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Tuesday, February 6, 2018

Episode 386 Dynamic Topography



What is dynamic topography? Well, it depends on who you ask. Dynamic topography is similar to other terms, like uplift, that have been used in so many different ways that you really have to look at the document you’re reading to understand what the author is talking about. This term has been applied to places around the world, like the Colorado Plateau in the United States, South Africa, the Aegean, and East Asia, which makes it even more complicated to tease out its meaning.

Most broadly, dynamic topography refers to a change in the elevation of the surface of the earth in response to something going on in the mantle. This “something” can include both the flow of the mantle, as well as differences in mantle temperature or density. For the purposes of this podcast, I will use a more strict definition: Dynamic topography is the change in the elevation of the surface of the earth in response to the upward or downward flow of the mantle.

How much higher or lower can dynamic topography make the earth’s surface? Well, that’s a matter of debate. Earlier studies have suggested that several kilometers, or over 6000 feet of modern elevations can be explained by things going on in the mantle. More recent work instead suggests that dynamic topography creates changes of at most a three hundred meters, or a thousand feet.

A good example of a place where this process is thought to be active is Yellowstone. As Dick Gibson discussed in the December 19th, 2014 episode, Yellowstone is thought to be a hot spot. That is, an area of the earth where hot material moves from deep within the mantle to the base of the crust, causing significant volcanism at the surface of the earth. Other well-known hot spots are located in Hawaii, and Iceland.

So how can a hot spot like Yellowstone cause dynamic topography? Well, you’ve probably seen a similar process at play the last time you played in a pool or a lake. Think of the surface of the pool like the surface of the earth. If you start moving your hands up and down under water, the surface of the pool starts to move up and down. If you ever tried to shoot a water gun upwards underwater when you were a kid, you probably remember it pushing up the surface of the water, and being disappointed that it didn’t shoot out at your friend or sibling. As an adult, you could try holding a hose upwards in a pool. Again, it probably won’t shoot out, but will gently push upwards on the surface of the pool.

Dynamic topography concept. © Commonwealth of Australia (Geoscience Australia)
 2017, used under Creative Commons Attribution 4.0 International Licence 
The principle for a hot spot creating dynamic topography is the same. The flow of the mantle pushes upwards, warping the crust and increasing the elevation of the earth’s surface above the hot spot. Near Yellowstone, this results in an area of high elevation which lies next to the Snake River plain.

But Dynamic Topography doesn’t just cause increases in elevation, it can also pull the earth’s surface downward. In North America, dynamic topography is thought to have been in part responsible for the creation of the Cretaceous interior seaway. 

As a reminder, the Cretaceous interior seaway was a shallow sea that covered parts of western North America, in middle to late Cretaceous time, about 100 to 79 million years ago. Its size varied, but at its greatest extent the seaway stretched through Texas and Wyoming in the US, and Alberta and to the Northwest Territories in Canada. It was widest near the US-Canadian border, where it stretched from Montana to western Minnesota.

Low elevations in western North America that allowed the ocean to flood in and form this shallow sea may have been caused by downwards flow in the mantle. This downwards flow was likely caused by oceanic crust that was subducted at the western margin of North America. That is, oceanic crust that went underneath the North American plate and into the mantle. Because this crust was part of the Farallon oceanic plate, it is often referred to as the Farallon slab.

As oceanic crust associated with the Farallon plate continued to sink into the mantle, it continued to cause changes in the elevation of North America. This drop in elevation likely decreased in size as the Farallon slab moved towards the eastern edge of North America, and deeper into the mantle.

Since Eocene time, or about 55 million years ago, dynamic topography associated with the Farallon slab is thought to have been in part responsible for lower elevations in the eastern United States. A wave cut escarpment called the Orangeburg Scarp is now located 50 to 100 miles inland from the coasts of Virginia, Georgia and the Carolinas. It formed at sea level and now lies up to 50 meters, or about 165 feet above the modern coast line. In fact, a good part of the southeastern US to the east of this escarpment contains marine sediments, and smooth topography as a record of its time underwater.

Differences in the elevation of the Orangeburg Scarp along its length suggest that rather than just going up and down, the Atlantic coast experienced a broad warping caused by mantle flow. The most recent phase of warping brought this area to modern elevations, as warm material moved into the upper mantle beneath the Atlantic coast. This warm material helped push the crust up to higher elevations, creating the southeastern US as we see it today.

This example also highlights an important part of dynamic topography: If you are already at really high or really low elevations, you might not notice it much. If you are near the coast, it can have a big impact as the sea starts to flood in and out due to changes in the mantle. Provided of course, you’re there for the millions to tens of millions of years it takes for the mantle to flow this way and that. That’s why geologists typically rely on the rock record to provide evidence for processes like dynamic topography.
—Petr Yakovlev

This episode was recorded at the studios of KBMF-LP 102.5 in beautiful Butte, Montana. KBMF is a local low-power community radio station with twin missions of social justice and education. Listen live at butteamericaradio.org.

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3 comments:

  1. I was very interested to find this post having just read a short overview of the Western (Cretaceous) Interior Seaway which suggested downwarping was isostatic--due to "over-thickening of the crust in the thrust belt to the west" ... "pushes the adjacent crust downward." How does this fit with dynamic topography? Is isostatic downwarping older thinking? I did some reading but found no reference to it in articles about DT and the WIS. Thx (overview was in the new "Ancient landscapes of western NA" by Blakey & Ranney)

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    1. I talked with Petr, and we agree with what you found that the foreland basin, where there is dramatic thickening of the Cretaceous section, is mostly the result of crustal downwarping because of the load from thrusts and sediments along the western edge of the mountain belt. Here in Montana, the Cretaceous Kootenai formation thickens from 300 or so feet in the Tobacco Root Mountains to more like 4000 feet at Sandy Hollow, just 50 miles to the west. Petr pointed out that the foreland of the Himalayas in India is a deep trough, but relatively narrow. The Cretaceous Interior Seaway extends from western Montana to the eastern Dakotas and eastern Nebraska, hundreds of miles, much further than the narrow trough that marks the axis of the foreland basin. It’s that gentle downwarping that Petr attributes to the downward pull of the Farallon slab plunging into the mantle. In the Nov 22, 2014 episode http://historyoftheearthcalendar.blogspot.com/2014/11/november-22-sevier-orogeny.html I pretty much attributed the seaway to the general downward pull of crustal warping due to the far-field effects of the foreland basin plus generally high sea levels. That’s probably wrong; the foreland basin influence probably doesn’t extend that far to the east (many hundreds of miles from the axis of the foreland basin), and the gentle, broad influence of downward flow in the mantle is a better explanation for the Cretaceous Interior Seaway, at least its central and eastern portions. We think. But the whole thing is likely a combination of the effects of downward flow in the mantle PLUS the downwarping of the crust because of loading in the western part of the basin. Thanks for a great question that gave Petr and me a chance to have a beer and geology talk!

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  2. Thanks for the detailed discussion. I'm glad you were able to enjoy a beer in the process :) It does seem that with such dramatic asymmetric foreland basins that two different processes could be at work.
    Also enjoyed the description of Sevier vs. Laramide in your 2014 post.

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