Planation surfaces of the Armorican Massif (western France): denudation chronology of a Mesozoic land surface twice exhumed in response to relative crustal movements between Iberia and Eurasia

Paul Bessin (1) , François Guillocheau (1) , Cécile Robin (1) , Jean-Michel Schrötter (2) , Hugues Bauer (2)

1 GR - Géosciences Rennes
2 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

The Armorican Massif, an extensive outcrop of Variscan basement in western France, is shaped by several planation surfaces of debated origin and age. We propose an evolution model for these landforms and their deformation based on detailed mapping of the planation surfaces, their relative chronology and their relationships with dated outcrops of sediments and weathering products. The Armorican landscape consists of six stepped planation surfaces (labelled PS1 to PS6) later incised by two successive river networks. These landforms are pediments and pediplains or polygenic landforms (Armorican Planation Surface – PS5) resulting from two periods of etchplanation. These planation surfaces are mostly pre-Late Cretaceous in age, based on the age of the sediments overlapping these pediments. The three older ones (PS1 to PS3) are pre-Pliensbachian (191–183 Ma); PS4 is pre- Bajocian (170-168 Ma), PS5 (here called the “Armorican planation surface”) is polygenic and ranges from the base of the Early Cretaceous to the base of the Bartonian (140–40 Ma); and the youngest (PS6), which is poorly constrained, is older than 40 Ma (base of the Bartonian) or 15 Ma (base of the Middle Miocene). Most of these “old” landforms are exhumed, i.e. they were buried by sediments and later re-exposed by denudation. At least two phases of burial and exhumation have been identified: (1) burial in Jurassic time followed by denudation during the early Cretaceous and (2) burial in late Cretaceous time followed by denudation during the latest Cretaceous to early Eocene. The depth of burial is unknown but is probably low due to the small amount of coeval siliciclastic sediments in the surrounding basins. The two periods of exhumation correspond to critical periods in the plate movements between Africa, Iberia and Eurasia. The first is probably related to the initiation and breakup of the rift between Iberia and Eurasia and the second to the convergence between these two plates.

Mots clés: Exhumation; Planation surface; Cenozoic; Armorican Massif; Mesozoic; Pediment

Domaines: Sciences de la Terre; Géomorphologie

Paul Bessin, François Guillocheau, Cécile Robin, Jean-Michel Schrötter, Hugues Bauer. Planation surfaces of the Armorican Massif (western France): denudation chronology of a Mesozoic land surface twice exhumed in response to relative crustal movements between Iberia and Eurasia. Geomorphology, 2015, 233, pp.75-91. ⟨10.1016/j.geomorph.2014.09.026⟩. ⟨insu-01116939⟩