20: Allen Nutman: “What if the boundary is folded?” – figuring out the structure of Earth’s ancient crust

In this episode, we hear more from Allen Nutman, Professor of Geology at the University of Wollongong in Australia, about how his mapping work together with Vic McGregor and Clark Friend led to the beginnings of a model for how the ancient rocks in the Nuuk region were formed as a series of distinct small continents that collided with each other about 2.7 billion years ago.

Transcript

Transcript

20: Allen Nutman: “What if the boundary is folded?” – figuring out the structure of Earth’s ancient crust

Based on interviews held on August 23–24, 2019 in Nuuk, Greenland

Note: Polar Podcasts are designed to be heard. If you are able, please listen to the audio, which includes emotion and emphasis that is not evident in the transcript.

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Allen 0:01

The main thing simply was the passion. It was a very important thing to try and sort out and resolve. And the interesting thing really was that it wasn’t that side A or side B were right or wrong. It actually turned out that there was a completely different explanation.

Julie 0:23

Welcome to Polar Podcasts, where you’ll hear stories from geologists who’ve spent their careers, their lives, exploring and studying the remarkable and remote geology of Greenland. Why did they become fascinated with Greenland? What were the problems and the discoveries that drove them? And what was it like working in these remote places, where few people venture, even now? I’m Julie Hollis.

In this episode, we hear more from Allen Nutman, Professor of Geology at the University of Wollongong in Australia, about how his mapping work together with Vic McGregor and Clark Friend led to the beginnings of a model for how the ancient rocks in the Nuuk region were formed as a series of distinct small continents that collided with each other about 2.7 billion years ago.

Allen 1:13

So in, 1984, I was up here and part of the reason for that was there was a very, very big controversy going on in geology at that time. This sort of geology is, should one believe what what expert field geologists are doing or should you believe what the isotope geochemists are telling us. Because there were some rocks down in Ameralik Fjord at a place called Kangimut Sangmissoq where Vic McGregor, I guess along with myself and Clark actually, we believed that these particular rocks were some of the very ancient rocks in the area, called, the Amitsoq Gneisses, based on their field characteristics. On the other hand, isotope geochemists from Oxford, Stephen Moorbath and Paul Taylor, they were presenting isotopic evidence to show that actually these rocks were much younger. So there was, if you like, a complete, divide of diametrically opposed views about what the field geologists were saying and also what the isotope geologists were saying. So we decided, actually, to go back and look at this completely from scratch.

So that was the idea in 1984 but for those of you around in West Greenland in 1984 will realize what a horrible summer it was. So once we’d started that particular work, we had, I think, only one complete working day out of a component which we’d reserved for that work of about three weeks we had one day when it was actually workable for the whole day. There was also another problem was that there was a very large amount of sea ice came up the west coast so that even around Færingehavn, Nordefar, and all around there, all the bays and so on were clogged with sea ice er, which had come up from the south. And I have memories of being on Vic’s boat in those particular situations where we would be basically in a little inlet because we didn’t want to be dealing with very big bits of ice and sometimes we’d have a rope across the inlet, you see, and then we’d be moving the boat from side to side to let the sort of icebergs pass, so we spent some nights so of, doing that kind of thing, which for this area here is just extraordinarily unusual.

But what we actually found, in the little time available to us, was what we might call, for the non-geologists, some kind of previously unrecognized great divide in the geology. And this went the first step in resolving this great difference of opinion between earth scientists with different backgrounds and skills. So what it turned out was that these rocks that we originally thought were a particular um, example of the very old rocks. They were on the other side to this previously unrecognized great divide from where all of the completely, agreed on and recognized old rocks occurred. They were actually just simply something completely unrelated, but looked very much like the recognized old rocks. So that was the start of the modernization of the geology of the Nuuk region, where one began to really develop plate tectonic scenarios for the evolution of the geology.

Plate tectonics, is where bits of unrelated bits of the Earth’s crust are moving around laterally, relative to each other, sometimes spreading apart with new oceans appearing in between, sometimes crashing together to build mountain belts, for example what is happening now where India is still crashing into the rest of Eurasia. So we started to recognize the several of these, shall we say, fundamental great divides, in the Nuuk region, which represent collisional zones between different chunks of continental crust.

Allen 5:58

So that was the first inkling that we went away with in 1984. 1985, we knew we actually had to do a lot more detailed work and at that stage, we were no longer being supported by the Greenland Survey. We had no grants. So actually in 1985, Clark and myself, we funded our own field work, basically a little bit of support with Vic and his boat. Out of the Greenland Survey’s kindness of heart, we did actually have a zodiac with an outboard motor lent to us, which we were very grateful for because the area that we chose to work on, because of our limited resources, meant that we had to chose a coastal area, where we could get around easily. And we chose the area around Færingehavn-Nordefar because that’s where actually we had realized a so-called great divide was passing through. The Færingehavn-Nordefar area, it’s a sort of flat-lying area of, of irregular peninsulas and large clusters of islands, a fairly short distance out of Nuuk. So that provided an ideal place to actually start to develop and investigate this new vision of the, of the geology.

At that stage, I was not married and didn’t have other financial or personal responsibilities, so from that perspective money I had was to dispose of entirely as I wanted to. But the main thing simply was the passion. We saw this, this divide between the field geologists and the, the emergence of the isotope geochemist. It was a very important thing to try and sort out and resolve. And, of course, the interesting thing really was that it wasn’t that side A or side B were right, or wrong. It actually turned out that there was a completely different explanation, with that explanation being that things could be explained by a previously unrecognized feature in the geology, in recognizing this great divide.

Just because you’ve got bits of the oldest rocks in one terrane, it does not mean they have to match the age of the oldest rocks in another terrane. And that really was the great importance of recognizing these initially very cryptic divisions in the geology. They are literally, in some places, no more than one or two metres wide. So particularly in regional mapping projects, they can actually be quite easily, missed.

In 1984 we had anchored in the bottom of Præstefjord. And we thought, we’re looking for this boundary, geological boundary, it already should be in there. By our reckoning, if we walk in from the head of Præstefjord about five kilometres we should come across it. So we kept on walking, walking, along the valley. No sign. And then, eventually, we came a river that one actually couldn’t just hop across the boulders. One would actually need to get yourself wet to go across. And Clark and myself said to each other, ‘Oh, you know, we’ve been testing this model, we should have come across it by now, but we haven’t. We should really turn back.’ And I remember Vic said, “Oh, you just don’t want to get yourself wet, do you?” Upon which, I took off my trousers, marched through the river, stood there and then marched back, and said, “It’s nothing to do about getting wet. We think that we’ve run out of testing this model.” 

And then on that walk back, I remember Clark said a very, very important thing. He said, “What if the boundary is folded?” Because previously, we had recognized that it was passing through the, sort of, Færingehavn area where it’s actually straight. And we’d been trying to follow it as if it was completely straight everywhere. And Clark said, you know, “What if it’s folded?” And that again was a magic moment. One was no longer looking for just simply straight great divides, which cut across the country but ones that wiggle across the countryside and that meant that the search took on a different perspective.

So what we decided to do then was there was a centre of a fold on the plateau on the north side of Buksefjorden where, if our model was right, the great divide would actually run round the shape of that fold up on the plateau. So what we did, we sailed round into the head of Buksefjorden and then we scaled the thousand metres to get up to the top of the plateau and then a bit of a walk in and lo and behold, we found the boundary, granulite facies on one side

Julie 11:28

Which means that the rocks contained minerals that showed they had been to temperatures above about 750 degrees Celsius, deep in the Earth’s crust.

Allen 11:38

the great divide and then amphibolite facies rocks on the other side,

Julie 11:43

Which means that the rocks contained different minerals showing they had been to significantly lower temperatures and therefore were from a different, shallower part of the Earth’s crust

Allen 11:53

showing that the great divide was folded.                       

During those years in Canada coming back to Greenland along with Clark Friend and Vic McGregor we continued to develop this terrane model, spreading it out from this little tiny area around in Færingehavn fjord, where we’d done detailed mapping first of all. And then extending it with complete stupidity throughout the whole of the Nuuk region.

Why was it stupid? Because we were three people with almost no logistical support and there was almost no geochronology in the area at the time. 

Julie 12:35

Meaning that very few of the rocks had been dated, which would have made it much easier to interpret the geology.

Allen 12:42

There was just a few, very few dates spotted around. And because of that one had to rely a lot more on appearance of gneisses.

One thing which is particularly important in geology is to actually know the age of a rock. Zircon is a mineral which takes in uranium, which of course then decays to lead. So these little crystals actually act as a little chronometer within the rock.

So besides having a very strong focus on the field geology and the mapping, my other speciality is actually dating of geological events by using these zircon crystals.

Julie 13:27

When we next hear from Allen Nutman, he talks more about his career as a geochronologist and his role in the discovery of some of the world’s oldest rocks. I’m Julie Hollis and you’ve been listening to Polar Podcasts.

Julie 13:47

In the next episode, we hear more from emeritus senior scientist Niels Henriksen about geological mapping in remote western North Greenland in the mid-1980s.