This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. To a first approximation, tectonic plates behave like rigid blocks of lithosphere. But this approximation breaks down near plate boundaries, where the interaction of a plate with its neighbors can compress it or extend it. The effects of compression, such as when two plates collide, are clear. The edge of a plate crumples and throws up a mountain belt. But when the interaction of a plate with its neighbors causes it to stretch, the effects are usually hidden. Stretching thins the lithosphere. The surface subsides, and the basins that form fill with sediment that buries the evidence. There are a few places where this isn't so, and prime among them is a region of the western United States and northwestern Mexico called the Basin and Range Province. Here, the stretching is recent enough and the land high enough that it remains on open view: dozens of regularly spaced mountain ranges between the Colorado Plateau to the east and the Sierra Nevada to the west, in a crust that has been pulled out to about twice its original width.

Brian Wernicke has devoted the bulk of his long and prolific research career to the study of the Basin and Range Province. He is an emeritus professor in the Division of Geological and Planetary Sciences at Caltech. Brian Wernicke, welcome to Geology Bites.

Brian Wernicke: Thanks. This is a great opportunity, Oliver, to discuss something that really changed the way people look at rocks in general, and that's evolved over the last 30 years.

Oliver Strimpel: Why is the Basin and Range such a good place to study lithospheric extension?

Brian Wernicke: Well, the main reason is that it's above sea level and exposed, so geologists are able to access it. But there are three main things that we can do in the Basin and Range. Because it's an active, extending area, we can first look at the geodesy, that is, the geodetic motions of crustal blocks, by putting out geodetic antennas and watching how they move with respect to one another. And then secondly, we can look at the record of ancient earthquakes that have occurred in the Basin and Range, which is a field that has flourished over the last 30 years. And now we have a detailed record of the earthquake history for much of the province, and that gets us back about 100,000 years to a million years. And we can compare that with the geodesy of the present day. And then we have long-term geological offsets, that is, features that predate the beginning of extension, where we know the pre-extensional geometry of these geological markers. And so we can reconstruct all the markers block by block and demonstrate that the province has widened very substantially over the last 20 million years.

Oliver Strimpel: OK. Let's talk about each of these three manifestations of extension, starting with the GPS monitoring. How does that work, and what are we able to see that way?

Brian Wernicke: Well, there was a technological advance that occurred, mainly by workers at MIT and Harvard, way back in the '80s, called space geodesy. And that's basically making interferograms, or records of how electromagnetic waves from space interfere with each other at different points on the surface of the Earth. And you can very sensitively evaluate how far two receiver antennas are apart from each other. So you bolt these antennas into the ground, and with space geodesy you use these interferograms. At first it was interstellar sources of radiation coming down, but more recently we have the advent of GPS satellite geodesy, and the satellites radiate the perfect wavelength for doing geodesy down to the surface of the Earth. And those interferograms give us very precise positioning. So the advent of GPS in the late '80s and early '90s allowed us to generate the first results, where we could get centimeter-scale positioning of two points on the Earth. So if I have Paris and New York, I can see them move 1 or 2 centimeters apart from one another on the basis of space geodesy and the earliest GPS geodesy.

And then Jim Davis at Harvard and I collaborated on a project to establish in the Basin and Range about a 70-site geodetic network of antennas, anchored very, very firmly to the deep bedrock. We drilled 30-foot-deep holes with cement-anchored posts, and those posts are isolated from the messy upper part of the soil and all that, and they come together in a tripod, and then we machine-bolt the GPS antenna on top of that, and those create ultra-stable monuments. So you see what the Earth is doing rather than what the soil is doing. And in the results of that network, we were seeing baselines change length by a millimeter per year, with only about a scatter of 1 millimeter in our daily measurements. And so when you look at all the points over the course of a year, your precision in the velocity between the two points is around 100 to 200 microns per year. So that's about a tenfold improvement on what the earliest space geodesy measurements were showing us. So we set that up all over the Basin and Range, and what we found was that there was a very smooth, active overall pattern of deformation, with the Sierra Nevada on the west moving away from the Colorado Plateau on the east, northwestward, at about 12 millimeters per year, plus or minus a fraction of a millimeter per year. So very precise.

Oliver Strimpel: OK, the second method you mentioned was paleoseismology. How can we actually look back at ancient earthquakes over the millennia and use them to establish a certain amount of extension?

Brian Wernicke: Earthquakes in the Basin and Range occur on what are called strike-slip faults, where one block moves directly past another block horizontally, but most of them are normal faults, where one block moves downward with respect to the other block. And that's what accommodates the ongoing extension in the Basin and Range. So there's a very detailed record of earthquakes over the last 100 years, when we've had seismic instrumentation in the Basin and Range, and those show earthquakes all over the place. But the big ones are concentrated in a couple of belts, one running along the east side of the province from Las Vegas to Salt Lake City, and the other one on the west side, running between Reno and southern California. And there have been about a dozen or so earthquakes that have been recorded instrumentally. And when those earthquakes move, we see the surface of the Earth break in a spectacular fault scarp. When two huge blocks move in an earthquake, they move about 2 meters in each earthquake, and that ruptures the ground surface. And along the ground surface, we see these roughly 50- to 100-kilometer-long breaks that occur during the earthquakes.

And those same breaks are very well recorded in the geological history. That is, even if you didn't have the seismological record, you can see the fault scarps upon which repeated earthquakes have occurred. From that point, you identify the fault scarp. There are literally hundreds of these in the Basin and Range, and maybe a few dozen big ones that go for hundreds of kilometers and are clearly the result of past earthquakes. And so we go into those and we dig a trench, usually 3 to 5 meters deep, right across the trace of the fault scarp. Now, when the earthquake happens and one block moves down with respect to the other block, what immediately happens is there's usually a small slump or landslide of material that comes off of one block and down onto the other block. You can date the sediments that come right up to before the slumping event, and then date the sediments on top of the slumping event, and pinpoint the time of that ancient earthquake. And this has been done over and over again in the Basin and Range. Something like 200 earthquakes have been documented on ancient Basin and Range faults, so we really have a pretty good picture filling in there.

Oliver Strimpel: So the third manifestation you mentioned were the geological offsets. Could you tell us about those?

Brian Wernicke: Yes. The Basin and Range has a rich history going back some 600 million years, and for much of that period of time, the Pacific was its own ring of fire, if you will. That is, there has been subduction of oceanic plates, with island arc volcanics on top. So there's a record, especially over the last 300 million years, of these arcs affecting the western margin of North America. And also during this time, there were a number of contractile tectonic events that pushed the crust together and thickened it up. So you have this long-term, continuous collision between the oceanic Pacific plates and the North American plate going on, steadily thickening up the crust of the North American plate. And the latest period of this is still going on in the Pacific Northwest, off Oregon and Washington, where we have a little piece of a plate left, subducting under North America.

So from that ancient history, pre-Basin and Range, we have a wealth of very tractable markers that allow us to take where the markers are now and restore them back to where the markers were prior to when the extension began. And so these markers include old thrust faults that recorded the crustal contraction. They include magmas, belts of magmas that were linear, that we can see are offset by the faults. And they also include what are called isopach lines, or sedimentary thickness contours, of the pre-extensional sediments. So when you measure a whole array of stratigraphic sections over a region, you see very systematic variations in the thickness of these sections. For example, the Ordovician period has a very strong variation, from zero thickness to about 1,500 meters thickness, going from east to west across the central part of the Basin and Range. And so you can draw contour lines between all of the Ordovician sections, and you can see the contour lines run into Cenozoic faults and are dramatically offset, some as much as 50 or 70 kilometers, from their trends on either side of the geological feature.

So the thrust faults, the plutons and the sedimentary contours do a very good job of allowing us to reconstruct the total amount of extension over 20 million years or more. In 1988, we published a reconstruction all the way from the Sierra Nevada to the Colorado Plateau that established a 250-kilometer total extension, the total motion of the Sierra relative to the Colorado Plateau. It's about a factor of three at the latitude of Las Vegas, Nevada. Where the Basin and Range is wider, it's the same net amount of extension, because the Sierra hasn't rotated. So up in the northern Great Basin, there's still 250 kilometers, but the province there is 750 kilometers wide, and so you get something like a factor of 1.5 stretching for the very widest part of the Basin and Range. So on average, a good rule-of-thumb number would be about a factor of two stretching for the crust, from Arizona all the way up to British Columbia.

Oliver Strimpel: Before we talk about how this enormous amount of extension was accommodated in the Basin and Range, let's step back and look at the bigger tectonic picture. I mentioned colliding continents as the context for plate compression, but what tectonic context is responsible for this stretching of the lithosphere?

Brian Wernicke: Well, there was an important switch in the driving force along the edge of the North American plate, and this switch occurred due to the interaction between an ancient plate that largely no longer exists, called the Farallon plate, and its conjugate twin to the west, which occupies most of the Pacific Ocean, now called the Pacific plate. So the general picture is that the two plates were separating from one another, divided by the East Pacific Rise. The rise was out in the middle of the Pacific Ocean, but through time it gradually moved westward, and the ridge itself first made contact with western North America about 30 million years ago, at the latitude of Los Angeles. And when it hit, the continent, rather than having the Farallon plate moving eastward underneath it, now had the Pacific plate moving west-northwestward away from it.

Oliver Strimpel: OK, because it was a spreading ridge, so the westward side of it was moving to the west and the eastward side was moving to the east. But when the east side was gone, you just have the west side left.

Brian Wernicke: Just have the west side left, and it kept, just as always, it kept on moving westward relative to North America. The plate, all of a sudden, rather than being pushed against by the Farallon plate, is now being pulled by the Pacific plate. And so that switch was a big deal as far as the geology in the interior was concerned. And the way that process happens is messy, in three dimensions. There was a tip of the ridge which came and hit at Los Angeles, and then the contact between the two began spreading apart from Los Angeles, one to the north and one to the south. Now, at the present day, the one heading northward, the so-called triple junction, is located in northern California, around Eureka, California, Mendocino, California. It's called the Mendocino Triple Junction. The other one migrated southward from Los Angeles and is now at the mouth of the Gulf of California. It's very hard to explain the plate geometry, but for this whole discussion, if you Google Tanya Atwater, UCSB, University of California, Santa Barbara, her web page on animations will come up, and you can look at a movie, several movies, of exactly what I just described and how it affects the geology of the West Coast. That movie very dramatically shows the switch between a convergent boundary and an obliquely divergent boundary due to the interaction of the plates.

Oliver Strimpel: And we'll put a link on the episode web page. So let's get back to the Basin and Range extension, then. And how was this enormous amount of extension accommodated?

Brian Wernicke: The crust in the Basin and Range is divided into a very crude checkerboard, if you will, or a sort of patchy pattern of blocks that are about 50 to 100 kilometers wide, within which there is basically zero extension, especially at first, when extension began between 30 and 20 million years ago, when this plate boundary change occurred. But the main pulse of extension divided the crust into what we call core complex domains and stable domains, this sort of checkerboard. So in that checkerboard, all of the core complex domains have had the upper crust stripped off of them, so that the stable domains move apart from one another in a wholesale fracture that penetrates down to the middle part of the crust. And the blocks essentially move apart, like tearing two halves of a sandwich apart, with all of the deeper crust rushing up in between the separating blocks. And so these form these 50- to 100-kilometer-wide areas where all of the rocks you see at the surface were originally in the middle part of the crust, underneath the stable blocks. So this sort of 50- to 100-kilometer periodicity was set up between stable, undeforming blocks that separated, with the deeper crust coming up underneath them.

Now, that pattern began to be disrupted starting about 10 to 15 million years ago, and what now dominates the Basin and Range is a pattern where the upper half of the crust is relatively uniformly tilting as a series of solid, rigid blocks. So the older terrains, the metamorphic areas uplifted in the early part of extension, now become parts of tilting rigid blocks, like a row of toppling dominoes. And all these dominoes have toppled over about 20 degrees due to the latter part of extension. Now, the early part of extension was relatively rapid, about 2 centimeters per year between 20 and 10 million years ago, but has since slowed down to about 1 centimeter per year now, and is largely accommodated by these huge tilting blocks, and their characteristic spacing is 30 kilometers, not 50 to 100 kilometers. Some of the old metamorphic domains that came up between the bigger separating blocks actually become tilted Basin and Range blocks themselves now.

The change from the widely separated blocks and core complexes to the 30-kilometer blocks is almost certainly rheological. It's a much-written-about subject. But as all of this crust becomes uplifted and the crust thins very dramatically, the medium that's being pulled on begins as a rigid elastic layer that overlies a thick ductile layer, and that thick, very weak ductile layer, as the blocks spread apart, rushes up and fills the gaps between the blocks. It's not entirely clear why it picked that particular wavelength of 50 to 100 kilometers, but that's what it is. But then the material that was at first extremely weak is being brought closer to the Earth's surface; the whole crust is coming up and cooling the whole time extension's going on. The response to the stretching then is to break it into a series of these 30-kilometer-wide dominoes that tilt over and form, on the upthrown part of the domino, the range, and on the down-tilted part of the domino, the basins. The uniformity of that is quite remarkable. That is, you see about 25 ranges between Salt Lake City and Reno, Nevada, on the east and west sides of the province, all tilted something like 10 or 20 degrees over the last 5 or 10 million years. And most of the earthquakes, and the faults that we look at to measure paleoseismology, occur on the fault scarps at the edges of the dominoes, between the dominoes.

These were recognized early on. Everybody likes to quote the line from Clarence Dutton, who was a very famous late 19th-century geologist. Dutton basically invented the concept of isostasy. He wrote a famous line in one of his memoirs that said that the Basin and Range topography reminded him of an army of caterpillars crawling northward out of Mexico.

Oliver Strimpel: Coming back to the big picture again, you'd expect a region that has undergone at least a factor of two extension would be thinned and therefore topographically low, but mostly the Basin and Range averages about a mile high. Why is it so high today?

Brian Wernicke: Yes, that is a seeming paradox that has preoccupied the community for many, many decades: if the stretching is really as high as we were saying it was, then why is the stuff still up in the air? Because everywhere else we know, you go to the Red Sea, you go to the passive margins, the areas of known plate tectonic stretching of crust, it's all under a bunch of sediment. And the reason is that the crust began its life very thick. Mesozoic California looked exactly like the Andes look now. That is, you had a very powerful subduction zone subducting all of that Farallon plate, and you created the great batholiths of the Sierra Nevada, the kind of rock that you see at Yosemite, the granites that the climbers love so much. And to the east of there, there was a thrust belt, an area of major horizontal contraction of the continental crust. And that's exactly the pattern we see in the Andes, where you have both the active magmatism happening inward from the Pacific trench, which is thickening the crust, and the horizontal shortening in the sub-Andean zone along the eastern margin of the Andes, a whole series of active thrust faults. And so this is exactly the ancient pattern we see in the Cretaceous in the Cordillera. And so one could easily see that the crust in the Cordillera was probably about as thick as the crust in the Andes is today. So that number is about 70 kilometers.

So now, if I take, say, Arabia and Africa and stretch them apart, I begin with about a 35-kilometer-thick crust, and the stretching creates the Red Sea and creates a couple of thousand meters of subsidence, and that fills with sediment, which makes even more subsidence. If you start with a 70-kilometer-thick crust and then you stretch it by a factor of two, you're at 35 kilometers at the end of stretching. And so the elevation of the Basin and Range, at about a kilometer, is because after you stretch the Basin and Range, you end up with a normal thickness of continental crust that isostasy will predict will lie well above sea level. There's another effect, which is that during the rifting you also thin the part of the lithosphere that's in the mantle, the so-called mantle lithosphere, and the stretching of that dense layer actually contributes to the buoyancy and the uplift. So everybody is very comfortable just with starting out with a very thick crust and thinning it down by about half.

Oliver Strimpel: I mentioned that most of the regions of extension around the world have been buried by sediments, and therefore they're hard to see, and that the Basin and Range is perhaps the best example that's exposed, where you can actually study all the manifestations of extension. But could you talk a little bit about some of the other places on the planet where we can actually see extension going on?

Brian Wernicke: Sure. It was very exciting, from the period of the late '70s, when the first core complexes were, if not understood, certainly described: this raising up of the middle part of the crust over a broad area, with the discovery of the Tertiary metamorphism especially being a big surprise and going against conventional wisdom. But then, once they were discovered and characterized throughout the late '70s and early '80s, it became clear that these kinds of things happened elsewhere in the world. And the premier areas where other examples were found were two areas of active extension, Tibet and the Aegean Sea and western Turkey region. And those have really good examples of stuff that looks very similar to the Basin and Range.

Oliver Strimpel: And do they have tectonic context that's similar, or is it very different? Especially in the case of Tibet, I imagine it's different.

Brian Wernicke: Yeah, they're extremely different. And the fact that they were different gave us a certain amount of confidence that the Basin and Range association was not a situational geometry. That is, the manifestations of extension were not a specific thing related to the overall tectonic situation. In fact, anywhere you're stretching something apart, you will see these things. So for example, in the Himalayas, you have a continent-continent collision going on. You've developed a 70-kilometer-thick crust underneath Tibet, and we're now in the early stages of continental separation, of stretching there east-west, while India is colliding into Asia. So there you have a continent-continent collision setting, and you're in the early stages of extension, and the topography is the highest in the world. In fact, both Mount Everest and a hard-to-pronounce range called the Nyainqentanglha Range in southeastern Tibet are metamorphic core complexes, yet they're associated with mountain peaks that range between 6,500 and 8,800 meters high. These are things that have experienced extension fairly recently, even though they're so high.

Now, if we move over to the Aegean Sea region, there you have a situation called slab rollback, where, as Africa collides northward with Eurasia, the remnant ocean of what is now the Mediterranean Sea has an old slab that's rolling backward toward Africa relative to Europe. So Africa's not moving very fast relative to Europe, maybe a centimeter per year at the most, but the slab attached to it is rolling back southward toward Africa at about 3 centimeters per year. So you see this sinking slab, and it's actually pulling the island of Crete and a lot of the rest of Greece southward toward Africa at an astonishingly high rate, much greater than the convergence rate. So behind it, what we call back-arc extension is stretching the continental crust in western Turkey and Greece to a very large degree. In fact, part of Greece is already underwater, right? The Aegean Sea itself. But there are also good exposures on land. Mount Olympus, for example, is a metamorphic core complex. Naxos Island, famous Naxos Island, is also a metamorphic core complex. The Central Menderes Massif in western Turkey is probably one of the biggest core complexes on the planet.

So I listed off the famous geological examples that have been studied in Turkey and Greece and Tibet, and those are in radically different tectonic situations. One's an ocean plate rolling back underneath; the other one is a continental collision moving toward it. In the Basin and Range, we have an oceanic plate shearing obliquely away from a continent, so that's yet a third manifestation.

Oliver Strimpel: Just to remind listeners about slab rollback, that we discussed that in a couple of earlier episodes, one with Laurent Jolivet on the Mediterranean in particular, and also with Claudio Faccenna. But just coming back to the Aegean and Tibet, do we think that any of these regions, or maybe even the Basin and Range itself, will eventually rift and result in Red Sea-type spreading?

Brian Wernicke: Yeah. Obviously, passive margins all were preceded by an end stage that established seafloor spreading between two continents that had rifted entirely apart and started to create ocean basin in between them. So now, in the case of the Basin and Range, there was an event that actually did exactly that. That happened well into the history of Basin and Range extension, about 6 million years ago, so something like two-thirds to three-quarters of the way through the whole stretching process. The Baja Peninsula, which is a big batholith just like the Sierra Nevada, was attached to North America. But at precisely 6 million years ago, the plate boundary hopped over in between Baja California and the rest of Mexico and tore Baja open, and made the other great example of a continental rift evolving into an ocean basin, which is the Gulf of California. So in that particular case, you haven't ripped the two big pieces apart like in the Red Sea, but you've taken a ribbon of the lithosphere and transferred the plate boundary and torn open a new ocean basin.

Now, it's easy to see that happening in the future in the Aegean Sea. If Crete makes its way all the way down to Africa and washes up on the shores of Libya, then it's hard to imagine you wouldn't form a secondary spreading center. There's just about a 20- to 30-kilometer section of crust underneath the Aegean Sea right now, and it's hard to imagine it surviving another 200 kilometers of motion of Crete southward toward Africa. So it probably would open up into a true oceanic back-arc basin. In Tibet, I don't see much hope we'll see anything like a Red Sea rift very soon. It has a lot of crust to tear through still. All rifts don't have to form ocean basins. Most rifts, in fact, are failed attempts to make ocean basins. Much of Europe, for example, is underlain by Paleozoic failed rifts that never made ocean basins. And then finally, in the Mesozoic, several of them linked up together and managed to pull North America away from Europe.

Oliver Strimpel: What are you working on at the moment?

Brian Wernicke: The main thrust right now: we're working on the first radiation of animal life over the globe. It happened in a period called the Ediacaran, which just predated the Cambrian period, which is when essentially all the major phyla that we know today in the animal kingdom appeared in an explosion. Before the Cambrian, a whole ecosystem of animal life evolved, none of which is easily relatable to modern phyla or the Cambrian fauna, but nonetheless the Earth was crawling with animals. Every continent has examples of these animals. We know they're animals because they made burrow tracks, and we can tell from their body plans that they're bilaterians and so forth, and almost certainly oxygen-consuming animals. About 575 million years ago is when this radiation, the Ediacaran radiation, occurred. And so the question is, what drove that? What we think is that there may have been a big influence by a major meteorite impact. That impact is called the Acraman impact, and it occurred between 585 and 590 million years ago. And you also see these other events at exactly the same time, including major global-scale glaciations and very hot interglacials. So the climate is wiggling back and forth at a level an order of magnitude greater than it is now. There might have been animals before that, and there probably were, but they didn't radiate. They survived in small niches that are only very fragmentarily preserved. By 575, they're all over the place.

In terms of making an analogy, the Acraman impact might have had the same effect as the KT impact had on the fact that we even exist, which is that the KT boundary killed off the main predators of mammals, which were the dinosaurs. And mammals, which had only been like little 1-inch mice up to that point, suddenly radiated very quickly after the dinosaurs died and occupied their megafauna niches. Probably, if the KT didn't happen, it's hard to see how mammals would have gotten a foothold in the ecosystem. So going back to the Ediacaran now, you're in a period where you're probably oxygenating the Earth enough that marine organisms have enough oxygen in seawater to be able to proliferate. And so the question is, did the impact have an effect that was vital for making animal life, as was the KT event in the radiation of mammalian life?

But nonetheless, you can see the glaciation, you can see the oxygenation, you can see the animals and the isotopic anomalies in ocean chemistry, if we can just fit them together. The precise chronology is very poorly known, because Ediacaran sections are famously difficult to date. You don't really have detailed fossil control, and the geochronology has proved extremely difficult to pin down, at the million-year time level, which event occurred when. So the practical importance of the Acraman impact in Australia, if it can be searched for, discovered and recognized in all the other Ediacaran sections, is that it would provide at least one solid timeline that you could identify and use to correlate all the different sections together. Right now, the cause and effect of all these relationships is a mess. And if we can just get to the point where we have chronological markers and better geochronology, we might be able to come up with a model as informative as the KT boundary extinction models.

Oliver Strimpel: Brian Wernicke, thank you very much.

Brian Wernicke: Well, thank you.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.