Alaska's Brooks Range)

Hello from Ellensburg, Washington, USA. This is the Nick Zettner Geology Podcast. Today's topic, Alaska's Brooks Range. Thanks for listening and for watching. This is a dress rehearsal. This topic will be featured on stage with chalkboards next week, April 10th, a Brooks Range geology lecture. And I mostly have the lecture written, but I thought that I would share in this format some of my ideas and just kind of work out the kinks. So let's go 30 minutes or less with the Brooks Range geology. If you're watching this instead of listening, you can follow along with some of the diagrams that I have under the document camera. So I have everything laid out using specific chalkboards and I have a rough outline for myself. Let's get right into it. If we think about the geography of the Brooks Range, where is that? It's a major mountain range in extreme northern Alaska. It's above the Arctic Circle. So it's north of Anchorage, north of Fairbanks. It's just south of the North Slope, famous for the oil pipeline. Bering Sea is to the north. Sorry, Arctic Ocean is to the north. Bering Sea is to the west. So it's this kind of general east-west trending mountain range known as the Brooks Range, which I really didn't think that I was going to focus on at all with the Alaska A-Z series this past winter. That was from November to February. But before I knew it, I was digging into the Brooks Range and here's the first major point of today, dear listener. The Brooks Range is a time capsule. The Brooks Range was created more than 100 million years ago. That's far older than the Rocky Mountains of North America. And yet nothing has really happened to the Brooks Range in the last 135 million years. It's just sitting there. You're like, well, what do you mean nothing's happened to it? Well, it hasn't eroded away. It's still a mountain range. Maybe it was a little bit bigger when it was at its peak power, but it's still there. But more importantly, it has not been screwed up by major strike-slip faults. The Brooks Range has not been screwed up by lava flows coming later and burying portions of the mountain range. It hasn't been invaded from below by major plutons that are younger than the Brooks Range and screwed things up that way. We haven't had a whole series of sedimentary basins to get developed on top of the thing. So that's important because if we think of the rest of the mountains of Western North America, down through Yukon, down through British Columbia and Alberta, down through the lower 48 states and into Mexico, there's been all sorts of geologic activity since 100 million years ago to significantly screw up everything. And so of course there's major debate about what happened in the early days of the Rocky Mountains and other things like the Basin and Range Provinces extended everything. The flood basalts of the Columbia River Basalt Group, the German Chocolate Cake has buried a bunch of this stuff. Major strike-slip faults have taken half of a mountain range and sent it north more than a thousand kilometers. But the Brooks Range is still intact. So I think it's not only worth today thinking about the Brooks Range as a mountain range and just kind of talking about what field geology we have up there and what tectonic story has been told involving westward subduction by everybody, I might add, not just a few. Everybody agrees westward subduction of an ocean plate subducting westward underneath a volcanic arc out in the ocean, everyone agrees on that. And you're like, okay, well, what's the big deal? Well, nobody agrees that westward subduction, hardly anybody agrees that westward subduction is a big part of the mountains down here, south of Canada or even Canada. So why is the Brooks Range viewed so differently than the rest of the mountainous Cordillera of North America? And is it possible that there is some westward subduction in a major way down here because that's what the Brooks Range is telling us? Yeah, that was the bold open. I don't know if it worked for you. So yeah, we're going to the Brooks. And if we look at a map showing a little bit more structure, we can see that there's some beautiful orocline, some big, beautiful, big, major swooping folds that go through the entire state of Alaska, the big Z or the big Z as it's known, and how much of the geology in Alaska continues over to Siberia and Kamchatka and things like that. That's not really our topic today. But when we realize that the Brooks Range is a major collisional mountain range, that's the first point. And I just said it, I just spilled the beans 10 minutes ago or five minutes ago. It was a collision. Everyone agrees that the Brooks Range is the result of a major collision 135 million years ago between an oceanic island arc in the Pacific, a linear oceanic island arc, think Japan, with a continent. And yes, it's a portion of North America that was originally a part of Northern Europe and Northern Asia, Baltica as it's known. But that collision or that addition of Baltica with North America happened long before 135 million years ago. I didn't have that until a couple of days ago. I emailed Sarah Ruskey, University of California, Davis, and she set me straight. So it is North America that's colliding with this island arc. So if we realize that's the story, it's the geometry of the ocean plate that's involved that is crucial. So down here, again, coming back to that major message, down here, everybody loves, especially because the California geologists back in the 1960s and 1970s were hammering this idea that the Farallon plate was subducting eastward beneath California and therefore eastward beneath much of North America and the California triad with a four arc basin and a volcanic arc and a subduction complex at the margin. And depending on how you sketch the thing, eastward subduction at the Western margin of North America has been the dominating paradigm, the dominant scene, the master company line, eastward subduction of an ocean plate to make the Sierra Nevada mountains, eastward subduction underneath Western North America to explain damn near everything, including the Rocky Mountains, including the Cascades. Okay, well, let's not get nuts here. The Cascades here in the Pacific Northwest, where I live in Ellensburg, Washington, we're just immediately east of the Cascade volcanic arc. Yeah, that is eastward subduction. We've got the geophysics to prove it. We've got the imaging, we've got everything. So 46 million years of eastward subduction of a Farallon plate that's now known as the Juan de Fuca plate is the story. So yes, eastward subduction is a thing. But pretty quickly, if we think about the Brooks Range, it's not an eastward subduction story. It's a westward subduction story. So let me try to describe this to you as you're listening and driving down interstate, whatever. If we have westward subduction, that simply means that the Western margin of North America is continental crust. Okay, so can you picture that off to the east? Okay, west and east. Okay, this is going to be too much, right? I'm showing the viewers who are watching this on YouTube, I'm showing them a little sketch. So I'm just going to try to use my words to describe the sketch. So on the left side of the sketch, there is a volcanic arc out in the ocean, an oceanic island arc. And that means that there's composite cones that are erupting regularly on this oceanic island arc. It's arc magmatism. And arc magmatism means we need a subducting ocean plate, but we're out there in the water in this sketch. Okay, off on the right part of the sketch is continental crust, it's thicker crust, it's buoyant continental crust, it's part of North America, a longer history of North America, leave it alone. There's a body of water, there's an ocean between this lonely oceanic island arc out in the water to the left, and the North American continent to the right. We good? But what I'm showing in this sketch is an ocean floor, in other words, an oceanic plate that is moving west with western North America. Now that's a surprise if you're just trying to visualize this, isn't it? Again, if we think of eastward subduction, we have a plate boundary between the ocean floor and the continent. We have a plate boundary. We have two different plates. In other words, we're going back to eastward subduction. There's one ocean plate that's diving beneath a different plate that's got continental crust on it. Well, this is radically different. Westward subduction is saying, yeah, there's a plate boundary between two tectonic plates, but the plate boundary is out at the oceanic island arc. It's not at the western margin of North America. Can you visualize that? Here's another way of saying it. If we're contemplating the concept of westward subduction, which is what we're doing, there is no plate boundary right at the western margin of North America. Therefore, the continent of North America and the ocean plate that's west of it are all one plate. And you're like, is that even possible? Can you actually have continent and ocean crust on the same plate? Yes. Today, the North American plate has what? It has half Atlantic ocean floor crust on it and half North American plate. Continental stuff. So, if we're moving North America westward, the continent of North America in this westward subduction sketch, and the ocean plate is connected to the North American continent, that means both the ocean floor and the North American continent are moving at the same rate because it's the same tectonic plate. And we're literally pulling the leading edge of the ocean floor underneath the oceanic island arc. Therefore, the subduction to make the arc magmas is from westward subduction. I hope you can see it now if you're visualizing. And yeah, there's a four arc basin with an accretionary wedge just to the east of the volcanic arc out there in the water. Trench, magnitude nine, quakes, the whole thing. Well, if you just keep, let's finish the thought then, if you keep pulling this ocean plate under the oceanic island arc, the distance between the North American continent and the oceanic island arc is going to get smaller and smaller. In other words, we're going to start slowly but gradually diminishing the size of the ocean basin until what? Until we finally close the gap and have the continent of North America pulled right next to and underneath the volcanic arc. And what we have then is a collision. It's a collision between a volcanic arc and a continent. That's what made the Brooks Range in northern Alaska. And you're like, really? There's no ocean up there now. But there was. Before 135 million years ago, there's evidence that there was an ocean, a vast ocean, and we have a name for it, the Angiuchem Ocean, which was between the Koyukuk volcanic arc out in the water, out in the Pacific, and the North American continent. So if you followed my description of the westward sketch, now we're just adding place names or geologic names that are now intimately part of the history of the Brooks Range. One more time, the Koyukuk volcanic arc was the line, that was the Japan thing, the Koyukuk arc. And there's now data that says that that Koyukuk volcanic arc out in the Pacific Ocean, before 135 million years ago, was more than 3,000 kilometers long. It was a major microcontinent, essentially, but let's just say volcanic arc. So that's the Koyukuk arc. The Angiuchem Ocean is the body of water. The floor, the oceanic plate beneath, the oceanic crust beneath the ocean water was the Angiuchem Ocean floor. And then North America is North America. Okay. So it's the timing now that we want to try to work with and try to convince ourselves that this sketch, we're kind of doing it backwards. I'm not starting with the field evidence. I'm doing kind of the story first, and then we'll go and look at the evidence. But the story is when we finally get rid of the Angiuchem Ocean, and therefore when we finally get rid of the Angiuchem Ocean floor, getting it subducted all through or all down this oceanic trench underneath the Koyukuk arc, we're finally going to have a collision between the Koyukuk arc and North America. And it's that collision that creates the Brooks Range 135 million years ago. And then I'll finish my thought here that that continental crust doesn't just kiss the Koyukuk arc. The North American continental crust gets pulled down the trench as well. And you're like, wait a minute. I didn't think continental crust can subduct because it's too thick and lightweight and buoyant. And that's true. So you're like, what? No, that's true. Continental crust does not subduct and go all the way down into the mantle because of its thickness and buoyancy. So this continental crust is going to get pulled into the trench. It's still connected to the ocean plate, but we're eventually going to snap off the ocean crust, the Angiuchem Ocean floor from the North American continent because this continent is not going to want to go down. And when we snap that off or when we break the ocean plate away from North America's continent, then there's a boinging back up. There's a dramatic uplift at the suture between the volcanic arc and the continent. And that ultimately is why we're having this uplifted hinterland of the Brooks Range. Okay. So that's the story. And you've heard the number a couple of times now, maybe more than a couple, 135 million years ago. And 135 million years ago is one of many, one, two, three, four, five, six, seven, eight different collisions. If you listen to Robert Hildebrand, and I've been listening to Robert Hildebrand closely this past few months, Bob is a collisional tectonics person. And Bob says, if you look carefully in the history of the North American Cordillera, what did I just say? I forgot eight. There are eight separate collisions that are recorded in the rocks. And you're like, well, I've never heard that before. Well, this is a new way of looking at the geology of the American West. And yet it's an old way, if you realize that Alpine geologists and Himalayan geologists have been working this out before. What they know how to look for is what I call a collisional suite. Let me try to describe this for you. So if we look at a map of a series of features that individually don't seem like much, but collectively have amazing power, that's what we want to try to visualize here. So if I'm trying to back up this story, and I am, if I'm trying to back up this story, that there was an arc continent collision 135 million years ago to make the Brooks Range, I better have some compelling evidence for it. Well, I've got two kinds of evidence. I've got surface geology evidence from USGS geologists who did their work 40, 50 years ago. Maybe they were mapping and didn't even understand the significance of what they were mapping. I don't know. And then there's more recent mantle tomography where we can find the ocean slab that went down the tubes. We can find the Angiuchem ocean floor. It's still down there. And they work beautifully together. The mantle tomography in this case with the Brooks Range and the surface geology in the Brooks Range is a beautiful marriage, which is something that has not been easy to do down here, a little closer to Ellensburg. Okay, so let's do the surface geology first at the 18 minute mark. The collisional suite, as I call it, let me try to From west to east. No, let's think of it north. Sorry. Let's think of it south to north in the Brooks Range. Okay. So if the Brooks Range is an east-west mountain range, and generally it is, let's go from south to north. Let's go from the Fairbank side to the north slope side, and just hike up and over the Brooks Range. Can you do that with me? The first thing we're going to run into are some plutons, magmas that invaded from below. And those plutons are arc-related. Those are the magmas of the Koyukuk volcanic arc. Those are the magmas that were created from subduction, westward subduction, before 135 million years ago. So let's think of the timing of this. I guess I can't hold it. I'm going to go back and forth between the tectonic sketch, the interpretation in other words, and the field data itself. So the Koyukuk volcanic arc lavas, and also the Koyukuk volcanic arc underground plutons have an arc signature. They have an arc magmatism because of the subduction of the ocean plate. And they're older than 135 million years ago. Does that make sense to you? If we have subduction, we haven't done the collision yet because we have the ocean. So it's this game of working out the timing as well as the geometry here. So if we have, it works. If we have arc magmas that are older than 135 million years ago, we haven't closed the gap yet. We haven't done the collision. But as we hike from south to north across the Brooks, we're going to eventually get into some other magmas a little bit further north that are younger than 135 million years old. Those are post-collisional magmas. Those are magmas that are forming after the collision. And when we break and snap the ocean floor from the continent of North America, at the boundary between those two plutons, the arc magmas that are older than 135 million years old, and the post-collisional magmas a little bit further north in the Brooks range, which are younger than 135 million years ago, at the suture, there are some green rocks, an ophiolite suite. And I'm hoping to stick the landing today, talking a little bit with some exciting new stuff about the ophiolite interpretations. Let's keep going north though. In that post-collisional slab failure magmatism, there's also some uplifted metamorphic rock, mid-crustal rocks that are now up at the surface. Further north yet, there are some major thrust faults that are from the collision. And finally, further north yet, are some sedimentary rocks that are being shed away from the uplifted guts of this collision. So if we're doing that on the map and visualizing those different kinds of magmas, uplifted mid-crustal rocks, thrust faults, and sediments, if we think of all those things together, and the ophiolites between the two magma bodies, in cross-section, it maybe works a little bit better. From the side view, in other words, that collisional suite, left to right, is arc magmatism, suture with an ophiolite, slab failure magmatism with uplifted mid-crustal rocks, thrust faults further to the right, and sediments being shed away from the uplifted area. And we have terms called hinterland and foreland, which give us the idea that at the true guts of the collision, we have an uplifted scene. Those were the mountains were nice and big. So the Brooks Range was clearly taller than it is now when it was active 135 million years ago. But it's asymmetrical. This sketch does not have the suture right in the middle of the sketch. It's right off to the left. I'm looking at the cross section now with the viewers. And so most of this scene, two-thirds of the collisional suite on the right-hand side of the suture is the continental signature. And it's only the left third of the collisional suite that's the arc itself. But everything is forming because of the collision, because of the closing of the oceanic basin. And in the case of the Brooks Range collision, we have a range of about 10 million years of all this post-collisional magmatism, uplift in the hinterland, thrust faults to the north, and sedimentation. Whereas again, the arc magmatism is older than 135 million years ago. So that's the surface evidence for the Brooks Range. Can I do the final parts of this in five minutes? I'm going to try. There's mantle tomography that backs this up as well. And the timing works. So how does that work? Well, if we're looking at a sketch again of the North American continent inching closer and closer to the Koyukuk arc, we know that now. What happens to the Angayutam ocean floor when it subducts? It doesn't disappear. It doesn't melt. It continues as an oceanic plate and starts folding on itself like ribbon candy. And that ribbon candy continues all the way down into the lower mantle. But there's a beautiful and important story with the Angayutam ribbon candy. It's been beheaded. In other words, we were depositing ocean plate into the ribbon candy to make the Angayutam slab wall. But when we did the collision of North America with the Koyukuk volcanic arc 135 million years ago, we ran out of ocean plate to subduct, right? And so then suddenly the Buick, which has hit the moose, is now going to just keep motoring down the highway with the Koyukuk arc splattered all over the windshield of the North American continent. And this slab wall made of Angayutam ocean floor just continues to sink down below the surface. Here's the wild part. Karin Siglok, mantle tomography geophysical expert, has been able to work very carefully with this Angayutam slab wall. The depth from the top to the bottom of the ribbon candy, which is physical evidence of what used to be the ocean floor between North America and the Koyukuk arc, she has the depth of that Angayutam slab wall between 1,350 kilometers deep, that's the top of the slab wall, and 1,800 kilometers depth at the base of the slab wall. 1,350 and 1,800, those are kilometer depths of this ribbon candy. Again, it's got no connection to the surface anymore. What's the message there? The depth of the slab wall is directly equivalent to the age of when the ocean floor was piling up in the mantle. And because there's a steady sinking rate that Karin has devised, 1,800 kilometers depth at the base of the Angayutam slab wall equates to 180 million years ago, which is the beginning of the subduction of the Koyukuk arc and the collision to finally get rid of any ocean crust subducting underneath the oceanic island arc is at 1,350 kilometers below the surface. That equates to what? 135 million years ago works out, it's almost too perfect. The correlation, the match between the timing of the forming of the slab wall down in the mantle and the timing of the arc continent collision with the collisional suite in the Brooks Range when you hike around with your boots and you bang on a bunch of rocks and you eat your Cheetos for lunch on top of some of these rocks, including the ophiolite, that's the story. And it's a westward subduction story. We finish with this. The ophiolites, which are at the suture, I call them green rocks because many of those rocks were originally igneous, but they've been metamorphosed a fair amount, so they typically are literally green, peridotites, serpentinites, and so on. They're beautiful green rocks that have been found all over the place, and they typically represent part of the ocean floor that's been caught in this collision. For my whole career, I have taught that ophiolites form at mid-ocean spreading ridges, and that was the company line for how to interpret an ophiolite suite that you can find on land. You just visualize it's some sort of ocean spreading ridge out in the middle of somewhere, an East Pacific rise, a mid-Atlantic ridge, something like that. Well, there's a new tectonic interpretation that is growing among geologists who think about such things, and includes Michael Eddy, the guy that does a bunch of creative science, and Mike and I are close collaborators at this point. Mike, watching the Alaska A-Z series this past winter, has been inspired to write a new project on the Brooks Range, and he wants to get a date on the ophiolite that's in the Brooks Range. Why? Because the new tectonic interpretation of ophiolites is that they form at the beginning of subduction. You need a spreading regime for an ophiolite, but Mike and Bill Stearns and others convincingly say that when you first start an ocean plate subducting—in other words, you take an ocean plate that's just passive, and then you kind of lower the drawbridge, and you start to subduct the ocean plate beneath a volcanic arc—that's an extensional environment in the forearc basin, right before a volcanic arc out in the water. Mike Eddy says, how about the age of the ophiolite marks the age of the beginning of the Koyukuk Arc? Let's test that. If this timing is so great, with the mantle tomography and the collisional suite on the surface using the magmas and everything else, what's the number? 180. What's the number for the beginning of the subduction of the Angiuchem ocean floor beneath the Koyukuk Arc 180 million years ago? Mike Eddy says, I want to go up, even though it's remote, I want to find some exposures of this ophiolite in the Brooks Range, and I want to see if the ophiolite's 180 million years old. And if it is, we have even more evidence that we have the timing of the formation of the Brooks Range—that's the collision—but even the beginning of the Koyukuk Arc. And that even works better and bolsters the idea that there was no Koyukuk Volcanic Arc until 180 million years ago. So the important dates for today, 180 million years ago, is from many different lines of evidence, the beginning of the Koyukuk Volcanic Arc. And at that time, 180 million years ago, North America is not even close to this lonely island arc. But between 180 and 135 million years ago, we slowly close the gap and we slowly narrow the Angiuchem ocean until finally 135 million years ago, there is no more ocean, there is no more ocean floor, there is no more subduction, and that ophiolite gets caught between the arc and the continent. And it's still there. All these rocks are still there in the Brooks Range of northern Alaska. Have I gone more than 30 minutes? Yeah, I have. But that's the layout that I have for this chalkboard lecture. And I have to confess something. First of all, thank you for making it to the end. Second of all, that was way harder than I thought it was going to be. But the visuals are really going to be needed with the chalkboards. And you know, the format I have with these downtown lectures is I do 30 minutes of chalkboards and 30 minutes of, you know, animations and maps and other things. I don't know, maybe it's going to be 40 minutes of chalkboards and 20 minutes of slides because I do feel like I really need to lay this out on the chalkboards the best I can. Halfway through this, dear listener, dear viewer, starting like, boy, how am I going to do this? I don't know. I'd be curious, especially here on YouTube. If that worked for you, can you leave a comment down below? Or if there are parts of what you heard that didn't quite work for you? I've got a week to read your comments before I do it for real. And yes, that lecture on Friday, April 10th, 2026 will be recorded and filmed for YouTube. So you'll be able to see this lecture in polished form sometime later in April, I guess. Dress rehearsal is done. I'm feeling good to get this out here. And I thank you much, dear listener, dear viewer, for watching and listening to what? A podcast episode in the Nick Zettner Geology Podcast Library called Alaska's Brooks Range. Thank you. Love you. And goodbye from Ellensburg, Washington, USA.