Formation of the Alps: Detaching and uplifting, not bulldozing

Date:
September 17, 2020
Source:
ETH Zurich
Summary:
Researchers have used a computer model to test a new hypothesis
about the formation of the Alps while simulating seismic activity
in Switzerland.

This will help improve current earthquake risk models.



FULL STORY ==========================================================================
For a long time, geoscientists have assumed that the Alps were formed
when the Adriatic plate from the south collided with the Eurasian plate
in the north.

According to the textbooks, the Adriatic plate behaved like a bulldozer, thrusting rock material up in front of it into piles that formed the
mountains.

Supposedly, their weight subsequently pushed the underlying continental
plate downwards, resulting in the formation of a sedimentary basin in the
north adjacent to the mountains -- the Swiss Molasse Plateau. Over time,
while the mountains grew higher the basin floor sank deeper and deeper
with the rest of the plate.


==========================================================================
A few years ago, however, new geophysical and geological data led ETH geophysicist Edi Kissling and Fritz Schlunegger, a sediment specialist
from the University of Bern, to express doubts about this theory. In
light of the new information, the researchers postulated an alternative mechanism for the formation of the Alps.

Altitude of the Alps has barely changed Kissling and Schlunegger pointed
out that the topography and altitude of the Alps have barely changed over
the past 30 million years, and yet the trench at the site of the Swiss
Plateau has continued to sink and the basin extended further north. This
leads the researchers to believe that the formation of the Central Alps
and the sinking of the trench are not connected as previously assumed.

They argue that if the Alps and the trench indeed had formed from the
impact of two plates pressing together, there would be clear indications
that the Alps were steadily growing. That's because, based on the earlier understanding of how the Alps formed, the collision of the plates,
the formation of the trench and the height of the mountain range are
all linked.

Furthermore, seismicity observed during the past 40 years within the Swiss
Alps and their northern foreland clearly documents extension across the mountain ranges rather than the compression expected for the bulldozing
Adria model.



==========================================================================
The behaviour of the Eurasian plate provides a possible new
explanation. Since about 60 Ma ago, the former oceanic part of the
Eurasian plate sinks beneath the continental Adriatic microplate in the
south. By about 30 Ma ago, this process of subduction is so far advanced
that all oceanic lithosphere has been consumed and the continental part
of the Eurasian plate enters the subduction zone.

This denotes the begin of the so-called continent-continent collision with
the Adriatic microplate and the European upper, lighter crust separates
from the heavier, underlying lithospheric mantle. Because it weighs less,
the Earth's crust surges upwards, literally creating the Alps for the
first time around 30 Ma ago. While this is happening, the lithospheric
mantle sinks further into the Earth's mantle, thus pulling the adjacent
part of the plate downwards.

This theory is plausible because the Alps are mainly made up of gneiss and granite and their sedimentary cover rocks like limestone. These crustal
rocks are significantly lighter than the Earth's mantle -- into which
the lower layer of the plate, the lithospheric mantle, plunges after the detachment of the two layers that form the continental plate. "In turn,
this creates strong upward forces that lift the Alps out of the ground," Kissling explains. "It was these upward forces that caused the Alps to
form, not the bulldozer effect as a result of two continental plates colliding," he says.

New model confirms lift hypothesis To investigate the lift hypothesis,
Luca Dal Zilio, former doctoral student in ETH geophysics professor Taras Gerya's group, has now teamed up with Kissling and other ETH researchers
to develop a new model. Dal Zilio simulated the subduction zone under
the Alps: the plate tectonic processes, which took place over millions
of years, and the associated earthquakes.



==========================================================================
"The big challenge with this model was bridging the time scales. It takes
into account lightning-fast shifts that manifest themselves in the form
of earthquakes, as well as deformations of the crust and lithospheric
mantle over thousands of years," says Dal Zilio, lead author of the
study recently published in the journal Geophysical Research Letters.

According to Kissling, the model is an excellent way to simulate the
uplifting processes that he and his colleague are postulating. "Our
model is dynamic, which gives it a huge advantage," he says, explaining
that previous models took a rather rigid or mechanical approach that did
not take into account changes in plate behaviour. "All of our previous observations agree with this model," he says.

The model is based on physical laws. For instance, the Eurasian plate
would appear to subduct southwards. In contrast to the normal model of subduction, however, it doesn't actually move in this direction because
the position of the continent remains stable. This forces the subducting lithosphere to retreat northwards, causing the Eurasian plate to exert
a suction effect on the relatively small Adriatic plate.

Kissling likens the action to a sinking ship. The resulting suction effect
is very strong, he explains. Strong enough to draw in the smaller Adriatic microplate so that it collides with the crust of the Eurasian plate. "So,
the mechanism that sets the plates in motion is not in fact a pushing
effect but a pulling one," he says, concluding that the driving force
behind it is simply the pull of gravity on the subducting plate.

Rethinking seismicity In addition, the model simulates the occurrence of earthquakes, or seismicity, in the Central Alps, the Swiss Plateau and
below the Po Valley. "Our model is the first earthquake simulator for
the Swiss Central Alps," says Dal Zilio. The advantage of this earthquake simulator is that it covers a very long period of time, meaning that it
can also simulate very strong earthquakes that occur extremely rarely.

"Current seismic models are based on statistics," Dal Zilio says,
"whereas our model uses geophysical laws and therefore also takes
into account earthquakes that occur only once every few hundreds
of years." Current earthquake statistics tend to underestimate such earthquakes. The new simulations therefore improve the assessment of
earthquake risk in Switzerland.


========================================================================== Story Source: Materials provided by ETH_Zurich. Original written by
Peter Ru"egg. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Luca Dal Zilio, Edi Kissling, Taras Gerya, Ylona Dinther. Slab
Rollback
Orogeny Model: A Test of Concept. Geophysical Research Letters,
2020; 47 (18) DOI: 10.1029/2020GL089917 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200917105413.htm

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