This is Geology Bites with Oliver Strimple.
With the help of plate tectonics, we have a relatively
straightforward picture of how major mountain chains formed.
For example, at a very high level, the Himalaya resulted
from the collision of two continental plates, India and
Eurasia. And the Andes are a volcanic
chain that formed where the oceanic Nasca.
Plate subducts beneath the South American plate.
But what about the Alps? They are the most.
Intensively studied of all mountain chains being readily
accessed from the geological research centers of Europe.
But despite this, there remains considerable uncertainty as to
how the Alps were formed. One of the major challenges in
Alpine research is to understand how rocks deform on various
scales. From individual mineral grains
up to the scale. Of mountains.
But the picture is complicated by the presence of several
sedimentary basins and tectonic fragments along the southern
margin of Europe in the Eocene epoch, about 40 million years
ago, when the Alps started to form.
Rob Butler has devoted much of his research career to the Alps,
and particularly to discovering how defamation within Alpine
rocks has been concentrated over the past 40 million years.
It turns out that while recent defamation is clearly visible as
folds and faults in the rocks today, it was the structure of
the southern margin of Europe before the Alps started to form
that played a critical role in the early evolution.
Of the Alps, Rob Butler is Professor of Tectonics at the
University of Aberdeen. Rob Butler, welcome to Geology
Bites. Thank you.
It's great to be here. Since I just mentioned the
importance of the situation on the southern margin of Europe
before the Alps started to form, let's start with that.
What did the geography of the region look like in the EAR
scene about 40 million years ago?
Yeah, it was a great starting point.
I mean, first of all, we have to be quite clear what we mean by
Europe at that stage. And the Italians won't like
this, but we're not going to include them in Europe at this
stage. They're going to be part of a
small continental block that geologists generally referred to
as Adria or Apulia. And that was a distinct fragment
of similar continental blocks that decorated or got caught up
between the two major continental masses of Africa and
Europe. So when we're talking about
Europe at that stage about 40 million years ago, in terms of
land masses, the bits, I'm interested in Western Europe.
So that's very much SE France if you like running around into
Western Switzerland or at least the northern part of Western
Switzerland. So we have that being an area of
low lying ground, some of it above, some of it below sea
level, some very simple bits of proto mountain range beginning
to develop in there, nothing very substantial either side of
that. You've got mountain ranges
beginning to develop in the sense of topography and ranges.
So in the east and outs running into Austria and so forth,
modern day Austria and beyond, there would be more conventional
types of mountain range, perhaps like the modern day Caucasus or
somewhere like that. Not a big white belt, but narrow
strips. Similarly a sort of proto
Pyrenees between Spain and Liberia and southern France.
So we have a complicated surroundings of proto mountain
ranges, but in the western Alps we've got essentially a very low
lying area, most of it underwater.
So what events set? The mountain building process in
motion, so probably the preceding 50 + 1,000,000 years.
There's been convergence between Africa and Europe and the bits
of these small content blocks are getting shuffled and pushed
together. So there's an area of general
convergence and reshaping the geological Europe, not the land
that we would call it. So if you were potching around
in the ESC you'd see the seaway. But actually the geology of
Europe continued and begun to subduct or get carried beneath
this northern margin of Puli or Adria.
So that's the preceding part. As with any mountain range,
pretty much there's a period of seduction of the ocean areas and
thin bits of continent being taken down into the upper
mantle. So that's the preceding game and
you just let the tape run so that the convergence continues
and eventually you collide mountains.
And that's a Himalayan situation.
That's the same in any collision mountain belt.
But in the ESSE, we're not quite at that stage yet in the out.
So 40 million years ago it's all set to go.
So wait a minute. So you have this shallow sea
that you talked about, this low lying area, part above sea
level, part below and that is subducting in which direction?
Broadly southwards or South and E underneath this continental
fragment that you can think of as Italy, modern day Italy or
Adria as you called it. Quite so.
OK. So that then swallowed up that
low lying area. So during this period there was
no actual topography created and then it was only when that area
got fully consumed that you then started a more conventional
mountain building process. Yeah.
So we need to think a little bit about what this area that's
already going down the plug hole if you like what that actually
consists of and that's thin crust.
The old continental margin people think rather like the
modern day Atlantic margin of Iberia or of Western France.
So you have a an area of thin tapering wedge of content
lacrosse sedimentary basins that have been gradually taken down
into the system. So you've got thin crust and a
big root of mantle underneath it, and that's the driver that
allows the content with crust, the thin crust to be taken down
and seducted in contrast, as you go further out into what you
might think is stable Europe, the normal Europe, which if you
like would be the sort of massive central of France in
terms of what you see in the outcrop today, that's normal
thickness continental crust. And the change will happen when
that enters the seduction zone and chokes it so that thicker
crust is too buoyant, too thick to be carried down by its mantle
root into the mantle. And it's then that you'll start
thickening that up and making mountain ranges okay.
So now we've wound the clock forward to.
About 25,000,000 years ago. Would that be in the early Mayo
scene? Roughly yes.
And deligacy in early Mayo scene is when there's evidence in the
geological record for a change in the surrounding basin areas
that are going to collect the detritus that erodes off the
proto Alpine chain. So as you begin to the
mountains, they poke up in the air, they erode and the signal
of that is detritus being washed into the surrounding areas and
that really gets going into the early Miocene, virtually nothing
before that. Is the initiation of the
topography in this region still driven by what's happening with
the subduction zone and how it gets.
Clogged up with this thick crust?
Or are we talking about the more regional convergence of Africa
up north towards Europe? Well, it's both really, because
that convergence happens because you're able to have the
seduction going on and the likelihood is that the motor for
the convergence as it continues is the pull of that seducted
slab of mantle lithosphere that lay under the grifted margin and
beyond. So that that's an extra pull
that pulls the material in together.
So that's so-called slab pull process, which is one of the
main drivers of plate tectonics. Anyway, It's much easier to
think of processes in terms of slab pull rather than if you
like a car crash of continents okay.
So you have this slab pool operating which consumes these
low lying areas and then you say get choked up by the massive
santral type thick crust. So what happens next?
That thick crust is not completely thick.
It actually consists of slightly weaker rifted basins, perhaps
like the margins of intercontinental rifts like the
North Sea for example, so that it's got some scope to deform
and squash and to thicken up process we call inversion
tectonics because what was basins have now become
mountains. And in bygone days people really
didn't recognize the presence of these old basin sections in the
Alps. And the reason we now know this
is because of a lot of detail work done by Alpine
strategraphers and paleontologists, actually not by
structural geologists who recognized earlier rift basins,
The sort of things we now know exist because of seismic and so
forth, places like the North Sea.
And we recognize those in the stratigraphy now squashed up in
the car crash of the Alps. So you've thickened the crust by
squashing together those old basins.
Okay. I'm still a little confused
about the mechanism by which the basins get squashed together
and. Form the topography.
If the subduction process is essentially ground to a halt,
the subduction is continuing in terms of what's happening for
the upper mantle, rather like pulling a rug.
And the material on the rug, which is the crust, gets rammed
together. So you can imagine having a rug
on the floor and you're pulling, Let's imagine you pull a rug
under a bed. The furniture that's on that rug
gets bashed into the bed frame. That's the concept of crust
being bashed together. But the pull the rug, the slab
pull continues until, of course, the amount of furniture has
rammed up against the bed and you can't pull the rug anymore.
And that's essentially what's happening in a collision
process, that the crust that's coming in clogs the system up.
But the process continues to the extent that it then creates
topography and the squashing as you refer to it.
Yeah, because the crust is thick, it doesn't take much
extra squash to generate mountain ranges.
The interesting feature of the thin crust that preceded it in
the system is that it looks like that can be just carried down
into the mantle and doesn't really do much topographically.
It doesn't thicken much, it just gets in Trained in and we see
that recorded by high pressure metamorphism in it, the record
mineralogically of the material being taken down to great depth
and changing character mineralogy.
It then returns in that seduction zone by its residual
buoyancy and it sort of squirts back up rather like toothpaste
if you like being squirted out of the toothpaste tube, but
that's a seduction process and isn't doing very much to
generate topography. It's certainly making some neat
structures in the rocks, but isn't really manifest in
landscape processes. So let's wind the clock forward
a bit. Well, that process we can date
it radiometrically from cooling gauges and crystallization of
minerals so that return flow, most of it seems to have
happened by about thirtyish million years, so towards the
end of the league of scene and that represents the change when
that thicker concept of crust is arriving on the scene.
So you get this return flow. You pack stuff up in the upper
crust as it crushes in, and at the same time the thick content
crust arrives and begins to thicken up as well.
And that process is clogging the whole system up.
Content crust is having a hard time getting down the subduction
zone. Now you've got too much of it.
It's too buoyant. And so that thickens.
And now you start creating a range topographic mountain
range. So you elevate the rocks that
have come up this channel. You start seeing metamorphic
rocks at the surface. They generate detritus that
appears in the surrounding sedimentary records.
And so you can chart not only that but also the bits of
detritus coming off the thickening continental crust as
well. And you see them all coming out
together into the surrounding areas.
And where do we see that detritus today?
One of the really great records of it's actually is in a package
of ROTS called the Manos Aranessa which is a Miacene
succession in Northern Italy as you could probably guess from
the name of notwithstanding my bad pronunciation and it's
caught up in what is now the Northern Appanines and that
package is a sequence of circle turbotites which are relatively
deep water rocks. That area was low lying and is
essentially submarine fans building out into what is to
become Northern Italy. That's the smoking gun for these
processes is that part of the Poe Basin today it went
underneath the Poe Basin. The Poe Basin is yet younger
still to try to that's come off the Alps and N Napanines and is
still accumulating today. I said earlier that the
preexisting structure along the southern margin of Europe played
a critical role in the early evolution.
Are you referring there to the transition from the thin crust
to the thick crust or is there more fine grained detail that we
see reflected in the deformation of the Alps today?
That is a testament to that situation 40 million years ago.
You're right first time. The general distribution of
crustal thickness is fundamentally related to a
rifting process in the Mesozoic which was forming tethers, the
precursor ocean to the Alps and other chains.
But in detail you can see that quite a lot of the contractual
structures, the folds and so forth.
You see it certainly in parts of the Alps like the Ekram National
Park area to the east of the city of Gronog.
The folds there, lots of the folds there.
You can relate to rocks being squashed up against fault
blocks, pre Jurassic age fault blocks.
So the structures you see when you wander around in the Alps,
many of them have the spatial distribution control by these
preexisting basin structures. And the areas that didn't have
very many preexisting normal faults have a much simpler
stratigraphy, much more like a continuous layer cake.
And those areas permitted thrust slip to go out rather like
having a tablecloth moving out across a table top.
If that table is broken up and has lots of different steps on
it, that's much harder of course.
So the areas that have those big so-called de Como are unsticking
tectonics are places where there were very weak developments of
these preexisting rift basins. And what regions would those
correspond to today? The classic example of the Jura
Hills in northern Switzer running around to that corner of
France, that's the continuation, so outboard from Geneva, if you
like, going right round towards outboard of Zurich.
So that's the classic example. But you can see hints of it
elsewhere in the geology of the Alps, where strata that
deposited at a large scale across the top of blocks have
been peeled off and carried out as thrust sheets.
The term that people use all the time to describe these sheets
are naps. That's what you're referring to,
is it? Yeah, nappy sheet.
So that's basically a tablecloth slipping around the place.
Now, of course, if you squash any rocks enough, they can sort
of squirt out. So that's not the exclusive
reason why you get thrush sheets, some of the sedimentary
basins when they're squashed a lot.
Essentially, you're squirting the contents like a jam
sandwich. You sort of squirt the contents
out, and those can become far trouble thrush sheets as well.
It's like an inelegant sort. Let's talk a bit about the
different deformation styles then SO you already mentioned.
How the juror are this tablecloth?
Or I don't know if there's a more technical term for it,
tectonics. Could you just describe the
different parts of the Alps a little bit in terms of how
they've deformed and how that manifests in what we see there
today? Yes, take Cornwell.
Tectonics would be the fanciest way you'd save for tablecloth.
Takes on ungluing detachment tectonics.
And that means just like a tablecloth, if you make that
slip surface slightly sticky, it can rock up and make folds, and
the folds have a very simple type of form, kilometer wide
structures, they make the hills of quite a lot the juror.
The landscape actually mimics the bedrock geology as you go
deeper into the outs. Those rocks have had material
taken off the top, they were hotter.
Therefore, when they deformed, they were once deeper
underground. And in those situations you get
different types of folds. Shorter wavelengths for much
shorter wavelengths. Folds.
I've already touched on the idea that if they're preexisting
normal faults, that the sedimentary strata in the little
basins get squashed up against normal faults, and that will
generate much shorter wavelength upright faults, much more
intense folding than you see in the decor, more systems of the
juror. So there's a fold scale and fold
shape variation that relates to these different settings, in
part related to where they lay on the old wifted concept of
margin, whether they were normal faults or not.
And in part where they lay in the crust or in the roots of the
mountain range, what level they lay, which controls some of the
ductility of the rocks when they were deforming.
So these jewel controls, it is quite difficult to tease these
out actually. What are the relative importance
of of each Part of the issue is that structural geologists
traditionally have only been really that bothered about the
structure and haven't thought very much about the basins, the
preexisting basin stratigraphy. These days, of course they do,
but you go back 50 years when a lot of the ideas of folds
relating to ductility were developed, the idea there were
riff basins in the Alps was barely thought about.
Is that partly because the structural features are so much
easier to see? I mean, even the untrained eye
like mine, you can see the glorious thrust, for example,
It's just a great big line across the mountain.
Is that because there's a selection effect based upon what
it's easy to see when you go hiking in the mountains?
I'm sure that's true. And there's a tendency also for
people to work on things that they can recognize, things you
can't necessarily make your mind up about straight away.
I think there's a tendency for all of us to to put to one side,
and often the stratigraphic relationships make for slightly
more complicated structures because the layering wasn't nice
and continuous to start with, so people have tended to overlook
them. Nowadays people are going after
them all over the place because those are the key and it's an
interesting challenge in an investigation of any region.
Something we have in the sciences, of course, is that the
the world is our oyster, but it's a big world and a lot of
oysters. And so the question is which
bits do you concentrate on, which you think are important?
For me, I think the important stuff is always the stuff you
can't understand. It's the stuff that doesn't leap
out at you, because that's almost certainly where you're
going to find the solution. The things that are most obvious
are usually not the most important, and in this case, the
less obvious stuff is the preexisting stratigraphy.
It's the areas where there's preexisting stratographic
variation, those in Rift systems, those will happen most
dramatically from one side of a normal fork to another, from a
fork block to a basin. So you want to find those.
It's a holistic approach. You want to try and put that
strategically together at the same time as you are trying to
unravel the structure one of the ways in which people.
Try to disentangle the various contributions to this
accommodation, if you like, Of the one continent moving next to
another one is really the overall amount of crustal
shortening that's taken place. And at least in the case of
Himalayas, there's often a debate about to what extent
that's accommodated by huge boundary faults, or by integral
shortening through faults and maybe smaller faults on many
different scales. Is that a similar debate in the
Alps with respect to the fact that these early major faults
were detected first and now we're finding out much more
about the finer scale structure, understanding distributed
deformation. An individual hand block won't
have that much convergence between Italy and France
recorded in it, but you put a lot of blocks together.
That's a lot of convergence. And the catch is to capture that
as an insight, it's quite difficult.
You can't take a photograph of it.
You can take a photograph of a little bit of it, but you can't
capture the essence in the same way as you can by taking a
photograph of the Glarus thrust. So I think in structural
geology, I think this goes for many other scaledependent parts
of the science. There's a tendency to deal with
the things that you can capture the photograph or capture with
the view, not necessarily capture the distributed nature
of defamation. That's .1.
However, the question really is whether you can evaluate the
amount of shortening in mounting belts to actually get the
convergence. It's something I tried a long
time ago. In fact, my first post doc was
to try and do exactly that in the outs, and I came up with a
number of hundreds of kilometers shortening because if you
unravel all these structures, that's the sort of number you
get. The people have had similar
numbers for centuries, but how much of that is because of the
return flow in our subduction channel.
So you since you're double accounting, you've got the
material that's gone down off seduction zone.
So that's recording true plate conversions.
But if you also add in the return flow coming back up,
you're counting it twice because it's come back up.
So I think there's been an awful lot of double accounting going
on and the challenge is to recognize it because there's
obviously real convergence as well.
So separating those two components, well, I don't think
you can and maybe you try and find other explanations or other
tools to establish the far field tectonic convergence in the
system. What kind of tools might those
be? For example, the Himalayan
collision with Tibet and the rest of Asia.
We understand how that works because we can reconstruct the
plate motion on a global scale and reconstruct India's relative
motion through the opening of the Indian Ocean basin for
example and tied into a whole of plate tectonic model.
And we can do a similar thing between the whole of Africa and,
if you like, stable Europe. But what you can't do is then
define the movements of the individual little blocks caught
up in the car crash between which is the Alps, the other
Mediterranean chains. You need to sort all those out.
And that's tough anymore. That would be to if you have a
car crash and to work out where a particular piece has come out
of the car and where it's gone to.
You couldn't forecast where that wing mirror went when you had a
car crash. Not easily.
So that's the challenge we have is having a marker we can work
at. It's a scale that's appropriate
to the scale of Alpine system. So then what you're suggesting
is that there were many more fragments that came into play in
determining what happened in this crash than there were in,
for example, the Himalayan case. Let's take that in two parts.
Certainly in an Alpine Mediterranean context, there is
great debates about what the distribution of the various
pieces of older concept of fragments.
By all do I mean pretethis. So that's Triassic and older
fragments, what their distribution was.
You can identify where they are. There's a huge controversy about
where, for example, the Calabria that the foot of Italy exactly
sat nowhere near where it is now.
Some of the other fragments that make up the Italian peninsula,
where do those sit, let alone the southern margin of the Alps
going into northern Italy? Where precisely was that
precisely in this case is ±100 kilometers.
Well, 100 kilometers is quite a lot of shortening in Alpine
context. So that uncertainty of where
you're going to put it feeds into the far field
determinations of strain in the mountain belt and the price is
in that value, not it's all about the 10s of kilometers, not
the hundreds of kilometers when it comes to understanding the
structure at the scale of those mountain ranges.
Now the question is how much of that is also true of the
Himalayas, and we just don't know it yet.
People talk about the presence of volcanic arcs that were
between India and Eurasia and whether those collided with
Eurasia first or collided with India first.
But we're not really talking about volcanic arcs.
Yeah, we're just talking about where all these little terrains
came to be after the previous phase of rifting.
That's right. But the question is then what do
you mean by terrain? So you could think of an
individual fault block which has had a bit of stretch away from
another bit of fault block on a rifted margin as being a
microcontinental block. It was all joined together by
bits of continent. But understanding where all
those fragments exactly sit is the prize, and so you need to be
able to unravel that. So it okay in a Himalayan case.
What did that rifted margin look like that's been caught up?
The amount of geological investigation of that problem in
the Himalayas is miniscule compared to the understanding of
what's been built in the Alps, where there's been two centuries
of stratigraphic studies rather than a handful of expeditions
into rather remote parts of the northern Himalayas.
Coming back to the Alps, then, what are the most pressing
research themes that people are engaged in today?
There's a lot of effort trying to add geochronological
precision to the timing of things, trying to get better
estimates of the conditions in the subduction zone, partly
because subduction zones are continuing to be a major topic
in a science research. Understanding how continental
fragments get entrained into subduction zones is a global
issue, and the opposite is a place that you can try and look
at that. Part of the challenge though is
that many of these works rely on quite historical reconstructions
of the large scale structure. So the question is, it's all
very well having a very precise number on the defamation, but if
you're imprecise about where it is in space, it doesn't
necessarily help you that there remains this problem of how you
deal with the diversity of data you need to solve these sorts of
problems. And you've alluded to the idea
that one of the app's great assets is the sheer amount of
data. How do you handle it?
Most of it's not real data. Most of it's first order
interpretation derived from observations and caught up by
all those issues we've been touching on such as selection
bias. And so how do you see through
all that? It's a challenge.
I mean it's a challenge I have in trying to teach Alpine
geology. How do you really get into what
is knowledge, what is inference, what is known, what is unknown,
what do we think we know? But actually we just assume and
I think there's a real challenge with knowing too much or not
being able to filter. Are the apps still growing today
going up? Yes, they are.
They're also wearing away and sending detritus down the River
Poe whenever it rains, which this year it did a lot.
So there's still Africa convergence going on that is,
that is squeezing the crust a little bit, although hardly at
all. And most of those earthquakes
are not in the Alps there in places like Sicily and elsewhere
in the Mediterranean area. But there's still a weak
convergence going on. There's still earthquakes
because of rebound rebounding crust is coming up and that
that's not a steady process. It's creaky.
So you get earthquakes, you get mountains that are growing.
We know that not only because of geological research, it's also
had a loss of topographic surveys for centuries, but also
over the last 30 years from G PS:.
Records. But you can also see the uplift
in some of the long railway tunnels in Switzerland, which
starts off horizontal and now bowed, so that there's
relatively short kilometers wavelength differential uplift
happening across the Alps as well.
And that uplift has a tectonic origin.
The principal driver is actually erosion, so that as you erode
material and take it away from the Alps, you're taking a load
off the Alps, which means the bits that haven't eroded, which
are the mountain peaks rather than the mountain valleys, rise.
So it's an isostatic response to erosion that's fascinating.
So actually, by wearing down the mountains, we're just really
creating more relief. We're not actually wearing down
the peaks. Yeah, we're not wearing down the
mountains. We're eroding the valleys, the
valley sides, particularly in a glaciated time.
Then you're wearing out to the bottoms and the sides of the
valleys and the spiky bits go higher.
Obviously, they fall down catastrophically every now and
then. But actually, overall you
generate exactly more relief. But that's really because
overall, the Alps are still fairly young.
I mean, eventually they'll be flattened like the Urals or
something, won't they? Presumably once you've taken all
the load off your ship, it just goes back to where it was.
So yeah, are you currently engaged yourself in any Alpine
research? One of the things I've been
playing with is trying to use the sedimentary rocks and the
strata that deposited around the time, particularly of the Eocene
Oligocene to reconstruct relief. But it's submarine relief
because these were submarine deposits, the turbulites, and
they were fed into kilometer deep Seaways that surround the
Alps. The things I mentioned before,
the earliest ones weren't derived from the Alpine chain
because the Alps wasn't generating topography, they were
derived from other places and it came into this seaway.
And so if you can reuse this record, you can potentially get
a very detailed and quite subtle story of how bathymetry
submarine topography varied around the edge of the Alps.
I think it will provide information about the slab
processes, the seduction zone processes that were just coming
to an end at this sort of time, and it will tell us potentially
how those slabs coupled with the adjacent, not so rifted margin
of Europe. I'm a believer in serendipity
and the importance of doing curiosity research.
I don't know if it'll have any tectonic value at all, but if
you don't do it, you'll never find out.
Whereabouts are you actually looking at these sedimentary
structures? I'm looking at it in a system
that's Germany, known as the Great Dano, which is quite a
famous set of turbodites in southeast France.
And you can find its continuation around that rim
through the Dauphin and maybe even up to that corner of
western Switzerland, right around the western Alpine Ark.
It's a nice place to go as well. So it's always good to have a
hobby that provides good wine. Rob Butler, thank you very much.
Thank you. It's been fun to see pictures
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