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

Abstract

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 (labeled 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 preBajocian (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 break-up of the rift between Iberia and Eurasia and the second to the convergence between these two plates.

Planation surfaces as a record of mantle dynamics: The case example of Africa

“…(2) A map was made of these undulating etchplains based on their physical continuity at the scale of each morphological unit and in between these units. These studies were based on the analysis of Digital Elevation Models (SRTM -see Bessin et al, 2015 for a presentation of this mapping technique) which were verified by field studies and/or numerous published local observations on landforms and…”

Planation surfaces of the Tian Shan Range (Central Asia): Insight on several 100 million years of topographic evolution

“…Their relative chronology is usually inferred from (1) their geometric relation with dated rocks (volcanic, plutonic or sedimentary rocks; e.g. Bessin et al, 2015), and (2) low-temperature thermochronology data constraining the timing and amplitude of major exhumation/burial events and thus providing information about the maximum or minimum age of erosional landscapes (e.g. Jolivet et al, 2007;Japsen et al, 2014).…”

Becklesia maulnyi sp. nov.: A new cycadean species from the Lower Oxfordian (Upper Jurassic) of Écommoy (Sarthe, NW France)

“…The fossil described here occurred on the eastern border of the Massif Armorican, an old Hercynian massif widely flattened since the Lower Jurassic (Bessin et al, 2015). At this time, two main domains can be recognized in this area: the Normandy domain to the north and the Le Mans domain to the south, separated by an hydrodynamic barrier formed by the positive epeirogenic tendency of the Perche promontory (Dugué, 1989(Dugué, , 1991Mégnien et al, 1980b).…”