Stage 14: Tectonic training camp

For today’s stage, Earth went on a tectonic training camp to work on the strong forces needed to create those Pyrenean climbs. You know by now that mountains are made by Earth’s tectonic plates pulling apart. They are colliding and sliding past each other, propelled by forces acting within and beneath the plates. The fabled climbs over the magnificent Pyrenees facing the riders this week, and especially today, are the product of these mountain building forces acting on southern France over hundreds of millions of years!

Twists and turns

The Pyrenees are predominantly a contractional mountain range. This is when Earth’s uppermost mechanical layer, the lithosphere, is shortened. In the Pyrenees the most recent mountain building episode, or orogeny, took place 55 million years ago. The Pyrenean Orogeny created the Pyrenees. It’s a good name.  However, older mountain-building events made the rocks of southern France ready to bend, slide, and break in specific patterns when space got tight and stress got high. It’s all related. The climbs the riders face today would not have been there without the geology.

tectonic training camp
A rider amongst the granite peaks in the Axial Zone of the Pyrenees on the way to Col du Tourmalet. Photo credit: The Col Collective.

Our stage is 152 kilometers long. Or short. It is estimated that this is the same distance Iberia and France moved towards each other to make the Pyrenees. How exactly did this shortening happen? Like the race itself, it is a tale with many twists and turns, upsets, and victorious pushes. Let’s take a look at the tectonic training camp!

Fly away, Iberia

Iberia was priorly attached to France (Armorica) back when the supercontinent Pangea was all the rage at the end of the Permian (~250 million years ago). Like all trends, Pangea eventually fell out of fashion, and by the Jurassic period ~155 million years ago, the continents were well on their way to new places. Time to say goodbye.

At this time, Iberia was located farther north and west of its present day position. The northeastern part of Iberia, now in the eastern Pyrenees, was located near Biarritz and the northwestern top of Iberia was located south of Ireland.

Position of modern-day nations within Pangea, 300 million years ago. Source: Massimo Pietrobon.

So you can see it coming. Iberia was located right where the Bay of Biscay is today, and was occupying half of the young Atlantic Ocean. The other half of the Atlantic ocean didn’t exist yet. Portugal was located against Newfoundland (eastern Canada). Riding a bicycle from Pamplona to Madrid would take you to the west rather than to the south as you go today. Fun fact: right to the east of Iberia was Adria. It is the previously “lost” continent that is now below the Adriatic Sea, northern Italy, and the Dolomites. Read about that in our Giro d’Italia blog. The city of Pau in southern France was right next to…the Po Plain in Italy! 

Tectonic training

The tectonic plates went on quite a training camp. North America, Iberia, Europe, and Africa separated. The North Atlantic and the Bay of Biscay opened in the west and north of Iberia. Then another ocean that we call the Alpine Tethys formed between Iberia and Adria in the east. Are you still with us? Iberia broke away from France, but the geometry of this break (what geologists call a rift) wasn’t ideal in the grand scheme of how the North Atlantic was opening.  

At top, paleogeographic maps drawn at 121 million years ago (at left) and 79 million years ago (at right). Green indicates oceanic domains, Iberia is in yellow. (via Sciencedirect)

So, change of plans. A new rift formed west of Iberia that would become the northern Atlantic Ocean, stranding Iberia offshore of western France. As the Atlantic continued to open, Iberia moved to the south and east together with Africa. Around 120 million years ago in the Cretaceous period, Iberia suddenly rotated counterclockwise over 40° and the northern Iberian tip came close to its modern position adjacent to southern France. It almost moved towards North America but then, luckily for us, did not.

But, like the finish line, we’re not quite there yet in the formation of the modern Pyrenees. Africa headed south and being a good teammate Iberia followed suit. Together they formed a big basin where the Pyrenees are today. This was between ~110 and 90 million years ago. Some ninety million years ago, Africa and Iberia started to move together again, and after a long circuitous journey Iberia began the final push into southern France.

Hello Pyrenees

As Iberia collided into what is now southern France, it used the east-west trending faults that formed previously in the ‘Variscan’ mountain building phase ~350 million years ago during Pangea days. We mentioned before, the Pyrenean Orogeny continued on older events. Iberia was pushed below France, the top of the continent was scraped off and piled up. These scraped off rocks form the ‘Axial Zone’ and the southern Pyrenees (see map).

Simplified geologic map of the Pyrenees, showing major zones and the orientation and ages of the rocks. NPF = North Pyrenean Fault. (Adapted from Vissers and Meijer, Earth Science Reviews, 2012).

The southern (and northern) Pyrenees display an assemblage of marine and terrestrial sedimentary rocks that formed on Iberia in the last 200 million years. Keep an eye out for layers of buff colored sandstones and often fossil-rich limestones on the way into the mountains from Pau. You can also observe exhumed mantle rocks (peridotite) from the failed rift basin.

Example exhumed mantle rock in the Northern Pyrenees, the remains of the failed rift. (From Satterfield, Rollinson and Suthren. Geology Today, 2019).

Mountains and more mountains

The Axial Zone shows the rocks that were located below these sedimentary rocks in the “basement”. They are older and come from the ‘Variscan’ mountain building phase. The granites and gneisses forming the glittering gray massifs on the route today formed prior and during the collision. They melted and deformed under intense heat and pressure as the continents rammed together. It might almost be the same amount of watts a sprinter needs to get over the Tourmalet.

This whole stack of rocks was pushed northward over southern France. The sedimentary rocks on the southern margin of France were bulldozed up along ancient faults that are much steeper than on the southern side of the Pyrenees. An example is the North Pyrenean Fault. This in part explains why the steeper and higher climbs of the Tour are located in the northern Pyrenees. Thus, the distinctive topographic grain of the northern Pyrenees is the product of many mountain building events. It was a tectonic training camp of the ages.

One last metaphor

Similar to the slivers of crust along the southern margin of France, the riders will start like Pangea. They are a coherent bunch moving together as a supercontinent peloton. As the race progresses, riders will begin sliding past each other as they work their way up into the mountains. You will see a breakaway, and a gap may open up. Just like Iberia, the gap may close. When you think it’s finally over and everything is in its right place, an attacking climber vying for a stage victory might open a new gap quickly and form yet another big hole in the lead group.

The strong riders on either side of the gap are the basement uplifts, getting higher, but the basin between will fill quickly with loosely packed and excited spectators. The bigger question is, will an attack and victory today give shape to the overall structure and outcome of the general classification when the riders reach Nice? We’ll have to be patient and observe the race on small and large scales to see how the details fit together. And at the end of the day, all this activity got us 150 km closer to Nice!

NB: Blogs in other languages than English are all auto-translated. Our writers are not responsible for any language and spelling mistakes.





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