This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Plutons are bodies of igneous rock that crystallize from magma at depth below the surface. But even though the magma never makes it to the surface, it still has to travel many kilometers up from its source near the base of the crust to the upper crust, where plutons form. How does it make that journey? And what is the resulting structure of plutons and the intrusions that feed them?

Keith Klepeis is a field geologist who studies such questions. Recently, he has focused on one region in particular that provides an exceptional window into the origin, evolution and structure of plutons: the southern Fiordland region of New Zealand's South Island. He is a professor at the College of Arts and Sciences at the University of Vermont. Keith Klepeis, welcome to Geology Bites.

Keith Klepeis: Thanks very much, Oliver. I'm happy to be here.

Oliver Strimpel: In what tectonic context do plutons form?

Keith Klepeis: They form in a variety of different environments, but one of the most common is above subduction zones. Above a subduction zone, when the oceanic plate goes beneath a continental plate, a Cordilleran-style magmatic arc forms. We see those at the surface, say in the Andes or the Cascades, as a series of volcanoes. But if you think about what's below those volcanoes, there are these magma chambers that are made up of individual plutons and intrusions, and those are the parts that we're trying to study in this project.

Oliver Strimpel: Where do we find the most prominent plutons that are exposed at the surface today?

Keith Klepeis: Well, there's a variety of different environments. Usually you have to look at ancient systems, systems that might have been active tens to hundreds of millions of years ago. For example, in the Andes, especially the southern part, there's a very large structure called the Patagonian Batholith, and that was one of these large bodies of igneous material that fed volcanoes at the surface at one time, but that surface material has been eroded. There's a bunch of other places. The Cascades in the United States is another place. And New Zealand has a wonderful deep exposure in the southwest part of the South Island called the Median Batholith, and that's the one we're looking at.

Oliver Strimpel: Let's talk about the processes that lead from the subducting slab to the formation of magma at the base of the crust, and then to the eventual emplacement of a granitic pluton. Let's start with how the magma forms at the base of the crust.

Keith Klepeis: That begins with the process of subduction. What we mean by that is an oceanic plate, made up of basalt, sediments and lithospheric mantle, dives down beneath a lighter plate, which can be another oceanic plate or can be a continental one. When that happens, the material that makes up that subducting plate starts to change, and it dewaters. So some of the basalt and the sediments release their water. It filters upwards into the mantle above the subducting slab, and it reduces the melting temperature and causes melting. Then that melt migrates upwards in a series of conduits, and it eventually hits the upper plate, the base of the lower crust. That is the part that we're really interested in, because we're trying to understand how you go from melts in the mantle all the way up through the crustal column and eventually into volcanoes at the surface.

Oliver Strimpel: So do we first get pools of liquid magma forming at the base of the crust?

Keith Klepeis: As you go up through the crustal column, above the upper mantle wedge, there are these horizontal boundaries. One of them is the Moho, which forms the base of the continental crust, and that is one where we see, in particular, magma as it's coming up tends to pond and pool. Now, there's another kind of boundary that's steep, and usually it's characterized by fractures and fault zones and shear zones, into which magma can also seep and migrate. These horizontal boundaries and these vertical boundaries combine to influence the shapes of plutons.

Oliver Strimpel: You mentioned the other kind of boundary. OK, you have the Moho, which is, broadly speaking, following the surface at a certain depth. What is this other kind of boundary that you mentioned, down at the base of the crust?

Keith Klepeis: Continental crust, it turns out, has a very long memory. During past orogenic belts or events, and during past magmatic episodes, you create structures that become long-lived. For example, in Fiordland, in New Zealand, we can see some of these old structures. They're big, giant, steep zones, and we've been able to date them. They extend back hundreds of millions of years. And it turns out these are conduits, or pre-existing ancient weaknesses, along which rising liquids and magmas like to filter and penetrate. They come up in these conduits, and they interact with other boundaries, like the Moho and other compositional boundaries, and they can fan out and form these mushroom shapes. But they are originally fed up through these conduits that we can actually see.

Oliver Strimpel: So this magma that makes its way to the base of the crust, is it really entirely melted, or is it a kind of mixture, partially molten, partially crystallized? What would we see if we actually drilled a hole down there and had a look?

Keith Klepeis: If you could actually see it as it's coming up into the base of the crust and ponding, it would be a mixture of crystals and liquid. And the liquid we're seeing, the melts, are of an andesitic composition. So it's different from the original melts that were produced in the upper mantle wedge, and what that tells us is that the magma is evolving as it rises. Crystals are accumulating over time, and that liquid melt is changing. What we're trying to figure out is how those crystals and melts interact, how those mushes, those magma mushes, are forming, how they change as they migrate up through the crustal column, and whether they assimilate other material. Ultimately, what we're trying to figure out is what happens from that source of melt into those conduits, and, as melt migrates further, how it creates magma chambers in the shallow crust that feed volcanoes. That process is a combination of physical things that are happening and also chemical things that are happening.

Oliver Strimpel: That reminds me of one of the early Geology Bites podcasts, in which I talked to Steve Sparks, and he talked about mush and was definitely a proponent of that model. Is that the prevailing view now, that we actually have this sort of part liquid, part crystalline mush in large volumes at the base of the crust before it ascends further up?

Keith Klepeis: Yeah. Let's explore that a little bit. Let's go back to the older model, the one that predated the mush idea. This older model is referred to as MASH. It's an acronym, and MASH stands for melting, assimilation, storage and homogenization. In that older model, you get a bunch of magma coming in from the mantle. It gets injected into the base of the crust, and it starts to change and differentiate through a process called fractional crystallization. The other thing that happens in that model is that it starts to interact with the host, the material into which it was emplaced. So there might be some melting of the host that gets incorporated into this magma chamber, and that changes its chemistry. So that MASH model is really about how that one batch of magma starts to change in composition and starts to migrate out.

Now, that has fallen out of favor, and the new model is referred to as mush. It's not an acronym, but it refers to that magma mush. One of the main differences is that it doesn't rely on that interaction with the host to change the chemistry. Another major difference is that it requires multiple batches of magma, this mafic, iron- and magnesium-rich material coming from the mantle, getting injected into the crust multiple times. What that actually does is reheat what's already there, that magma chamber; it mechanically mixes it, and it releases some of the trapped melts that are in there. And that, we think, is one of the major processes that produce granites, which then migrate to shallow levels and form some of these shallow magma chambers that feed volcanoes. So that's the mush model, and we see great evidence for that in Fiordland.

Oliver Strimpel: OK, so let's talk a little bit more, then, about what we think happens next, to go from the mush at the base of the crust and then cause the magma to rise up.

Keith Klepeis: Yeah. OK, so let's get into some of the details of Fiordland. One of the advantages we have in Fiordland is that the Median Batholith, especially the western side, is tilted, and it's very deeply exhumed. The reason it's like that is that that piece of continental crust has been uplifted and eroded by motion on the Alpine Fault. The Alpine Fault is a plate boundary right there. It's got a big component of compression across it, and that has resulted in this exposure. So one of the things we can do in Fiordland is see those deep, lower crustal levels. In addition to that, we can see these individual plutons, these big, giant bodies of magma, where you can see the boundaries. And the unusual situation is that we can see the top of them and we can see the bottom of them. So we can actually walk physically from the bottom to the top and map the interior, the architecture, of those plutons. That's the basis for what I'm going to tell you about what actually happens.

We've been doing this for a little while, walking around Fiordland, and we've been able to find up to five of these very large vertical conduits, those kinds of discontinuities into which magma from the mantle is intruding and forming these plutons. And we can see how it interacts with magma that is already in the lower crust. One of the big things that happens is you get these multiple injections, and they start to mechanically move some of the magma that's already there and release some of these granites. These start to form intrusions and dikes, steep-sided intrusions that start moving material from deep levels into shallow levels. We're trying to trace, chemically and through isotopic dating, but also through physical structures you can see, that path from the mantle source all the way up through the pluton into shallower levels.

Oliver Strimpel: What creates these conduits, and are they basically channels where there is more melt than there is in the surrounding regions?

Keith Klepeis: The best interpretation we have so far is based on a long history of work in New Zealand, especially the South Island. What we've discovered there is that the ancient continent of Gondwana, which Zealandia was a part of, was constructed from a series of terranes. Some of these were oceanic terranes that accreted onto the margin and built out the continental crust of Gondwana. During that accretion episode, you formed these giant faults and boundaries between those terranes, and those persist in the geological record. We've found at least three of them. You can see them physically; you can map either side. You can see chemically how they're different on either side. And these seem to be the conduits that feed from the mantle all the way up into the upper parts of the crust. What we see is that these conduits are up to 5 to 10 kilometers wide. We see magma entering them as dikes and intrusions, and starting to spill out into the lower crust and then migrate to shallower levels.

Oliver Strimpel: You mentioned just a moment ago that you can see in the Median Batholith in New Zealand both the bottom and the top of these individual plutons. How do you know when you're at the bottom, and how do you know when you're at the top?

Keith Klepeis: Over the past few decades, the geologists who have worked in Fiordland have been able to collect really accurate data on metamorphic pressures and temperatures and mineral assemblages, and these metamorphic mineral assemblages have been well dated. What that means is that we can take pressures, in particular metamorphic pressures, that belong to a certain time interval, so we can be sure that we're not mixing different time periods. And we can reconstruct what Fiordland looked like in ancient times, in this case the Cretaceous, about 120 million years ago. Based on those changes in metamorphic pressures, we can tell which way was down and which way was up. And it turns out, because of the mapping that's been done in Fiordland, we can see the base of these plutons and prove that it was at lower crustal levels, maybe 45 to 50 kilometers depth. And we can see the upper part of the pluton, the upper boundary, and prove that it was at shallower levels, maybe 25 kilometers depth. So that full 15- to 20-kilometer thickness is visible and is well constrained by this metamorphic pressure framework.

Oliver Strimpel: And so if you were to go below the bottom or above the top, would you be moving into different rock types?

Keith Klepeis: Absolutely. At the top, it's very dramatic. You see all of this andesitic, dioritic pluton below, and then suddenly, at the top, you start moving into the metasedimentary rocks that dominate the middle crust. There are granites and all sorts of material up there, but it's a very sharp compositional boundary. And then at the other side, the bottom, you see the same dioritic or andesitic magma, and immediately below it you see sheets of mafic and ultramafic rocks, stuff that was derived directly from mantle depths. So those are also compositional boundaries. Having those exposures at the top and the bottom is a huge advantage, and it's allowing us to answer these questions in a new way.

Oliver Strimpel: And so you can go from the bottom to the top, you say, particularly in the Median Batholith in New Zealand, because it was tilted. Talk to me a little bit more about how that geometry enables us to get such a good view of what's going on.

Keith Klepeis: If you actually look at a map of Fiordland, it's divided into different provinces. In a general sense, there's a western province and there's an eastern province. What we've been able to show is that these provinces are bounded by reverse faults, faults where one part went up relative to the other. And if you walk from outside Fiordland towards the Alpine Fault, you can see that those deep levels get progressively more exposed at the surface. So you get deeper as you approach the Alpine Fault. Because the Alpine Fault itself has a component of compression across it, these thrust faults that we see in Fiordland seem to be genetically linked to the formation of that boundary. So as we walk across Fiordland, and these wedges of crustal material get popped up by these reverse faults, we can see deep into what was lower crust during Cretaceous time. We're taking advantage of that architecture, piecing back together what that magmatic arc, that Cordilleran-style batholith, used to look like.

Oliver Strimpel: And that tilting, then, you say, is really a direct result of this location, right next to this major fault, this plate boundary.

Keith Klepeis: It is, and it's one of the big things that we have to focus on. Even though looking at those faults and the origin of the tilting is not part of our current project, we have to address it, because we have to be certain that we know how to reconstruct the crustal fragments into what they used to look like. I'm a structural geologist, so one of my jobs is to pay close attention to those faults and record how they're moving and how they moved in the past, so that we can remove their effects and look at what that column used to look like. So that's a big part of this project.

Oliver Strimpel: Have you been able to determine the three-dimensional shape of the batholith, and how that shape relates to what's inside it?

Keith Klepeis: Doing that is very laborious. If you look at Fiordland, because of the results of the Alpine Fault, it is physically possible to walk from what used to be the upper crust down through the middle crust into the lower crust in a single traverse. But if you actually go to Fiordland, it is a temperate rainforest. It is covered by vegetation and deep valleys with moss and swamps, and the topography is extraordinarily rugged. So practically, what we have to do is take advantage of the fjords. We hire a boat and cruise up and down these fjords, which is a wonderful situation, because the sides of the fjords at sea level provide virtually 100% exposure of rock in a continuous way. So we can get these profiles by going from fjord to fjord. We get these two-dimensional profiles deep into these exposures, but that's two-dimensional, right? We have to supplement that. So we take helicopters to the mountaintops, above the tree line, where there's also 100% exposure, and we do that across several ridges between fjords. So we have data down at sea level and continuous data from the ridgetops, and we piece together a three-dimensional geometric model that provides the framework for the processes we're talking about.

Oliver Strimpel: What kind of shape were you able to come up with, if you were able to generate a three-dimensional shape?

Keith Klepeis: We had some surprises there. If you look at Fiordland from a map point of view, lots of these plutons look oval-shaped or oblong, not quite circular. But if you look at them in three dimensions, they have a distinctive mushroom shape. When you look at the bottom and the top and piece it together, it has this big, giant flap, and sometimes one side is much, much bigger than the other, so it's kind of an asymmetric mushroom. That flap of the mushroom is the part that goes laterally into the lower crust. And you can see that mushroom start to converge at the base of the pluton into the steep conduit. In the case of the Misty Pluton, it's a 5- to 8-kilometer-wide vertical conduit, and inside that conduit, you can actually walk around in it. You see these vertical intrusions of mafic and ultramafic magma coming right from the mantle, and they're feeding into these mushroom shapes in the lower crust. Now, we did not know that going in, and you can't see it from looking at the map. You have to be able to reconstruct it from these three-dimensional architectural studies. And it turns out it's not just the pluton we're looking at, the Misty; it's a bunch of these other ones too. So it seems to be a common pattern at the base of the crust.

Oliver Strimpel: So if you were to go from the bottom of the crust up to where the pluton was emplaced, would you just have one big, steep-sided conduit with a big mushroom? Or does it go through various conduit-mushroom, conduit-mushroom cycles on the way up?

Keith Klepeis: It turns out it's quite complex. So far, this is what it's looking like. You do have these vertical conduits down at the deepest levels, and they feed these mushroom-shaped plutons. The thickest one we see is about 15 kilometers thick, these mushroom shapes. You can see the top, and as you go up through the top, you tend to get into more steep-sided zones in the middle crust. So it seems like you are feeding some of these steep-sided intrusions at middle crustal levels from these mushroom-shaped plutons deeper down. And then as you go up further through the column, it gets even more complicated, and you get shallow-level plutons, which can approximate circular objects or flat objects, and can be quite complicated. So it's kind of a mixed zone in the upper crust. It actually turns out that the deep, lower crustal parts are more geometrically organized than what we see at shallow levels. But you do go back and forth between the steep zones, the shallow zones, and back to steep, as you're moving material towards the surface.

Oliver Strimpel: Do we have a sense of the time scales involved? How long did it take to emplace the pluton, and how rapidly would the magma have risen up?

Keith Klepeis: It turns out it is, in a geological sense, much more rapid than we imagined. This Misty Pluton, which is one of the largest in the Median Batholith, is about 700 square kilometers, and it's got a sister pluton called the Malaspina, which is about the same size. It turns out both of these were emplaced simultaneously in about 1 to 3 million years, which sounds long from a human point of view, but in a geological sense is very quick. We used to think it took 10 million years or more to build up some of these large plutons, but it's very quick, and you can tell this by dating zircons using the uranium-lead method. Titanite also helps to date some of these individual intrusions. So 1 to 3 million years seems to be the rule rather than the exception.

Oliver Strimpel: I'm reminded of a previous podcast in which I talked to Richard Ernst about large igneous provinces. I'm wondering if the magmatic flare-up that you're talking about here bears any relationship, in scale or nature, to the enormous volcanic outpourings of flood basalts that form the large igneous provinces.

Keith Klepeis: There are some important differences, and a couple of similarities, but let's talk about the differences first. If you look at the original definition of a large igneous province, it is usually thought of as a very large outpouring of mostly mafic magma, magma like basalt that's rich in magnesium and iron. And in scale, the largest ones cover millions of square kilometers. If you add up all of Fiordland, we're talking about thousands, and individual plutons are hundreds. So the scale is much, much less than a large igneous province, and the composition seems to be quite different. The similarity, however, is the pace at which these intrusions are building large piles of magma. In some cases, large igneous provinces are emplaced very quickly, in 1 to 5 million years: this huge outpouring in a very short period of time, forming these plateaus. They're immense, amazing things that can influence climate. And it looks like batholiths can also accumulate at speeds that are similar to those; it's just that the compositions are different, and maybe the scale is different.

Oliver Strimpel: You and your team have compiled a large body of observations in the field, in New Zealand, for example, but probably elsewhere. Are you planning to tie all your results together with a physical model, to better understand the way the various physical processes operate to create the plutonic rocks we see?

Keith Klepeis: Yeah, absolutely. In this project, where we're looking at the Misty Pluton and parts of the Median Batholith, we're in our second year, so we're getting ready to go into the field again next month. We're still gathering those architectural data, and the samples we need to date things and to look at things geochemically. But we are building towards making a physical model, and integrating that with the geochemistry to illustrate the processes of building plutons: how you make plutons and how you move them around.

Oliver Strimpel: Are there any particular outstanding questions that you're hoping the model will answer, or that you'd like to address with further fieldwork?

Keith Klepeis: Yeah, we want to really explore this mush model in more detail and see if we can track individual batches of magma. Just to remind you, the mush model requires multiple batches being injected, and we think we can track over time how they interact and how they migrate. The other part is that we are now identifying large source regions of granite in the lower crust, and we want to track how that moves, both in terms of its speed and how it changes chemically, and how it interacts with other batches as it moves up through the column. So we're collecting samples, building up the architecture, and we're now getting to the stage of doing isotopic dating, to see if we can get rates, and looking at the chemistry to see how things are changing. So that's a major question.

There's one other that I'm especially excited about. My colleague Elena Miranda is an expert on how things deform. One of the things we'd like to get into more is that as magma is moving, it is deforming. It can be brittle, it can be ductile, and we want to know the mechanisms by which that happens. It turns out those mechanisms can change as you go from deep levels to shallow ones, and we have access to these very deep samples. So we're now exploring on a micro scale how those minerals are deforming. That has implications for many other fields, including how plate boundaries form and how shear zones behave, in all sorts of applications.

Oliver Strimpel: Keith Klepeis, thank you very much.

Keith Klepeis: Thank you, Oliver. It's been a pleasure.

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.