The Limagne fault
– 35 million years
The fault lies parallel to the Chaîne des Puys volcanic alignment, and it can be traced over a distance of 30 km from Enval to Ceyrat, passing through Sayat and Royat. It takes the form of a vegetated scarp around 700 m high, and marks the junction between the Plateau des Dômes and the Limagne plain on which the city of Clermont-Ferrand lies.
The Limagne fault is a major geological and tectonic feature of the application, which originated some 300 My after the Plateau des Domes.
Its origin is also linked to continental movements : as a direct result of the formation of the Alps (from 40 – 25 My) the continental crust at a distance from this new mountain chain became thinned and suffered fracturing in places. This led to the development of a large-scale fissure system throughout western Europe (from the Massif Central to Bohemia in the Czech Republic) known as the west-European rift. The Limagne fault forms part of this rift system.
If extension had continued then the continental crust would have disappeared and an ocean would have sprung up in place of the present Limagne Plain.
Formation of the Limagne fault
40 My ago, the crust became thinned and underwent extension, leading to the formation of the large western European grabens in which the ground level dropped by several kilometers. The Limagne graben floor dropped to around sea level, and was inundated by a shallow body of water. Over a period of several million years sediments were deposited which slowly infilled the graben.
When the infill was completed, around 25 My, the sediments reached a depth of nearly 3,000 m, raising the level to that of the Plateau des Dômes. By this point the water had completely disappeared.
At the same time a forerunner of the Chaîne des Puys volcanism appeared in the sedimentary zone. Puy Crouel and Montrognon were active at this time.
Somewhere around 3 – 1 My the basement and the sediments were uplifted together, triggering another intense, prolonged period of erosion which removed more than 500 m of sediments.
At the start of the Quaternary period (1 My) the fault was exposed by erosion, resulting in the current scarp relief. The sediments were not entirely eroded from the plain, and their remains give rise to the fertility of the wide Limagne Plain which covers the former graben.
The Limagne fault, which is defined as a ‘high quality forestry environment’, also serves to emphasis the Chaîne des Puys by acting as a natural geological pedestal.
The fault scarp has remained unspoilt due to the steepness of its slope, making it the home to a rich biodiversity. The appealing landscape, coupled with the proximity of the city, make this scarp an increasingly attractive place for walkers.
In addition, the fault relief forms a natural limit to encroaching urbanisation, which now abuts this geological boundary to the east.
Did you know ? The greatest depth of the Limagne fault is at Riom, where the sediments are 3,000 m thick.