’ve talked about enough complications that you know such an ‘event’ is anything but instantaneous, nor is it always the same in all places. This one is even more complicated, perhaps in part because it is young enough that we see a lot of its effects and therefore understand it a little better.
First let’s try to deal with some nomenclature issues. What do I mean when I say “Laramide Orogeny”? The question has to be asked because sometimes geologists use the phrase to refer to a particular time interval – in this case, mostly late Cretaceous into the early Cenozoic, around 75 to 40 million years ago – but some use the name Laramide to refer to a general geographic location, from Canada to Mexico, but generally in the eastern section of what we call the Rocky Mountains today, and extending as far east as the Black Hills of South Dakota. The expression is probably best in Wyoming, and the orogeny is named for the Laramie Mountains of southeastern Wyoming.
|Double Lake, in the Precambrian of the Wind River Mountains,|
a Laramide uplift. Photo by Richard Gibson (1982)
|Map of Laramide structures by Hamilton (1981), USGS. |
Contributions to Geology - University of Wyoming, V. 19, no. 2.
That’s really a huge amount of offset along a fault. The Wind River Fault slopes off to the east, under the Wind River Mountains, so the Precambrian rocks have been brought not just up, but up and over much younger rocks, including rocks of Cretaceous age. That’s how we know that the faulting dates to late Cretaceous time – we know the age of the rocks underneath the upthrusted Precambrian because those rocks have been encountered by drilling for oil and natural gas. The Precambrian granite and metamorphic rocks form a big triangular wedge faulted up over the entire stratigraphic section, from Cambrian at the bottom to the early and middle Cretaceous at the top.
So the continental crust is supposed to be strong. The word ‘craton,’ which we talked about back in the Precambrian, in January, means ‘strength.’ Wyoming is part of the ancient core of North America, dating back to more than 2.5 billion years. What could happen to break that supposedly strong crust? This is probably the biggest enigma in our understanding of the Laramide Orogeny – what force could break the crust, especially the crust that’s really pretty distant from the active collision, which was out to the west?
Out to the west, last month, the Jurassic, we talked about the collision that produced a long-lived magmatic arc whose roots are the granitic rocks of the Sierra Nevada Batholith today. That’s the action we’re talking about – a collision producing subduction – more or less standard subduction. It’s more than 1000 miles from the Sierra Nevada and California Coast Ranges to the Black Hills of South Dakota, so how was the energy transmitted that far, far enough to produce unusually huge breaks in the continental crust?
Probably the most common explanation is that the way the oceanic crust was subducting changed. The typical drawings you see of subduction zones show the oceanic crust diving down beneath continental crust at a moderately high angle, about 50°. This has been very well documented in subduction zones around the world, by accurate measurements of the positions of earthquakes not only with respect to the map of the surface, but also in the third dimension, depth. That really is the geometry of a typical subduction zone. The idea for the Laramide is that the angle of subduction changed from a high-angle dive to nearly horizontal – that the oceanic crust was pushing along more or less at the base of the continental crust rather than deeply beneath it.
If that happened, there would be two consequences. If the subducting oceanic slab did not reach depths where temperatures were high enough to drive off water, which migrated upward to melt rocks, we’d have a reduction in the amount of volcanic and magmatic activity – and this is observed in the rock record. Also, if that slab was being pushed almost horizontally beneath the continental crust, frictional drag along the base might have been enough to deform the overlying crust, breaking it along the huge faults that we observe, like the Wind River Fault.
So there’s a mechanism – but why would the subducting slab go horizontal? That doesn’t usually happen. One idea is that there was a change in the rate of subduction – the relative speeds of the colliding North American continental plate and the subducting oceanic plate increased. That’s not unreasonable, as rates of sea-floor spreading do change, sometimes significantly, and we find plenty of evidence for such changes in oceanic crust. We can’t verify that here, because the potential evidence in the oceanic crust is gone with the subducted plate.
Another possibility is that the oceanic crust that was subducting became thicker. I’m not suggesting that it somehow increased its thickness, but that an already existing, thicker portion reached the point of subduction. Thicker oceanic crust might be somewhat less dense, or stronger, and less prone to be pushed down into the mantle in the standard way. This is also reasonable, because there are in fact variations in the thickness of oceanic crust, although the variations are not usually huge. Typical oceanic crust is around 5 to 10 kilometers thick, 3 to 6 miles, but most of it is pretty close to 6 km or 4 miles thick. Some exceptions occur where mantle hotspots have poured more magma of basaltic composition, the same as parts of the oceanic crust, onto or into it. So, possibly a zone in the subducting oceanic crust that was thickened in that way reached the west coast of North America 80 million years ago or so, and because it was thicker, it went down at a lower angle, even close to horizontal, and therefore broke the overlying continental crust in Wyoming and South Dakota.
I think the general consensus is that some variation on the theme of a low-angle subducting slab is the basic cause of the Laramide Orogeny, but there’s plenty of debate about the details.
One other mechanism was proposed by Warren Hamilton with the US Geological Survey. He suggested that the Laramide deformation in Wyoming can be explained by avoiding the problem of transmitting the stresses 1000 miles or more from the point of collision in California. Hamilton suggested that the Colorado Plateau, a block of continental crust that’s more or less centered on the Four Corners area where Colorado, Utah, Arizona, and New Mexico come together, operated as a discrete, independent block at times. If the Colorado Plateau moved to the east, or rotated slightly clockwise, then you’d have a continent-continent collision that could provide a strong enough force to break the adjacent continent. The Colorado Plateau does have some differences from the main mass of the North American continent – it’s somewhat thicker, and in many places its margins have piles of volcanics that might suggest the Plateau’s boundaries are weak zones. The eastern side of the Colorado Plateau today is marked by the Rio Grande Rift, an active break, but one that dates to much more recent time than the Laramide Orogeny. But it might help us believe that the Colorado Plateau could operate as a relatively independent continental block.
There’s a thing that geologists do in bars, called the Wet-Napkin Experiment. Take a nice square multi-layered napkin, get it wet with beer. The consistency is important – more than damp, but not dripping wet. Put your fingers together and on the lower left (southwest) part of the napkin. Your fingers are the Colorado Plateau; move them gently to the right, colliding the Colorado Plateau with North America. Usually, this collision will produce wrinkles in the North American part of the napkin that have a distribution remarkably like the Laramide. You get the Uinta, Wind River, Beartooth, Bighorn, Laramie and Front Ranges. Often even the Black Hills and the smaller ranges show up. Note that there are plenty of mechanical issues with the Wet-Napkin Experiment, in terms of real life geology, but it’s kind of fun.
The mountain uplifts caused by the Laramide Orogeny coincide pretty well with the modern mountain ranges, but there’s more to it than just remnants of Laramide mountain ranges. The present-day mountains have a later story that we’ll get to next month.
—Richard I. Gibson
Links & References:History of Laramide Orogeny
The Most Mysterious Mountains in Wyoming
Green River Basin
Plate-tectonic mechanism of Laramide deformation. 1981, Hamilton, W. Contributions to Geology - University of Wyoming, V. 19, no. 2.