Eocene Farallon Lithospheric Chalupa

Also see my second post relating to the Farallon Plate here.

This past Friday I was talking with some of the people in my research group and we ended up discussing our favorite published figures. This is easy for me, hands down, it is the Eocene Farallon Lithospheric Chalupa of Humphreys (1995). I'll explain why I love this figure in a minute, but first...

I don't think I'd be out on a limb to say that when many geologists first look at a paper, they begin by studying the figures. Many branches of geology are very visual, the ultimate goal often being complete 4 dimensional reconstructions of complicated events. As such, most earth scientists spend a great deal of time learning how to make good figures. For many geologists, the steep part of this learning curve is field camp, when they first learn how to make geologic maps.

I think if you want to consider the information density of a figure, nothing can beat a geologic map. I am continually amazed at the amount of information that trained geologists can gather from geologic maps. The amount of information packed into a geologic map of course depends on the quality and scale of the map itself, but really good maps can detail billions of years of faulting, folding, erosion, magmatism, deposition, and metamorphism.

And one more "quick" aside. I think that my general reverence for geologic maps has played into my slight annoyance with Edward Tufte. This is probably entirely stupid and absurd on my part; I apologize in advance and fully accept criticism, but I am continually amazed that Tufte doesn't spend more time singing the praises of geologic maps. I had a chance to attend a public lecture by Tufte a few years ago. In general, I am a big fan of many of Tufte's ideas. I think that many people, geologists included, need to think seriously about how they visually represent their information. I think that geologists already have to spend a lot of time doing this, and so some of the examples seem kind of basic. I think this molehil of an annoyance of mine became a small end moraine of annoyance when I saw Tufte speak. First the background, this was a public lecture attended by about 600 people. Although Tufte is not categorically against things like PowerPoint or slides, he is much more in favor of archival quality paper. Unfortunately, these are expensive, and impractical for 600 people. So, the first 200 or so people who got into this lecture had beautiful large images to look at during the talk, images on such incredible paper that they are undoubtedly as bright and colorful today as they were 2 years ago. The rest of us got to crowd a dozen or so people around a single piece of paper, trying to follow along with his message. I would have loved a crappy PowerPoint slide complete with kitten background and lightning sound effects. I guess I am saying that I think it is important to strike a balance between quality and accessibility.

Long story short, I get a little bit stuck-up when it comes to geologists and visual representations of data. That is why I've decided to start blogging about some of my favorite figures.

So to start we have the Eocene Farallon Lithospheric Chalupa, published in 1995 in a paper by Eugene Humphreys (Geology, v. 23, n. 11, pp. 987-990). What this figure shows is his preferred model for the removal of the Eocene Farallon slab. The idea is that during the Laramide orogeny, the Farallon slab was subducting at a very shallow angle, effectively transferring plate boundary forces into the interior of the continent during the Laramide orogeny (making some of the Rocky Mountain structures). Beginning in the Eocene this slab was somehow removed, allowing hot asthenosphere to rise up and introduce a great deal of heat and basaltic magma into the western North American lithosphere. This led to the "ingimbrite flare-up," a period of intense volcanic activity in western North America, from Washington in the north down into northern Mexico. Volcanic activity did not begin everywhere at the same time, in fact it seems to have started at the northern and southern margins of the province in the Eocene. The volcanic front moved south from Washington, and north from Mexico, eventually meeting around Las Vegas in the Miocene (Vegas baby!) Many geologists interpret this to be the result of the gradual peeling away of the Farallon slab from both the north and south at roughly the same time.

So, here is the figure, the Eocene (~35 Ma) Farallon Slab!



So, with that background, what Humphreys attempts to explain in this paper is that pattern of magmatism, and how it could be related to slab removal. My favorite figure is his preferred model for Farallon slab removal. Why do I love this figure? Well.

1. It make sense. It takes a while to wrap your head around, but it explains the removal of the slab, and the associated patterns of magmatism.
2. The perspective. The view is looking up from the mantle somewhere underneath eastern Canada. The view is looking to the west, with the subducting Farallon slab coming right at you. I am used to map view and cross-sectional views, but I love the moxy to use this perspective. I wish google earth had this view.
   
3. The approximation. I like the fact that this is simplified and hand made. I don't think Illustrator has a feature that makes buckled subducting slabs. In my geologic education I have definitely felt pressure to make diagrams crisp and Illustrator-ific. I can imagine this figure originated on a sketchpad or (if I were to write this as a legend) cocktail napkin, and didn't get caught up in the refinement process.
4. It reminds me of a chalupa. Not that I consider Taco Bell excellent cuisine, but the buckled Farallon slab is a dead ringer for an enormous chalupa. And, it should be noted, that downing chalupas can also lead to flare-up events. I love mexican food, and Taco Bell is a distant cousin of mexican food, so that makes me smile.
   



5. I think my favorite thing though, is the fact that I didn't buy this figure at first. I first saw it in a seminar on Cordilleran geology when I was in graduate school. I remember spending a great deal of time trying to figure out what it was representing, and why this was the preferred model. At first I thought it was absurd and overly complicated. I am not an expert on slab removal or the ingimbrite flare-up, but I am now pro-Farallon Chalupa. Every time this figure comes up people who've never seen it are always unsure. Geology can be complicated. Geologic maps are complicated if you aren't used to looking at them. So I like that this image makes brains work. I don't think anyone immediately accepts this diagram, which is good. It is immediately interactive.

So what are your favorite figures? Any nominations? Feel free to nominate yourself if you have a figure you are particularly proud of.