sciences de la vie et de la Terre

Présented by Chloé ANFRAY, Ronan CALVEZ, Guillaume DEFOIX, Vincent LE GALL, Romain NEUVILLE, Clémence PAGNOUX and  Victor PRAUD.

Teachers : François CORDELLIER, Biology and geology, Annick LANGLAIS, Mathematics, Bertrand MABILLAIS, Physics and chemistry.
With help of Mélanie GUILLET (English teacher) for translation.

How can we represent and simulate the European lithosphere’s deformations after measuring them with computed GPS data ? French version

Project origin :

Summary :

Data and proceedings
Latitudinal and longitudinal absolute velocities
European plate horizontal internal deformations
Lithospheric vertical movements
Modelling post-glacial rising
Modelling alpine orogenesis

Data and proceedings

The GPS

The GPS system (Global Positioning System) is used to determinate the position of a point with a high precision. Receivers from the station or mobile compute the position using a satellite constellation running around the whole Earth. The satellites broadcast a radio signal to the receivers. With sophisticated receivers as shown opposite, millimetric precision can be reached. All year long recording of the position of the lithosphere fixed receivers makes it possible to estimate the plate's velocities with a good precision. Opposite : high precision fixed receiver in northern Europe

With agrement of EUREF : http://epncb.oma.be

Velocities are calculated following tree directions : latitude (Vel lat), longitude (Vel long) and altitude (Vel alt). The latitude, longitude and altitude positions of the receiver are drawn according to time as shown opposite. The three velocities correspond to the rates of the regression lines. The NASA gives graphic and digital data on its web site. http://sideshow.jpl.nasa.gov/mbh/series.html For example the METS receiver, placed in Helsinki (Finland), moved 12.07 mm/y toward the North , 20.01 mm/y toward the East and rose of 4.56 mm/y in the past ten years.

Data proceeding

We downloaded data from the NASA web site : http://sideshow.jpl.nasa.gov/mbh/series.html They are in text format. We had to copy them in a spreadsheet in order to sort and select them. Relative speeds can also be computed on this spreadsheet.

We selected the European receiver’s lines and drew the velocities as vectors by adding the latitudinal velocity vector to the longitudinal velocity vector for each measurement point. Here is the velocity vector of the SFER receiver (In San Fernando in Southern Spain). We used Open Office 1.0.1 as computer drawing application. The results are presented on a map of Europe downloaded from the EUREF Web site : : http://www.epncb.oma.be/_networkdata/stationmaps.php

Different maps were drawn in the same way : Horizontal velocity, horizontal relative velocity and vertical velocity.

The european lithospher horizontal movements

This map shows the velocity of each receiver in Europe. We notice that the European plate moves roughly toward the North -East 6 or 7 centimetres per year. However the receiver in Ankara moves toward the North because it is fixed to the Anatolian plate and not to the European one. Comparing the velocities of the different parts of the European plate raises a question. Receivers are fixed to a rigid lithospheric plate. Why don’t they move exactly with the same velocities and directions ? Are there any internal deformations of the European plate ? With autorisation of the EUREF website : http://epncb.oma.be

To answer this question we have to calculate the velocity of each point compared with a fixed one.

The european plate internal deformations

Using the excel spreadsheet, we can calculate the relative velocities of each point compared to another one of the European plate which is now considered as fixed. Latitudinal and longitudinal relative velocities are given by subtractions following this rule :

Relative lat. vel. = studied point absolute lat. vel. - fixed point absolute lat. vel.
Relative long. vel = studied point absolute long. vel. - fixed point absolute long. Vel.

Relative velocities vectors are drawn by composition of latitudinal and longitudinal relative velocities vectors.

For this first map the fixed point was chosen in Trömsø in northern Norway. All the stations in the North western part of Europe show no significant movements relatively to Trömsø, but some of the South eastern part of Europe (i.e Turkey, Bulgaria, Romania and Greece) slips toward the North West . So, central Europe is compressed between this moving block and the stable North western Europe. The most stressed part seems to be the eastern Alps.

For this map the fixed point was San Fernando in southern Spain. The same phenomenon can be found in South eastern Europe and the Alps

The fixed point for this map is the NPDL receiver in the town of Teddington (UK) Comparison between the three maps shows the same global patterns for relative speed vectors. The European plate is compressed in the neighbourhood of the Alps. These mountains look like a collapsed structure between two parts of the European plate.

Lithospheric rocks are rigid. We must look for movements of vertical deformations to understand the lithospheric plate shortening mechanism.

The european plate vertical movements

We worked only on the vertical velocities (vel Alt) of the GPS stations For example the MTES receiver in Helsinki rose of 4,56 mm/y in the last ten years.

On the European map, white arrows correspond to subsiding movements and black arrows to rising movements.

	Subsiding and rising movements are both present in Europe with very different values.
	To emphasize global trends, rising zones (in red) and subsiding zones (in blue) are drawn on the map. Scandinavian countries, North eastern Europe and the Alps globally rise while the Italian Piedmont, western Mediterranea and a narrow zone in the Balkans and in South Germany subside.

Some bibliographical researches provide us with hypothese about the rising and subsiding mechanisms. For Scandinavia, post-glacial rebound could be pleaded and orogenic movements could explain the Alps rising. A lot of hypothese could explain subsiding movements such as rifting, sediment accumulation, water pumping, etc.

Post glacial rebound

The measured rising movements in Scandinavia can be more than 6 mm/y. It can result of artic Iceland melting at the end of the Würm glacial time. Excess of weight due to ice disappeared at that time and the lithosphere began to rise following Archimedes’ principle. This phenomenon is called “isostatic rising”. The very high asthenospheric mantle viscosity makes it very slow and this isostatic rising is still going on nowadays.
Analogical modellisation is possible if we use a high viscosity fluid to simulate the ductile asthenospheric mantle. We operate with a wood block floating on wallpaper paste. First the woodblock is sunk in the paste with the hand. It looks like the effect of iceland weight on the lithosphere. Then the block is slackened simulating ice melting. The block rises very slowly and this rising lasts for a long time after slackening.

see the video

Rising was followed by digital recording and a picture was extracted every 15 seconds.

	At t = 0 the wood block is slackened. The GPS is symbolised by a bullet on three legs. Relative altitude(A) is 0 mm
	t = 15 s A = 3 mm
	t = 30 s A = 8 mm
	t = 45 s A= 10 mm
	t = 60 s A = 12 mm
	t = 75 s A = 15 mm
	t = 90 s A = 19 mm
	t = 105 s A = 22 mm
	t = 120 s A = 25 mm
	t = 135 s A = 30 mm
	t = 150 s A = 32 mm
	t = 165 s A = 35 mm
	t = 180 s A = 37 mm

Altitude according to time can be measured on the scale of the block side.

Drawing the curve of altitude according to time shows that the function is affin. When approaching equilibrium the rising slows down and stops. As Scandinavia is still rising we concluded that equilibrium has not yet been reached.

This isostatic phenomenon can also explain why central European plains and the Italian Piedmont subside under sediment’s weight. But we can’t use it to explain why the Alps rise strongly in their central part.

Modelling of the alps orogenesis

Nowadays the central Alps rise along the border of two old tectonic plates. The alpine ocean disappeared and the two continental crusts are now joined but collapsing movements from South East to North West are in progress. They could be responsible of the mountain rising

	Layers of colored plaster are set in a transparent box with a piston on one side to represent a part of crust with sedimentary rocks.
	Pushing the piston from right to left deforms the layers. It simulates the movement of South eastern Europe toward North West.
	Rising and shortening are observed. Inverse faults and folds appear. This experiment confirms our hypothesis as faults and folds are really observed in the Alps.
	The shortening increases the crustal thickness and the GPS receivers fixed to the crust record a rising movement.

In spite of this good result our simulation is not perfect because it cannot simulate at the same time the isostatic adjustment due to the thickening of the crust.