Widespread extension in the core of the western Alps revealed by earthquake analysis

Abstract

The western Alps are an active collision belt whose current stress field is inhomogeneous [Müller et al., 1992]. We report new seismological data which significantly improve our knowledge of this stress field. About 1600 earthquakes which occurred in the western Alps during the last 10 years were precisely located, and 79 new focal solutions were computed. The analysis of this database shows that widespread extension affects all the internal zones of the belt. To better constrain the associated stress regime, six stress tensors were computed using the Gephart and Forsyth [1984] method. They show that the current tectonics of the western Alps are contrasted with close variation in the stress regime (transpression to the front of the belt contrasting with extension in the core of the belt). The extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone, suggesting extensional reactivation of this inherited crustal discontinuity. Such widespread extension within the inner part of an actually ongoing collision belt cannot be explained by simple collision‐related tectonics. We propose that intrabelt buoyancy forces, such as those produced by a slab retreat or slab break‐off, interfere with the boundary forces driven by the ongoing Europe‐Africa convergence.

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

“…4 by the counterclockwise plate rotation of Apulia, which induces clockwise block-rotations along the dextral fault system following the Western Alpine arc (Mont-Blanc/Belledonne fault system to the North, High-Durance/ Bersézio/Argentera fault system to the South). The black and white arrows give the current stress field inferred from inversion of focal mechanisms (S1 black, S3 white, after Sue et al 1999;Delacou et al 2004). Abbreviations: see ical distribution, when compared to the geometry of the alpine arc and its tectonic architecture, leads us to propose a coherent structural interpretation.…”

“…5). This transcurrent dextral fault system runs from the Aiguilles Rouges strike-slip fault, possibly connected north-eastward with the dextral seismic alignment of the northern Valais (Deichmann et al 2002), towards the southern front of the Belledonne massif (Thouvenot et al 2003) up to the High-Durance and Bersézio/Argentera faults Southeastward (Sue et al 1999;Sue & Tricart 2003).…”

“…This regime fits well with the model proposed and numerically tested by Delacou et al (2004;2005b) and Sue et al (2007a) for the current geodynamics of the Western Alpine Arc. Gravitational re-equilibration of the belt due to buoyancy forces (Sue et al 1999;Delacou et al 2004, Sue at al. 2007a, probably enhanced by erosional processes (Champagnac et al 2007), could account for the generalized extension in the highchain, contrasting with compressional areas at the foot of the belt.…”

“…1, modified after the review of Delacou et al, 2004; see also Ménard 1988;Eva & Solarino 1998;Sue et al 1999;Baroux et al 2001;Kastrup et al 2004). In this paper, 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).…”

Late Neogene extension in the vicinity of the Simplon fault zone (central Alps, Switzerland)

“…Nevertheless, the larger Valais area is one of the most seismically active zones of the Alps. The strain/stress analyses performed in this area (Maurer 1993) and at ther larger scale (Sue et al 1999, Kastrup 2002 clearly indicate a present day extension in a N-S direction, at a high angle to the strike of the chain. Such a radial extension seems to be active all along the internal parts of the northwestern Alps today.…”