This is Geology Bites with Oliver Strimpel.
One of the most appealing aspects of geology is the way it
can reveal the common origin of rocks that today are separated
by thousands of miles. In a recent episode with Rob
Strachan, we saw how a single, albeit extended orogeny, the
Caledonian orogeny, formed rocks now in the northern British
Isles, eastern Greenland, and western Norway.
But the Appalachian Mountains are also part of this same
story, even though they now lie across the Atlantic Ocean.
These mountains stretch for over 2000 miles all the way from
Newfoundland and Canada to central Alabama in the United
States. Case Van Staal has been studying
the Appalachians for over 35 years, focusing especially on
the large scale tectonics of their formation.
He is a Meritus scientist at the Geological Survey of Canada and
an adjunct research professor in the Department of Earth and
Environmental Sciences at the University of Waterloo in
Ontario. Case van Staal, Welcome to
Geology Bites. Thanks for inviting me.
Since I said that the formation of the Appalachian Mountain belt
is related to the Caledonian neurogeny, let me quickly recap
the very high level tectonic context of thaterogeny.
It took place between the final breakup of the supercontinent
ridinia, about 600 million years ago and the formation of
Pangaea. It could in some ways be viewed
as a first stage in the assembly of the Pangaean supercontinent.
So around 430 to 400 million years ago, three of the large
continental masses that had rifted apart from Virginia
collided and formed La Russia. Those colliding continental
masses where Laurentia, which corresponds to present day North
America, Greenland and Scotland, Baltica which corresponds to
Scandinavia, and Avalonia and Ganderia which correspond to the
various terrains preserved in Ireland, Wales and England and
parts of mainland Northwest Europe.
These collisions comprise the main phase of the Caledonian
neurogeny. So getting back to the
Appalachians, where these 3 continental masses also
responsible for forming the Appalachians to the West.
In part because Beltica was not involved in the Appalachians,
but certainly Avalonia and Gondira were.
But you forgot one thing which is actually quite important.
You forgot the Taconic Grampian event, which is an Order Vision
event that took place between 480 and 450,000,000 years ago.
And actually that is one of the arguments that the processes
that were taking place in the Caledonian orogeny and the
Appalachian orogeny are related. Because this arrival of a
terrain outboard of Laurentia with Laurentia on the lower
plate. So Laurentia was the down going
plate is very similar all the way from Norway into the sound
of Appalachians. And so this event involved the
abduction of Ophelites and the arrival of islet arcs to the
Laurentian margin. And it is after that event that
the subduction changed its polarity from South dipping
because Laurentia was in an east West orientation at that time
was lying very close to the equator and then subduction
started to go in the nice Laurentia from South to north.
So that event is the realization that the event that was very
similar was one of the linkages between the Caledonian and
Appalachian horogenes. OK.
That was the tectonic Grampian events.
When we spoke earlier, you said that you have probably seen more
rocks in Ireland and Newfoundland than anyone.
Was it the close relationship between the rocks on either side
of the North Atlantic that initially LED us to this
tectonic picture? Well, this is not a new idea.
People have been commenting on the similarities between the
geology and the British Isles and the Appalachians in North
America for many years. Some of the people you can
mention is Hank Williams in Newfoundland at Memorial
University, and John Dewey was a professor at Oxford.
And they early on, after the acceptance of plate tectonics in
the 1960s, they started to realize that it was important to
correlate these features because they saw that there was probably
a similar tectonic history. Can you tell us just how the
rocks on either side of the North Atlantic relate to each
other? And do those relationships
extend all the way down the entire length of the
Appalachians? The rocks are similar even on a
little logical level. But yes, certainly the rocks in
the Northern Appalachians. So that's the rocks which
stretch from Connecticut all the way to Newfoundland, and
especially Newfoundland, because it's the closest to the British
Isles, has a geology that is very similar to what you see in
the British Isles, especially Ireland.
So the correlations are based on tectonic setting of the rocks.
So better a rock is an arc or a rift or a margin.
And so we're correlating rocks of similar tectonic setting,
looking at the provenance, looking at the tectonic
histories of the rocks. So we're using all kinds of
tools for that, paleontology and fossils in the rocks, the ages
of the rocks. Let's talk a bit more about each
of these types of relationship. How is the tectonic setting
related across the Atlantic? Well, for instance, when we
determine a rock, let's say this rock is an island arc and we use
igneous geochemistry for that. And we see these rocks are very
similar to modern day island arcs like we see in the Pacific
Ocean like in the Solomons or in Fiji, whatever.
And so if we see an arc association, let's say in
Ireland, and we see the same arc association in Newfoundland and
they have very similar ages and their faunas in the associated
sedimentary rocks and everything else is very similar, then the
correlation is a valid way to go.
I understand that we see evidence of extension that took
place in association with this collision.
Can you talk a bit about that? Yes.
So during the opening of the the up this ocean, we noticed that
there was hyperextension, which means that the continents were
rifting apart without producing a lot of magnetism and going
into spreading very rapidly. And so during this
hyperextension eventually you bring the litospheric mantle to
the service on the ocean floor. We can see that today off the
coast of Spain and Portugal and also maybe off the coast of
Ireland. And so these rocks are preserved
and they are very similar bows in the British Isles and in
Newfoundland and in the rest of New England.
So we see an association of ultra mafic rocks and sediments
that were deposited very close to the Laurentian margin.
These rocks, we call them Ocean Continent Transitional rocks and
we have dated them. So we know that they formed very
early on in the history of the opening of the upatus.
We know for sure that they were not an interoceanic terrain or
whatever. They must have formed during the
rifting period, and the fact that these rocks are very
similar is another link between the processes that took place
during the opening of the Yaputis Ocean.
The ocean that existed in the early Paleozoic and then was
basically closed and then eventually another ocean was
closed after the arrival of Avalonia.
It was called the Rheic Ocean. But these linkages are important
to us to correlate and see whether the processes are very
similar. So this is one of the arguments
we have used. I also understand that we see a
particular kind of mafic igneous rock called a bonnenite.
Is that something that is correlated on either side as
well? Yes.
So Bonnenites are very rare, especially in the Appalachian
Caledonian origin, but they are relatively common in this first
terrain that arrived at the Laurentian margin.
You can trace these bonanites from New England all the way
into Norway. So that suggests a common
origin. And bonanites are a high
magnesium, high silica and highly depleted in high field
strength elements such as titanium, and they formed under
specific tectonic conditions. And so the fact that these rocks
are very common in this particular terrain, which we can
trace all the way along the length of the origin, is another
argument that these rocks are related.
You mentioned provenance, so how is the provenance on either side
of the Atlantic related? We studied the sediments which
are associated with the rocks involved in these collisional
events. And so we're looking at fossils
and you know already for a long time that these fossils are very
similar along the length of the origin.
So rocks that form close to orange have a very different
faunas than the rocks that come from other continents like
Baltica or Conduana. And so we also look at things
like the trial zircons. So we dated the trial zircons
with uranium lead methods and they tell us what the hint lead
is, where these rocks are coming from and the rocks that are
eroding and then the Tritus is basically being transported by
rivers to the continental margin and then deposited in sediments.
So they tell us these rocks are now a locked in us and they have
been through the mangle, they have been deformed, the
metamorphos, but they still allow us to determine what the
hinterland, what the source rock are.
So we can separate rocks that were having a Laurentian source
from rocks that had a Ghunwanan source, because the hinterlands
are very different. They have very different
basements and very different histories prior to the
Appalachian Caledonian origin. Is it the trace element
signatures that are different? It's the ages.
So for instance, Gondwana is characterized by many
Neoprotozoic activities. Neoprotozoic is the time period
between 1 billion and about 550,000,000 years ago.
And so this event is very rare or absent in North America, but
it's very common in Gondwana. So if you see a lot of these
Neoprotozoic zircon showing up in your sediments, that will be
an argument to say, well, these rocks didn't form close to
Laurentia, but they come from Gondwana.
So this is the kind of things we're looking at.
So other things is like for instance, 2 billion year old
rocks, we call them the Burnian. They are very common for
Gondwana, but they are not common in Laurentia.
So it's a kind of different age groups that show up in these
sediments that are typical for whatever they come from.
You also mentioned that Ophelites were abducted in the
Appalachians. Can you tell us about those?
Yes, some of the best ophelites preserved in the world occur in
the Canadian Appalachians, especially so in Newfoundland
and to a lesser extent in Quebec.
And these ophelites were very important in the acceptance and
the development of plate tectonic models for the origin
of the Appalachian Caledonian origin, which is one of the
longest studied origins in the world.
Are these offrelites in good condition?
Then? Can we actually see the
structures associated with them and say something about the
processes that took place? Yes, some of the offrelites
preserved in the western part of Newfoundland are extremely well
preserved. One of them is the Bay of
Islands Offrelite, very famous. It's very big and it's lying on
top of the Paleozoic laryngian margin.
So the whole process associated with the formation of the
ophelite and it's emplacement onto the margin is relatively
well preserved and all kinds of things are very important for
understanding how these ophelites formed and how they
were in place are in part preserved in these ophelites.
For instance, the metamorphic souls.
So this is a soul of rocks that were glued to the base of the
ophelite are well preserved. And so they basically demanded
that these rocks were in place while they were still very hot.
And so they were metamorphosed in the rocks that were accreted
to it at the base, and hence these rocks were metamorphosed.
Hence the word thermal soul, terminal, aureal.
And you can see the thermal, aureal and metamorphic soul in
Newfoundland Ophelites. Yes, very much on several of
them. These souls are very well
preserved and have been studied early on and very very important
in developing the ideas people had on how these Ovid lines were
in place. Where the Appalachians
originally formed as the Himalayas scale mountain range
and then are the rocks on the surface today the exhumed mid
crustal roots of those mountains.
No, the Appalachians didn't. Only the parts of the
Calavanides did in the Scandinavian Calavanides where
Baltica was in the thrusting Laurentia for a very long time.
But the Appalachians only in a few segments, may have been in
like the Himalayas, like in southern New England where we
have evidence that there was a long lived high standing
tectonic plateau like in Tibet. But in the rest of the
Appalachians, anywhere in northern New England and in
Newfoundland and in the rest of the Canadian Appalachians, the
origin was not like the Himalayas, It was more like what
you would see today in Iran along strike of the Himalayas.
So these are rocks were much lower grade we see here rocks
volcanic and sedimentary rocks of relatively low metamorphic
grade preserved and the reasons between this variation may be
due to the obliquity of the convergence.
It's more oblique to the north and the undertrusting in
southern New England by first Avalonia and then by Africa
during the closure of the Riyadh Ocean.
So that undertrusting was very long lived in the New England.
So I hang pushing up the overlying crust to high levels
and kept it in place for quite a long time, about 70 million
years. So this was Avalonia and
Gondwana under thrusting, similar to the way the Indian
plate is now under thrusting under Tibet.
Exactly. So the Indian plate has been
under thrusting underneath Tibet and Eurasia because Tibet forms
part of the Eurasia for about 50 million years.
So India is a sick continental plate, very buoyant and it has
been pushed underneath Eurasia. So you get double crust.
So you have relatively light crust.
It's the same process. If you have a plank of wood and
you put another plank of it, the amount of wood that will come
out of the water will be higher that that is basically the
principle of isosceles and that happened in Tibet because you
stuff something of low density in the knees Eurasia.
The highest point along the Appalachians today is just over
2000 meters high. Is the present day topography
what's left of the topography that was generated by the
orogenies that formed the Appalachians?
Generally no, because after the Appalachian orogeny was finished
there was about 200 million years of denudation until the
opening of the Atlantic Ocean and so the origin must have been
eroded back to sea level. So the present day topography is
largely due to the processes it first associated with the
rifting and opening of the Atlantic Ocean.
So you get wrist shoulders and magnetism that happened, the
Central Atlantic magnetic province basically pushing up
part of the overlying crust, so forming topography.
And then subsequently during the recent history in the last 6070
thousand years till about 10,000 years ago, we had ice ages and
ice ages were basically loading the underlying crust.
And when the ice disappeared, the crust started to rebound.
So producing topography and the amount of erosion hasn't had
enough time to completely erode this origin back to sea level.
So that's why we see all the various topography in the
Appalachian origin. So do the hills of the present
day Appalachian Mountains actually correspond to the
location of the original accretion and orogenies that
took place? And if so, is that to do with
structural inheritance, in which later stresses are accommodated
by movement along form of faults and orogenies where the rocks
are actually weaker? Reactivation of faults during
subsequent drifting? Most likely, yes.
But the hills have no relationship to where those
falls are, so the topography is to a large extent not related to
where these reactivated falls are.
But yeah, reactivation of the falls during the opening of the
Atlantic most likely occurred. But not exactly where the
Appalachians are today. Like along the seaboard for
example, but not in the hinterland.
Of course, the main rifting event took place along the
eastern seaboard, but also further back into the hinterland
of the Origin to some extent. The amount of motion on these
faults is not very well defined and the ones on the eastern
seaboard are largely buried beneath sea level.
So we don't really know exactly what happened to these faults,
but it's not a coincidence that new oceans form along existing
zones of weakness. It's a well known relationship.
We've seen that in many other origins as well.
Because these rocks are relatively weak, they have major
faults that may continue all the way to the lidosphere, and so if
you start rifting it, those songs of weakness are going to
be reused. Nature is like that.
It always takes the easiest way out.
But that's not directly responsible for the Appalachian
Mountains that we have today, then no.
But yeah, I mean, the rifting we can see, for instance, if you go
to the most western boundary of the Appalachians, you can see a
huge topographic difference between the North Shore of
Quebec and Labrador and the Appalachians.
And that's may have something to do with the rifting episode that
opened up the Atlantic Ocean, parts of Arctic Canada.
For instance, in Baffin Island they have relatively high
standing mountains, which are probably related to magnetism
that was associated with the opening of the Atlantic Ocean.
So the structures we see today then will consequent on much
later events, and some of those later events took place and had
an effect right there because of the adjacent rifting that took
place, which was indeed affected by inheritance.
As I mentioned, the Appalachians are over 2000 miles long.
Are there any analogues of such a long collisional mountain belt
anywhere else on earth? Well, the one comes to my mind
is the Central Asian originic belt in Eastern Asia, which runs
from Siberia and Kazakhstan through Mongolia and northern
China all the way to the Pacific Ocean.
I mean, it's this extremely long mountain belt and also extremely
wide and probably the longest accretionary origin in the sense
that it's an origin where many pieces were accreted to the
Siberian Craton coming from the South and forming a long lived
origin. And that mountain belt is much
bigger than the Appalachian and the Caledonians together.
You're still working on the Appalachians.
In fact, you're setting off for a Newfoundland field trip later
today. What questions are you tackling
now? So we know the, OR at least we
think we know the general tectonic framework, what
happened, but we want to know the details of the processes, so
how these rocks were exhumed and then starting eroding, because
as soon as they come to the surface, they will start
eroding. We want to check that by looking
at the syntectonic, the sedimentary basins that formed
during those processes. So we want to see the unroofing
history and that will tell us about the processes that were
responsible for the burial and then the subsequent exhumation
of these rocks. So we're looking at the
sedimentary bases and we sampling rocks through the
systedographic record and we will dating the minerals that
can be dated by uranium lead methods, minerals such as
rutile, titanite, monacide, appetite.
And so each of these minerals has a closure temperature and
formed at specific metamorphic conditions.
And so we can try to reconstruct the processes that were
associated with that exclamation.
And actually that's a very exciting and very promising new
line of research. Hopefully we will be able to
present this data in the near future.
So this is generating what people refer to as the time
pressure temperature kind of loops in that temperature
pressure space. Yes, exactly.
So we can determine approximately when the rocks are
buried to the maximum depth if we are lucky and maximum
temperature. But then we want to know is when
these rocks, how they got back. So that's basically part of the
PDT loops, as we say, the pressure, temperature, time and
that tells us something about the processes that were
associated with that, whether the rocks were heated up during
this process or they remained refrigerated.
The results that we have so far are very interesting.
I'm very excited about it. You've spent almost all of your
long research career on the Appalachians.
What made you interested in them initially?
That's serendipity. I came to Canada in 1980 to do
my PhD and I did my PhD in Appalachian rocks.
So I got very interested in those rocks and then I was hired
rapidly after the finishing my PhD by the Geological Survey of
Canada. And in their wisdom, the
Geological Survey gave me the task to basically figure out the
Appalachian origin. They gave me a lot of freedom
and I used that. And I was basically driven by
curiosity how these various rocks correlate.
And so it's like in any terrain, the relationships between rocks
are also the most important and they can only be preserved in
some places. So if rocks in Quebec give you
very good relationships, then they may explain relationships
in Newfoundland as well. Those kind of arguments.
And that's why I kept working in it.
And it's a fascinating origin. It's one of the longest studied
origins, but still there's lots of questions to be answered.
Such as? The most important outstanding
tectonic questions concerns the origin of the various terrains
that became isolated in the Yaputis and Rheic oceans.
Terrains like Gonderia, Avalonia and Maguma which became.
Sequentially accreted to. Laurentia, the evidence.
For these. Accretions is preserved in the
Salinic, Acadian, and Neoacadian origenes.
Which occurred between 4:30 and. 360 million years.
Ago these. Questions concerned where these
strains formed, why did they depart and became isolated from
the source continent, and how did they?
Travel. Across the oceans towards
Laurentia and exactly what happened during their odysseys.
And what kind of evidence might help us answer these questions?
Providence studies, pedimagnetism and tectonic.
Reconstructions. This may help answer other
outstanding questions such as why subduction?
Started or was. Ongoing coevenly at several
places in the Yaputas and Rig oceans.
And then, of course, all these lines of evidence will help us
to understand how these oceans. Exactly closed.
And eventually how the supercontinent Pangaea formed.
Keith Van Staal, thank you very much.
You're welcome, Oliver of us, a pleasure, Thanks.
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