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186.5 km |
Mountains |
France’s youngest mountain range |
After the crystalline massif and the ancient volcanoes of the Vosges, the peloton will cross the youngest mountain belt of France today: the Jura Mountains. In the Jura, the peloton will be surrounded by, below the lush vegetation, white rocks: limestones. These limestones were deposited in a shallow sea, just like in the Paris Basin in the northwest and in the Aquitaine Basin in the southwest of the country. And a considerable part of these limestones date from…the Jurassic, which was named after the Jura. The Jurassic formed the heydays of the ammonites, between 201 and 145 million years ago.
An isolated mountain range?
But why are these Jurassic rocks visible at the surface? We are close to the Alps, and the foreland of the Alps are covered by a thick pile of debris from the mountains. Why not the Jura? The answer lies in the subsurface. The Jura is half-moon shaped and lies parallel to the northwestern Alps, but is separated from the Alps by a wide vallei in Switzerland that hosts the Geneva Lake and the Neuchâtel Lake. In this valley there is a lot of debris from the - this formation is known as the ‘Molasse’ that was mostly deposited since about 25 million years ago. And this Molasse has also covered the sediments of the Jura mountains, but between 20 and 7 million years ago, the sequence of limestones and overlying sediments was squeezed, shortened, folded, and thrusted. Just like that?
The little, younger brother of the Alps
No, not just like that. The Alps foldbelt formed because Europe was diving below the African plate - we will explain more about that later during the Tour. In the process, the rock sequences of the European plate were offscraped and piles up, just like in de Ardennes 250 million years prior, and this process formed the Alps. The northern and western rocks in the Alps were the youngest to be added, to the bottom of the pile (foldbelts grow at the base). In the northwest, the sediment sequences of Europe were deposited on a thick pile of ‘evaporites’ (rock salt and gypsum that formed by evaporation of seawater) from the Triassic. These evaporites formed about 220 million years ago in a shallow sea under the desert climates in central Pangea. Rock salt and gypsum is very weak. So when the sequence of limestones that were resting on the gypsum started to be pushed below the Alps, the evaporites started to slide, like a banana skin. The limestones above the evaporites became part of the Alps, whereas the crust below the evaporites stated part of Europe and moved below the Alps. This process occurred below the valley between the Jura and Alps. But in the northwest, where the gypsum layer was thinner or stronger, the limestones did not slide any further, but were shortened, folded, and thrusted: the Jura mountains. In total, the Jura mountains were shortened by about 30 km and this amount decreases to zero to the northern and southern tips.
Modeling in the laboratory
To illustrate how this process of ‘detachment’ on a weak horizon works, and how one process can produce two mountain belts, we modeled the process in the ‘Tectonics’ laboratory in Utrecht (the TecLab) - especially for GeoTdF! In the movie below you can see how layer by layer a model was built that represents the limestones and the other sediments of the Jura. And how the shortening of this sequence can make two foldbelts separated by a valley. The non-climbers of the peloton would probably have preferred to do today’s stage also in our sand-box model!
I am a geophysics student who likes to be outside and to be physically active. I aim to use knowledge of geophysical tools to investigate shallow-subsurface structures, especially those with immediate societal or technological relevance. In the Tectonics Laboratory, we study plate tectonic processes such as mountain building or earthquakes. Check the Geo-TdF team.
Sjaak van Meulebrouck
As geologist I investigate processes that lead to deformation of the Earth’s crust and lithosphere leading to the formation of mountain belts or sedimentary basins. I do that by describing geological structures through field observations and by explaining these observations through building and running physical scale models.
Ernst Willingshofer
I am a geologist and I study plate tectonics and the driving mechanisms in the Earth’s mantle, mountain building processes, and the geography of the geological past. I enjoy geological fieldworks all over the world, and translating the results to science and a broad public.
Douwe van Hinsbergen