Stage 12 | Briançon – Alpe d’Huez / Bending, or failure and breaking on Alpe d’Huez

Today is the last stage through the Alps, and promises fireworks on the flanks of the Galibier, the Croix de Fer, and Alpe d’Huez. The riders will start the ride in the nappes of the Alps that were derived from the Valais Ocean and Briançonnais microcontinent (see stage 9 and stage 11) and will ride through the deformed margin of Europe that contains de Croix de Fer and Alpe d’Huez. Throughout the stage, the peloton will cross rocks that have undergone the two different mechanisms in which rocks and rock packages can deform, on millimeter-scale, or on scale of entire mountain belts: rock packages along the route are in places broken – for instance where ‘nappes’ were pushed (thrust-faulted) over each other (see stage 9), or where crust is pulled apart and crustal blocks slide off each other. This kind of behavior is known as brittle deformation and occurs when the stress on a rock overcomes its strength: the rock will fail and break. When rocks fail, break, and suddenly slide, this causes earthquakes.

On the other hand, rocks may fold, and make for pretty wrinkles that can be centimeter-scale, or kilometers wide. This is known as ductile deformation and occurs when the stress of a rock makes it flow. Ductile deformation is gradual, does not produce earthquakes, and is the common mechanism of deformation in the Earth’s mantle. But at the surface, depending on the properties of the rock and the rate and amount of displacement, rock packages may experience both types of deformation. And this is what happened along the route of today’s stage.

Galibier – an enormous fold above a fault

The Galibier is a mountain that is part of the Briançonnais nappe – the thick rock slice that was derived from the microcontinent (see stage 9) and that was buried to great depth (stage 11). This nappe is separated by faults from underlying and overlying units, but internally, it has been folded in enormous, flat-lying folds. In the sketch in the Figure, you can see where the Col de Galibier is located in major, flat-lying folds. The Alps are known for such enormous structures and these illustrate the enormity of wreckage that occurs when continents collides. Deformation like this requires that the km-thick rock slices behave like chewing gum accommodating tens of kilometers of deformation.

Folding on Alpe d’Huez

The famous Alpe d’Huez is probably the best-known col in the Tour de France, and one of the most heroic climbs of the race. The image of Alpe d’Huez that is shown here, you may not have seen before. French geologist Thierry Dumont and his colleagues showed that Alpe d’Huez is part of a major fold that is larger than the entire mountain. Rock layers of Jurassic (Liassic) age similar to the ones that we saw in the Jura Mountains, are folded to vertical along most of the route. And this fold is not a simple one: the detailed work of the French geologists has shown that folding occurred in at least four different phases and directions. The rocks look like a towel that you threw in the corner after using it. On the scale of the entire Alps, the fold of Alpe d’Huez looks like no more than a detail. But it is a detail that the fastest climber on the flanks of Alpe d’Huez (Marco Pantani) took almost 37 minutes to climb up on. And he was only halfway the fold! Let’s see what todays climb will bring us: who will fold, who will fail, and who will bend without breaking?

I am a geoscientist specializing in paleomagnetism, geochronology, and stratigraphy. I develop palaeogeographic, paleoenvironmental, and paleolandscape reconstructions. My current projects focus on: - ocean and marine connectivity, the evolution of ocean passages and sea-straits; - connectivity-driven environmental changes, in deep oceans and epicontinental seas; - landlocked basins - paleoenvironmental evolution and reconnections with the global ocean.

Dan Palcu

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