The south armorican shear zone

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In the town of Quimper, the course of Odet follows a narrow valley , oriented WNW-ESE which is the topographical expression of an important fault in the earth's crust. This fault , that geologist have named south armorican shear zone has played a major role in the setting of the Massif Armoricain during the hercynian orogenis, about 400 billions years ago.

Along the south armorican shear zone, the granite rocs which constitute the essential of the basement of the region have been broken, sheared , crushed. The cohesion of these rocks has of course been reduced, facilitating the erosion and the digging of a narrow valley by the fluvial waters along the area of the fault. The region of Quimper looked like , about 400 billions years ago to the actual California, cut from one side to the other by the fault of San Andreas, along which the coast of California slides in the direction of north, compared to the rest of the United States, at a speed of 3 centimetres a year! Concerning the south armorican shear zone, a similar situation (in its principle) existed there are several hundreds of millions years ago but the geological activity in the region is since that time very quiet. No risk of "big one" for Quimper! Nevertheless the south armorican shear zone reminds us from time to time of its presence by small earthquakes, usually too weak to be felt by the population but recorded on seismic instruments.