Stage 9 | Aigle - Châtel / Microcontinent Island Hopping by Bike

Today’s stage from Aigle to Châtel les Portes du Soleil takes the riders through many of the main geological units of the Western Alps, but the road will not be easy for either the riders or the geocurious. Expect a mix-up, with the riders testing their legs on the biggest climbing day so far in this year’s Tour, and with slivers of continents and microcontinents, continental margins, and deep oceans appearing and reappearing in anything but an orderly fashion. The riders will spend most of the day riding uphill into deep ocean basins and will come up for air only a few times on rocks from ancient shallow seas or isolated pieces of continental crust.

Microcontinent - The simple story

If we restore the last 50 million years of deformation in this region of the Alps, then we find a fairly simple geography that includes two continents (Europe & Africa (Greater Adria)), two marine continental margins, two oceans, and a microcontinent (Briançonnais) in the middle of it all. The subduction of the European Plate beneath the African Plate that led to the collision of these two continents brought all of the intervening units together, deformed them into spectacular sheets (nappes) of fault-bounded folded rock, and moved them into new configurations and locations. If this deformation was straight-forward, like a scrunched up sleeve of a shirt described in Stage 6, then it would be much easier to piece together these units: starting in the north and moving south, we’d encounter

• A. rocks of the European continent, including younger sedimentary rocks comprising the eroded and transported sand and cobbles of the Alps (the Molasse described in yesterday’s post, as well as moraines and outwash from once far-more extensive Alpine glaciers and ice sheets);
• B. marine sediments deposited on the southern continental margin of Europe (referred to as the Helvetic nappes);
• C. deep marine sediments and ocean crust of the ancient Valais Ocean (referred to as the Lower Penninic nappes);
• D. crystalline rocks of the crust of the Briançonnais microcontinent (referred to as the Middle Penninic nappes);
• E. deep marine sediments and oceanic crust and uppermost mantle of the Alpine Tethys Ocean (see Stage 11 for more information about these rocks, link) (referred to as the Upper Penninic nappes); and finally
• F. rocks comprising the northern margin of Greater Adria.

These units and their geography are shown in the cartoon below.

The story is rarely simple

In today’s stage, the riders visit all of the units described above, except for rocks from the African plate that lie in the southern Alps and Dolomites. But as the Tour travels south from Bülle to Aigle and up to Châtel, the riders don’t simply ride from Unit A to B to C to D and finish on E. Instead, they will pilot their bikes from Unit A to B to E to B to D to E to C to D to B to A to B and finally to Unit D. They’ll spend much of the day riding through deep, ancient oceans and only occasionally come up for air on islands of dismembered microcontinent. Will the neat and tidy order of the GC from the first week of racing also get mixed up by this mélange of continental collision?

Why so complicated?

How is it that these tectonic units are so mixed up and so seemingly disordered? When trying to reconstruct the structure and history of the Alps (and most ancient and modern mountain belts), it’s important to remember that erosion by rivers and glaciers has removed much of the record of Alpine deformation, and that many key features are hidden beneath the surface. Geological reconstructions of mountain belts are guided by geometrical rules and our understanding of rocks’ material properties during deformation, as well as observations made and understanding gained from studying other mountain belts. Analog models of thrust belts and rock deformation (like those described in Stage 8, as well as computer models and subsurface imaging by geophysical techniques help us test and refine these rules and interpretations. Remember from the description of Stage 8 that nappes and fold belts typically grow at their base, such that material accreted to a plate early in a continental collision (such as the Units C, D, and E) can be far transported, refolded, and even dismembered by units, folds, and faults added to the plate more recently, and closer to the plate (such as Units A and B). Erosion often removes the continuous trace of units or faults on the surface and can even create ‘islands’ (klippe) of exotic rocks at the tops of mountains and ‘windows’ through one nappe into another. In this way, a once continuous nappe of microcontinental crust appears on the surface as isolated slivers of rock amongst a sea of earlier and later accreted oceanic sediments. With all of this riding across oceans, perhaps Le Tour should add a pedal boat stage!

I am an Earth Scientist who harnesses the unique information encoded in the magnetic properties of geological materials to study tectonic, climate, ecological, and environmental processes.

Pete Lippert