Active tectonics of the Western and Central Alps inferred from seismotectonic approach
by
Sue Christian
Geological Institute, Neuchâtel University, CP2, 2007 Neuchâtel, Switzerland.
Coauthors: Bastien Delacou, Jean-Daniel Champagnac, Cecile Allanic, Martin Burkhard

The western and central Alpine tectonic regime is characterized by ongoing widespread extension in the highest zones of the belt and transpressive/compressive tectonics at the external limits of the belt [Sue et al., 1999, Delacou et al, 2004]. This contrasted tectonics is examined using a global synthesis of 389 reliable focal mechanisms covering the whole belt. In term of deformation state, we used an original method of regionalization to precisely map the different tectonic modes in the belt. Extension appears as the main feature of the current activity in the western Alps, and affects their inner areas as a whole, following the arcuate geometry of the chain. Shortening is limited to local areas at the outer limits of the chain. Strike-slip is observed in the whole alpine realm. In term of stress analysis, we inverted the whole database of focal mechanisms in homogeneous areas determined using our regionalization analysis. The stress state is confirmed to be radial both concerning s3 in the inner extensional zones, and s1 in the outer transcurrent/tranpressional zones. Extensional areas are correlated with the part of the belt which presents the thickest crust, as shown by the comparison with the Bouguer anomaly map and the smoothed topography of the belt. The overall geodetic strain corresponds also to a radial extension across the western Alps [Calais et al., 2002]. Indeed, there is a quite good qualitative coherency between seismotectonic and geodetic approaches. To compare the seismic strain with the overall geodetic strain, we attempted to quantify the seismic part of the deformation. We used the regionalization of the seismic deformation to determine sub-areas of tectonically homogeneous seismic strain. In each sub-area, we computed the total seismic moment tensor, and the associated yearly seismic strain rate. This rate allows to compare different areas of the belt, in which the seismic catalogue may cover various range of time. Thus we can estimate the seismic strain per sub-area in the whole belt. Only some percent to some tens of percent of the geodesy-related deformation could be explained by the alpine seismic activity. This result rises up the issue of aseimic deformation in the Alps. This approach brings new quantitative elements to understand the ongoing geodynamic processes in the alpine belt. The low seismic strain rates we obtain in a belt characterized by a high tectonic contrast in a quite limited area, could suggest that the Alps are currently in a meta-stable tectonic state, ruled by isostasy/buoyancy forces rather than European/Apulia plate tectonic collision.

REF: Calais et al., 2002, Geology, 30-7, 651-654; Delacou et al.,2004 GJI 158, 753-774; Sue et al., 1999, JGR, 104, 25611-25622.

Date received: September 6, 2004

Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caon-34.