The French Massif-Central, example of a not so inactive intraplate region
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Date
2021-11-17
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Australian Nuclear Science and Technology Organisation
Abstract
Plate tectonic theory postulates that intraplate areas are geodynamically inactive regions, active geologic and seismic deformations being concentrated along more or less narrow areas: the plate boundaries. The tectonic plates are supposed rigid, allowing stress transfer from one boundary to another. Therefore, no deformation is expected within the intraplate regions. True inactivity of intraplate areas is however refuted by evidences of active deformation. Many cases of intraplate earthquakes are known as for instance the lake Muir earthquake in 2018 (Mw = 5.3), the Botswana earthquake in 2017 (Mw= 6.5) or the New-Madrid sequence in 1811-1812 (4 events with Mw > 7). Recent propositions tend to decouple plate tectonics dynamic from intraplate earthquakes (e.g. [1]). In this case, the processes generating the stresses are local (e.g. fluid migration) or transient (e.g. GIA) and therefore long-term intraplate deformations are unlikely to happen. Significant intraplate deformations are however easily recognizable at the earth surface using the topography. Indeed, an important regional relief (Mountains) is the first evidence of earth surface long-term deformation and examples can be found for almost each intraplate region, for example: the Appalachians mountains (Northern America), the Great Dividing Range (Australia) or the Guiana highlands (Southern America). The origins of these topographic features are
highly debated and almost every explanation has been given: tectonic stress, past tectonic frontier with old relief, dynamic topography, etc. In many cases, the lack of absolute dating precludes the determination of the landscape evolution rates leading to inaccurate and sometimes physically unsound geomorphologic models.
Using the example of the French Massif-Central, we study if the long-term surface processes (erosion and
sedimentation) can be responsible for intraplate deformation. Quantification of the surface erosion and incision rates were performed using Terrestrial Cosmogenic Nuclides (TCN), mainly 10Be and 26Al. Using both mean watersheds derived denudation rates (covering the last ~ 15 ka) and long-term incision rates using endokarstic infilling (covering the last ~ 5 Ma) we show, despite local variations due to specific morphology and possible climatic variations, that the region is affected by significant erosion (s.l.) with a mean denudation rate of ~ 60 m/Ma of and an incision rate of ~ 90 m/Ma. Given the current ~ 300 m depth of the valleys, we conclude that this mountainous region is the consequence of a Plio-Quaternay uplift and therefore that intraplate area can be associated with active long-term processes leading to consequent finite deformation. First order numerical model addresses the question of driving processes and show that a combination of thermal isostasy and erosion driven isostatic adjustment can explain both long-term uplift rate and distributed volcanic activity of the
area. Such relatively constant long-term uplift is expected to be at the origin of long-term stress concentrations and therefore intraplate earthquakes could be associated with low-frequency seismic cycles modulated by transient or local processes.
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Keywords
France, Plate tectonics, Earthquakes, Geologic faults, Geomorphology, Deformation, Lakes, Stresses, Topology
Citation
Malcles, S., Philippe, V., Ritz, J.-F., Fink, D., & Cazes, G. (2021). The French Massif-Central, example of a not so inactive intraplate region. Paper presented to the 15th International Conference on Accelerator Mass Spectrometry, ANSTO Sydney, Australia. November 15th – 19th, 2021. (pp. 87). Retrieved from: https://ams15sydney.com/wp-content/uploads/2021/11/AMS-15-Full-Program-and-Abstract-Book-R-1.pdf