While it was commonly accepted that the seismicity of the Pyrenees and the Western Alps must be due to the spreading of these chains under their own weight (gravity collapse), French researchers from the Geosciences Montpellier (CNRS-INSU, University of Montpellier), Géosciences Rennes (CNRS-INSU, University of Rennes) and IRAP (CNRS-INSU, University of Toulouse) laboratories show that erosion could be the real driving force behind this seismicity. This study is published in the journal Geology.
Intra-plate deformation and seismicity under the Pyrenees and Alps
Earthquakes occur in areas where no deformation is expected since these regions are within very stable tectonic plates. In Europe, for example, deformation rates are so low that they are not quantifiable at present. Indeed, deformation rates between GPS sites several hundred kilometres apart on either side of the Pyrenees and the Western Alps indicate deformations that are less than the uncertainties in the measurements (<0.3 mm/year). However, the Alps and the Pyrenees are the seat of frequent but moderate seismicity.
The mechanisms at the focus under the axial zones of these chains show extension oriented perpendicularly to their main topographic axis, whereas those in the external zones of the Alps indicate compression. All these observations have generally been used as evidence of the spreading of these chains (gravity collapse). However, geomorphological studies suggest that the Alps are over-extended and geodesic methods (GPS and levelling) confirm this by indicating vertical uplift rates of rocks (not topography) of up to 2 mm/year.
In the case of a gravity collapse, horizontal velocities are expected to be at least equivalent to vertical velocities, with the latter preferentially showing subsidence at the core of the chain, which is not the case. It is therefore necessary to consider other processes that may cause this deformation.
The effects of erosion on mountain ranges
The removal of topography by erosion is a process that allows rocks to be brought up by isostasy. These two processes, erosion and isostasy, have been incorporated into numerical (finite element) modeling to test the effects of erosion on a mountain range. The geothermality of the lithosphere was defined based on measurements of heat fluxes in the lithosphere and the depth of seismicity in the mountain ranges. The rheological properties of the crust are defined on the basis of experimental studies.
In a first step, the stability of the model was tested without erosion. The results show no deformation and therefore do not confirm the hypothesis of gravity collapse. Once erosion is taken into account, vertical movements are generated at the heart of the chain and these vertical movements are accompanied by slight horizontal movements, which result in horizontal extension below the axial zone of the chain and compression in the peripheral zones of the foothills. Surprisingly, this extension is observed for chain shortening rates of up to 3 mm/year provided that erosion is sufficiently strong (~1mm/year).
The questioning of the gravity collapse and the revision of the seismic hazard in France
These results show that gravity collapse is not the process responsible for the deformation of the Western Alps, and reveal the role of erosion in activating both vertical and horizontal movements. The seismic hazard models in force in France only take into account horizontal deformations associated with plate tectonics, to the exclusion of any other process. For the authors, it is urgent to rethink the causes responsible for deformation and seismicity in metropolitan France. This is particularly true for mountain areas, where, against all expectations, erosion could be a major factor in seismicity activation.
This discovery shows that it is important to further study the effects of erosion on reliefs by developing 3D models taking into account topography, potentially active faults and the distribution of erosion. At the same time, erosion rates and their evolution over time need to be quantified in order to determine more precisely the share of erosion in relation to post-glacial rebound and deeper phenomena.
- Erosion-induced isostatic rebound triggers extension in low convergent mountain ranges, P. Vernant1, F. Hivert1, J. Chéry1, P. Steer2, R. Cattin1, A. Rigo3, Geology, Volume : 41, Février (2013)
- 1-Géosciences Montpellier, CNRS-Université Montpellier 2
- 2-Géosciences Rennes, CNRS-Université de Rennes 1
- 3-Institut de Recherche en Astrophysique et Planétologie, CNRS-Université Paul Sabayier, Observatoire Midi-Pyrénées
- Alexis Rigo, email@example.com