Stage 19 | Castelnau-Magnoac – Cahors / The Aquitaine Basin and the Wilson Cycle

With the Pyrenees left behind, today’s stage will lead the peloton through the rolling hills of southern France. This region is known as the Aquitaine Basin and is today drained by the river Garonne and its tributaries. The landscape of southern France may be gentle, but the topography in the subsurface is much more spectacular.

Aquitaine Basin: as deep as the Marianas Trough

Along the northern perimeter of the Aquitaine basin are coral reefs from the tropical seas of the Jurassic, approximately 180 million years old, that overlie the crystalline massifs of the Central Massif and the Armorican Massif. But in the deepest point of the Aquitaine Basin, just north of the Pyrenees, that transition from crystalline massif to the Jurassic limestones, or Triassic sediments that lie underneath, is at a depth of 11 km. And at the top of the Pyrenees, we find those same crystalline rocks at an elevation of 3 km: a vertical difference of 14 km. For comparison: The Aquitaine Basin at its deepest point is as deep as the Mariana Trough, the deepest point of the Earth’s surface. Mt. Everest is not even 9 km high.

The Wilson Cycle, part 1: continental breakup

The Aquitaine Basin is actually a series of basins stacked on top that had different causes, and tell a story from the breakup and re-unification of continents: the so-called Wilson cycle. The oldest rocks of the Aquitaine Basin that we can see at the surface, the 180-million-year-old limestones on the northern and eastern margin, were deposited in calm-water, shallow-marine environments. But this calm environment is deceptive: seismic lines across the Aquitaine Basin show that during this period, a series of steep faults formed along which the crust was broken into large blocks that slid down, each next one deeper than the previous, with the deepest point in the south. Such a process of extension breaking continental crust is known as ‘rifting’, and is currently active for instance in the East African Rift Valley. These so-called ‘normal faults’ formed in a time that Pangea started to break up, which first led to the formation of the Central Atlantic Ocean that separated Africa from North America. To the north, the Central Atlantic Ocean developed two branches, one between Iberia and North America, which also connected to the Bay of Biscay, and the other went into the western Mediterranean region, between Africa and Iberia, separating Adria (to which the modern Po Plain of northern Italy belongs) from Europe at the location of the southern Aquitaine basin. When a mid-oceanic ridge formed in that new ocean that we call the ‘Alpine Tethys’ Ocean, extension in the Aquitaine basin ceased and the basin became filled by limestones and sandstones that were derived from the lands of Europe that formed a ‘passive’ margin of the ocean.

The Wilson Cycle, part 2: continental collision

Aging oceanic lithosphere becomes denser than the Earth’s mantle and will eventually subduct, and the subduction process will continue, in one or more phases, until a continent on the subducting plate collides with a continent in the overriding plate, and subduction eventually comes to a halt. In the case of the Pyrenees, this closure history was a bit less simple than in textbook models, and the closure and opening of basins occurred twice. How that occurred exactly is still heavily discussed in the scientific community that studies the Pyrenees, but normal faulting and extension occurred again in the Aquitaine Basin around 100 million years ago. But the oceanic basin that formed between Iberia, southern France, and Adria (Italy) in the Jurassic has now disappeared because of subduction. This subduction eventually led to the formation of the Alps, the Apennines, and, when the continental margin of Iberia was shoved below France, the Pyrenees. During the formation of the Pyrenees, the Aquitaine basin became a so-called ‘foreland basin’. The weight of the Pyrenees makes the crust around the mountain belt bend down, and the resulting depression is filled with debris eroded from the mountain belt. The products of erosion of the Pyrenees form a 2 km thick succession of rocks with a total volume of more than 50 thousand cubic kilometers. An increase in the rates of deposition of these materials took place some 25 million years ago, around which time the shortening in the Pyrenees stopped. The rocks of this age are exposed at the surface in the southern part of France next to the Pyrenees, and they have given their name to the interval in Earth history during which they were deposited, the Aquitanian (23-20 million years ago). And another Wilson Cycle closed its circle.

At the moment of writing of this blog, the start list of the Tour de France 2022 is not known yet. But it would be wise to bring a cyclist named Wilson for this stage: that must be today’s winner.

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. Check the TdF-team.

Douwe van Hinsbergen