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Lateritic paleoweathering profiles in French Massif Central: Paleomagnetic datings

Abstract : Geological records of the history of ancient land surfaces are scarce and often fragmented by unconformities. Determining the age of these surfaces has long been the limiting factor in correlating continental paleosurfaces with marine basin deposits. Age determination of the weathering product of continental paleosurfaces as well as weathered material within the crystalline basement, is of prime importance for geodynamic reconstruction in general and more so for assessing differential dynamics between basin and basement areas, erosion rates, etc. In this paper, we examine layers of ferruginous kaolinitic paleoweathering products overlying the basement rocks of the French Massif Central. These layers, which are mainly formed in situ and are azoic (devoid of fossils), have been classically related to the "siderolithic" (Eocene-Oligocene) period because of their geographic ties with Tertiary deposits. The paleomagnetic ages of these paleoweathered surfaces have been obtained by comparing their acquired remanent magnetization, converted into paleomagnetic poles, with the APWP (apparent polar wander path) of the Eurasian plate. Recovered ages indicate late Jurassic to early Cretaceous ages (140-160 Ma with a confidence limit of 10 Ma) which are much older than the previously assumed Tertiary age. The ages obtained in this study considerably alter the present knowledge of the geodynamic evolution of the French Massif Central. Our results suggest a lack of sedimentary cover at the Jurassic-Cretaceous boundary. This implies that at the time of deposition of the siderolithic formations, the basement was cropping out and devoid of any Mesozoic cover. Furthermore, the profiles are not compacted, indicating that they have never been buried beneath a significant sedimentary cover. The obtained ages point to a great stability of French Massif Central throughout the Mesozoic, with very low ablation-erosion, while neighbouring basins (Aquitaine and Paris) were subsiding about 2000 m.
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Caroline Ricordel-Prognon, France Lagroix, Marie-Gabrielle Moreau, Médard Thiry. Lateritic paleoweathering profiles in French Massif Central: Paleomagnetic datings. Journal of Geophysical Research : Solid Earth, American Geophysical Union, 2010, 115 (B10), pp.B10104. ⟨10.1029/2010JB007419⟩. ⟨hal-00741768⟩

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