This is Geology Bites with Oliver Stremple.
What events in the geological past were responsible for
shaping Central Europe? In earlier episodes I talked to
of Liniman about the assembly of Central Europe during the
Paleozoic and with Rob Butler on the origin of the Alps in the
much more recent past. Here we talk about two orogenies
that also played a key role in the story.
First, the Codomian orogeny that lasted from the late
Neoproterozoic to the early Cambrian, IE from about 700
million years ago to about 425,000,000 years ago.
And 2nd, the Veriskan orogeny that took place in the late
Paleozoic from about 380 to 280 million years ago.
Yuri Jacques has been studying the geology of Central Europe
for over 25 years, using methods ranging from structural studies
in the field to detrital zircon geochronology.
He is a professor at the Institute of Geology and
Palaeontology at Charles University in Prague.
Yuri Jacques, welcome to Geology Bites.
Thank you very much for inviting me.
I'm glad to be here in this podcast.
Before we talk about individual orogenies, let's try to place
this in the wider tectonic context of the assembly and
breakup of supercontinents. The so-called Wilson cycle that
you break up a supercontinent and then it reforms roughly
every 500 million years or so. This period that we're talking
about, how does that fit into the Wilson cycle?
This Wilson cycle would be positioned on a time axis
between the supercontinent Rodinia and supercontinent
Pangaea. So in the Mesoproterozoic and
Allenoproterozoic times, we have here supercontinent Rodinia
which broke up. And part of this, one of the
results of this breakup was Gondwana.
And then and we'll probably talk about it more, was Pangaea,
which formed during late Paulozoic.
So all the processes we will be talking about occurred in
between. So we are in terms of a
supercontinent cycle in between Rodinia and Pangaea.
In terms of Wilson cycle, it's a little bit more complicated
because Wilson cycle, it's only one of the forms of
supercontinent breakup and assembly.
So it implies that you create an ocean and then you close the
same ocean. Here what we have the record in
Central Europe, it was much more complex.
There are more oceanic basins, more terrains involved.
So the history is, say, more complex than the simple Wilson
cycle. But in terms of supercontinent
cycle, we can track processes which happened from the breakup
of Rodinia to the assembly of Pangaea.
So Europe today is part of Eurasia, so that is itself a
pretty massive continental block.
So during the assembly of Europe, was this leading to
greater and greater amalgamation of terrains that eventually
including the Caledonian Neurogene and then after the
Voriscon, sort of basically led to the formation of Pangaea?
Is that really this very high level picture?
On a larger scale you are right, but on a smaller scale we should
not forgot the Kadhomian Originae, which actually form
much of crust which we see now in Western and Eastern Europe.
And the younger Variscan Originae simply recycled this
material. So I would argue that the main
crustal forming event, maybe even most important crustal
forming event, was the Kadhomian Originae at the late Proterozoic
to Aleppozoic times. It really created new crust, the
Variscan origin. It was collisional and it
largely recycled this Cadomian basement.
OK, so let's talk about the Cadomian orogeny.
Can you walk us through a little bit more about what happened?
So first of all, it should be mentioned that although the
Kadomi and Origeniae was first defined in Europe, it was more
likely a global scale Origenic event.
Because it has been proposed that at the end of
Neoproterozoic and in the beginning of Palazzoic we are
talking about processes that happened about 750 to 500
million years ago. Almost entire Gondwana was
surrounded by broadly coeval and linked Origenic belts.
So what we now call the Avalonian Cadomian belt was only
one segment of a global origenic system.
And key feature of this global origenic was subduction of large
oceanic plates beneath the Gondwana mainland, forming sort
of subductions of girdle and in effect subduction controlled
formation of extensive tracts of accretionary edges, volcanic
arcs, back arc basins which were attached to the upper overriding
plate. So in terms of recent analogy,
this is pretty much similar what we see now in the Western
Pacific. In the Western Pacific, then
yeah, it's OK. You get subduction and you get
all the the consequences or corollaries associated with
subduction. So do we see those today?
Can we find the remnants of those in Europe?
I mean, can we see accretionary wedges or where volcanic arks
used to be, or remnants of mountain chains?
Yes, we do. And actually, if you look on a
geologic map of Europe, the fragments of the former
Avalonian, Kadumi and Belp are now scattered all over Western
and central Europe, with the main exposures being the Iberian
massive in Spain and Portugal, Armorican massive in France, and
of course the Bohemian must sit here in Central Europe.
However, we find them frequently also within the Alpine originic
system. So they were recycled during the
Variscan origin and again recycled during the Alpine
origin. And in this case they extend
farther east and South into the Alps, Carpathians, and even
farther into Bulgaria, Greece, Turkey, and even to Iran.
So in all these places we find the relics or fragments of the
former accretionary orogenic belt of the former Avalonian
Kadamian active margin. And I would argue that in here
in Bohemian Massif, we have one of the best preserved fragments
of this margin, because we have here a relic of an Ophelite, we
have a relic of an accretionary which very well preserved.
We have a relic of volcanic island arc and also probably the
arc basin, all in their original relationships.
So we can study very well the Caribbean processes here in the
Bohemian Massif. OK, there you have them still in
their original relationship. I was going to ask you about
that because it sounds like there's been an awful lot of
processing that's taking place afterwards, with all fragments
appearing from as far away as Spain to France and then to the
central parts of Europe. So how do we reconstruct what
happened during the orogeny? How do we kind of manage to wind
the clock backwards and see how all the parts fit together?
Yeah, there is several lines of evidence and there are several
sort of principal things we can recognize in the rock record.
First is what is called the Kadumian unconformity, which has
been discovered here in Central Europe.
It was noticed in second-half of 19th century and later formally
defined in France in 1921. So it it's the major evidence
that the Kadumian originator took place before Cambrian or or
the vision, because what we see is that we have folded and
deformed metamorphose basement complexes which are overlined by
Foscilliferose on the formed Cambrian or even auto vision
strata. Nowadays we have a lot of data
available, which consists of Geo chronologic data, usually in
sediments, the trital zircon ages, which allow us to define
what's called maximum depositional age.
So we can sort of put some time constraints on the age of
deposition and deformation. Even in these rocks which do not
contain any fossils, we have geochemistry which allows
especially geochemistry or volcanic rocks, which allows us
to interpret them as either volcanic arcs built on thin
continental margin or on oceanic rust.
We also have pieces or fragments of the subducting plate which
were incorporated into the accretionary wedge probably or
they form seamounts on the sea floor.
And of course during subduction it's very difficult to subduct
these asperities. So they they are created and
mixed with the terrigenio sediments of the accretionary
wedge. So there is enough pieces of
evidence to argue that it was a Pacific type margin with active
protracted subduction where an unknown oceanic plate which is
almost completely gone to the mantle was subducted beneath
Gunvana creating these accretionary wedges, island arc
etcetera. So now as compared to 20 or 30
years ago, we have a lot of data.
The Geo chronology also allows us to set a time scales or set a
time frame for this process. So if we focus on the Central
Europe here, the academia and origina and the subduction
started probably around 636 hundred, 20 million years ago.
And what's interesting, it continued into Cambrian.
So the end of subjection was probably at around 525,000,000
years ago. So at the moment now at this
stage of research, we have a lot of data.
What about paleomagnetism? And do we know roughly at what
latitude all these events took place?
Paleomagnetism doesn't work good in these rocks because they are
quite old. They have been remagnetized
several times. But if we want to estimate a
palaeolatitude, we can simply use the overlying strata because
we have Cambrian or auto vision on top of it.
So we can use palaeontological data, biogeography, but also in
upper auto vision which is directly overlying the Cadomian
basement. We have glacial marine sediments
which argue for proximity to Gondwana because there must have
been an continental continental glacier available.
So putting it all together, that's a good argument for
southern latitudes on the margin of Gondwana and based on mostly
the Trito zircon Geo chronology and comparison of the Trito
zircon ages in Cadomian basement with those in Africa and South
America. So potential source areas.
The most common view is that what constitutes Central Europe
was somewhere close to West Africa, so sourced from West
African Craton or Trans Saharan belt, so Northwest, Northwest
Africa. So we have pretty much good
constraints on where the terrains which are now in
Central Europe were positioned at the end of no Proterozoic.
So let's run the geological clock forward now, after this
period that you've been describing in the end of the
subduction, what happened next? Yeah, that's actually very
difficult question because how exactly the Kadomian origin
ended, it has long been a matter of debate and several other
contrasting models have been proposed.
And I would say that it has not been resolved until now.
For example, Damien Announce and Brandon Murphy proposed a model
of an interaction of the Caribbean active margin with
migrating mid ocean Ridge and a change to a transform regime and
later to a passive margin from the auto vision onwards.
And what also seems likely that Avalonia are rifted completely
away from its original position next to South America or
Gondwana opening a Rake Ocean, and then collided with Laurentia
and Baltica, what we now call the Caledonian origin.
However, great debate exists to what happened to the other
Karumean terrains, whether they also were completely detached
from Gondwana and drifted northwards as entirely separate
islands within the Rake ocean, or inmate attached to Gondwana
at all times during the early Apollo's weeks.
So this debate actually is rather important for the
reconstruction of Prevariskan Palo geography.
How many oceans and how many continental blocks were involved
in the subsequent variscanerogene?
The model I like and I prefer the moment is the one with the
migrating mid ocean Ridge which intersected the Caribbean
subduction zone because it's nicely explains the termination
of Arc activity which was diachronous from West to east,
but we cannot be sure. So again, to step back a little
bit, so we had these series of events which you say we now have
a fairly good evidence to be able to constrain what happened
during the Codomian androgyny and we can roughly say how
things looked at the end of that.
And you say what's open to debate is what happened next.
But at what point, running the clock still further forward, do
we feel like we have enough evidence to say, well, at least
now we know where all these terrains are positioned and what
happened, and it's just that we have to somehow figure out what
happened in between, starting with the end of the Kadoni and
Orogeny? If you look at the end of Kadoni
and Orogeny here in Central Europe, what we see is granitic
plutonism. So large bodies of granitic
magma intruding into the former accretionary wedge at about 525
till 500, which is a sort of strange because accretionary
wedges are typically very cold regions of Earth's crust because
they are underlined by cold subducting oceanic place.
So there is no heat source. So we need a heat source to
explain the granitic platonism. One of the models that's what I
was talking about is that Rich intersected the subduction zone,
which would explain the heat source.
So you can find more recent examples of mid ocean ridges
intersecting subduction zones and creating sort of what's
called slab window. So the so the slab sort of is
detached and you can get a hot asthenosphere into the base of
of cold accretionary watch and that's what we think happened
during Cambrian. So we have this, we need to
explain this heat source. And the next stage was the
passive margin which is evidenced by Auto vision or
Basil auto vision sediments, Basil auto vision beds which
overlie the academia basement. So we have a very good record
actually from the end of Proteosaic to the beginning of
auto Vision. So we see the end of horogeny,
some heat input and granitic plutonism and volcanism.
And then we see passive margin, clearly passive margin sediments
of earlier division age. But all these are indirect
inferences. So moving the clock forward a
little bit, then we come to the Veriskan Horogeny, right?
So if we move onwards to early Polozoic, the key geodynamic
process at the time was continental drift of Gondwana to
the north together with the former Cadomian terrains at this
margin. So Gondwana and anything which
was north of it. So all these terrains formed
previously during Cadomian origini moved N towards
Laurentia, Baltica and Avalonia which were assembled during the
Cadogian origini to form La Russia.
So now we have two major converging mega continents,
Gunvana and La Russia moving towards each other and closing
the rake ocean in between and other smaller oceanic basins.
As we discussed before, it's not entirely clear how many
continental fragments or microplates and how many oceanic
basins were involved in the various scan origin and in the
various scan collision. And it's another a matter of
debate. So it has been widely debated
for decades. Nevertheless, I would emphasize
that this process was significantly different from
Cadomia and origini because it was a truly collisional origini.
So we had a two major continents on each side of an ocean and
after closure of the ocean the continents collided and this
continental collision culminated during Devonian to Carboniferous
and resulted in the formation of the Variscanorogenic belt and in
turn in the formation of supercontinent Pangaea.
So what is now in Central Europe was right in the heart of the
Variscan belt and in the heart of Pangaea.
And at the time, Central Europe was positioned somewhere near
the equator because we have carbon ferrous coal basins which
overlie the eroded Variscan basement.
So you said that the Variscan then was really a kind of head
on collision of two continents, whereas the the Cadomian you say
was more accretionary, there wasn't actually the same kind of
perpendicular motion if you like between the plates that created
a Himalayan scale type mountain range as the Variscan might have
done at the time. Yes, exactly.
That's why we call it accretionary origin, because
there was probably a large oceanic plate subducting beneath
Gunvana, but there was no major continent on the other side.
So we have protracted oceanic subduction lasting for 10s or
maybe even more than 100 millions of years.
However, it should be noted that oceanic plates are never smooth.
So the oceanic plates which subducts carriers, for example
seamounts, it can carry oceanic plateaus, it can carry intra
oceanic volcanic arcs or sedimentary basins and all this
material is very difficult to subduct.
So it accretes, it makes the oceanic plate thicker and it's
buoyant. So it's difficult to subduct.
So it rather accretes to the overriding continental margin,
and this is exactly what we think might have happened.
During paduma and orogeny, in contrast, the Variscan orogenic
Lee levels collision orogeny. So everything we know from other
collision orogenic belts like the Alps and Himalayas.
So closure of an ocean, first high pressure metamorphism
creating high topography, high orogenic topography, maybe even
an orogenic plateau like in the present day Tibet up to final
collapse, erosion and exhumation of high grade metamorphic
complexes. So everything of this we have
recorded here in Central Europe. So and that's why the Vorisic
neurogenes sort of world classic area for orogenic research.
Yeah. So the best evidence for the
vorisic neurogeny is in Central or Eastern Europe, is that what
you're saying? I wouldn't say the best, but we
have very well preserved all the Eurogenic zones.
So if you would go across on collisional origin, you would
start in a four land and you would go to the sort of central
axial zone which was uplifted and then there is of course the
deepest erosion and we are very well preserved this origenic
architecture. So if you would go from north to
South across the various kanorogenic belt, you would
start in four land basins, very low grade thick successions of
silicyclastic sediments. Then you would enter a low grade
metamorphic complexes and then if you would be in deep and you
can step into the orogenic interior which is now exhumed
originally very deep rocks which formed the thickened orogenic
root with granulites and migmatites and other high grade
rocks. And all this is very well
exposed and very well preserved here in Bohemian Massif.
So that's and in Central Europe. So that's why it has been
studied for decades. So if you go from north to
South, you say you go through all these regions.
What kind of distance are we talking about?
If you were going to do your transect, do a trek from top to
bottom, where would you start and where would you finish?
You would start somewhere in very South of Poland and then
end up in Austria, northern Austria, and it's about, let's
say 300 kilometers. But then should be noted that
it's not the end of the origin. So it's the end of the exposure
of the origin, because the various can origin continues
underneath the Molas basin of the Alps and goes farther South
and of course continues into the Alps.
But there it was revoked during Alpine origin, whereas to the
north of the Alps the Alpine overprint was only localized and
it was more reactivation not affected by the Alpine
neurogeny. But the Alpine neurogeny was a
bit further West. So you say if you start in
Poland and you go down towards Austria, you said the Veriska
neurogeny extends to where the Alpine neurogeny is today.
So it actually extended further W as well as north to South.
Yeah, you are right. You know, if he would look at
the, let's say geologic map of the world in Carboniferous, the
Veriskan origin was rather extensive.
So the length could have been like 8 to 10,000 kilometers,
maybe even more. So it was a really extensive
origenic belt. So we found a remnants or pieces
almost everywhere. But I'm now talking about those
parts or fragments which which are exposed and which escaped
the Alpine over print. So you can see them emerging
from below, from, you know, largely covered in much of
Western Europe by younger Mesozoic and Tertiary sediments.
So you have several places, several, we call it massives,
which project upward from younger sediments and that's
where we can study the various kind of region at best.
Is it the case then that even though we have a Himalayan scale
mountain range at the end of the Variscan that none of the
topography that we see today either in Central Europe going
down from Poland to Austria or indeed in the periphery of the
Alps, none of that is actually as old as the Variscan?
That's all much more recently created.
Is that right? Yeah, that's right.
Nothing left from that because it has been about 300,000,000
years since the end of the various canerogenes.
So and even on geological time scale, this is a very long time
span, so long enough for erosion to completely vipe out the
original topography. So much of the present day
topography and much of the present day relief which we see
here in Central Europe is due to reactivation from Alpine
originally because we are Alpine fall and which is still ongoing.
So we see here several phases of compressional reactivation doing
credentials and Sinosaic. And also we had here an episode
of Tertiary rifting which led to the thinning of the lithosphere,
some volcanism and we have some rift related cold basins.
So that's the most recent relief.
Nothing left from the Variscan Originae, of course.
It's too long time. But the reactivation took place
along faults that may have been active during the variscan
because there were I guess zones of weakness.
Yes, exactly. Especially at the end of
Variscan Originae and there was a major phase of strike slip
faulting and some of these faults run for tents or even
several hundreds of kilometers and they penetrate through the
whole crust. So these are large features
which were there before the Alpine origin and they are easy
to reactivate. So compressional stress from the
Alpine origin reactivated these faults quite significantly in
some places. And we have a lot of evidence,
efficient track dating that there was also significant block
uplifts during Jurassic, Cretaceous and Tertiary.
So there's plenty of evidence for movements in the Central
Europe. But all this was along these
brittle falls. There was no metamorphism, no
magmatism and no ductile pervasive deformation related to
the Alpine origin. So that's why we have very well
preserved the older Origenic cycle, the various.
Oh, I see. So the Alpine and Erogeny,
although it completely shifted things around and there's a lot
of structural change, there wasn't so much metamorphic
change. Not at all actually, because the
metamorphism happened in the Alps, but we are too far from
the origin. But still the compression we we
felt it. So there was some movement, but
these were localized along, as you pointed out, along zones of
weakness which were pre-existing faults.
What are you working on at the moment?
With my colleagues, we are working on 2 main projects.
One is focused on what we're talking about a bit and it's
reconstructing of terrain provenance, which formed at the
northern margin of Ganwana using the tritozochondrial chronology.
And we have now new project which started this year, which
is focused on reconstruction of our key and supercontinent.
So the field work will be in Canada and Wyoming and Finland.
So 2 very different time periods.
Very different time periods and very different field areas, but
we enjoy both. Both are very interesting
problems to solve. Yuri Jacques, thank you very
much. Thank you very much.
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