Quantification of strain rate in the Western Alps using geodesy: comparisons with seismotectonics

Bastien Delacou,Christian Sue,Martin Burkhard,Jean-Daniel Champagnac,Cécile Allanic,Jean-Mathieu Nocquet

doi:10.1007/s00015-008-1271-3

Abstract

The contrasted seismotectonic regime of the Western Alps is characterized by radial extension in the high chain, combined with local compressive areas at the foothill of the belt, and everywhere occurrence of transcurrent tectonics. Here, we compare this seismotectonic regime to a large-scale compilation of GPS measurements in the Western Alpine realm. Our analysis is based on the raw GPS database, which give the measured velocity field with respect to the so called “stable Europe”, and an interpolated velocity field, in order to smooth the database on a more regular mesh. Both strain rate and rotational components of the deformation are investigated. The strain rate field shows patch-like structure, with extensional areas located in the core and to the North of the belt and compressional areas located in its periphery. Although the GPS deformation fields (both raw and interpolated) are more spatially variable than the seismotectonic field, a good qualitative correlation is established with the seismotectonic regionalization of the deformation. The rotation rate fields (both raw and interpolated) present counterclockwise rotations in the innermost part of the belt and a surprising continuous zone of clockwise rotations following the arc-shape geometry of the Western Alps along their external border. We interpret this new result in term of a counterclockwise rotation of the Apulia plate with respect to the stable Europe. This tectonic scheme may induce clockwise rotations of crustal block along the large strike-slip fault system, which runs in the outer part of the belt, from the Rhône-Simplon fault to the Belledonne fault and Southeastward, to the High-Durance and Argentera fault.

Highlights

Introduction and current seismotectonic settingSeismotectonic studies have shown the contrasted strain/stress seismic fields across the Western Alps, with the occurrence of both extensional and compressional regimes (Fig. 1, modified after the review of Delacou et al, 2004; see Ménard 1988; Eva & Solarino 1998; Sue et al 1999; Baroux et al 2001; Kastrup et al 2004)
In the light of recent seismotectonic studies, we propose a tectonic scenario for the complex active geodynamics of this part of the belt, where plate tectonics and buoyancy forces interact
In order to better visualize the spatial variations of the calculated parameters and to provide a comparison with the seismotectonic regime, we interpolated each cell (112 cells for the raw GPS dataset, around 880 cells for the interpolated GPS dataset), using the same color code as in fig

Summary

Introduction

Seismotectonic studies have shown the contrasted strain/stress seismic fields across the Western Alps, with the occurrence of both extensional and compressional regimes (Fig. 1, modified after the review of Delacou et al, 2004; see Ménard 1988; Eva & Solarino 1998; Sue et al 1999; Baroux et al 2001; Kastrup et al 2004). We make a new analysis of the published geodetic velocity in order to study the orogen-scale deformation of the Western Alps (Nocquet 2002; Nocquet & Calais, 2003; Nocquet & Calais, 2004). The strain rates associated to surface GPS velocities are precisely compared with seismotectonic data to test the consistency of each techniques when applied to the regional deformation analysis across the belt. Using 3–4 years dataset, Calais et al (2002) and Nocquet & Calais (2003) proposed an upper limit of 0.5 mm.y–1

Methods

Results

Conclusion