Present-day geodynamics in the bend of the western and central Alps as constrained by earthquake analysis

Abstract

S U M M A R YThe contrasted tectonics of the western/central Alps is examined using a synthesis of 389 reliable focal mechanisms. The present-day strain regime is mapped and interpolated for the entire Alpine belt based on a newly developed method of regionalization. The most striking feature is a continuous area of extension which closely follows the large-scale topographic crest line of the Alpine arc. Thrusting is observed locally, limited to areas near the border of the Alpine chain. A majority of earthquakes within the Alps and its forelands are in strikeslip mode. Stress inversion methods have been applied to homogenous subsets of focal plane mechanisms in order to map regional variations in stress orientation. The stress state is confirmed to be orogen-perpendicular both for σ 3 in the inner extensional zones and σ 1 in the outer transcurrent/transpressional zones. Extensional areas are well correlated with the part of the belt which presents the thickest crust, as shown by the comparison with the Bouguer anomaly and the average topography of the belt. In the northwestern Swiss Alps, extension is also correlated with currently uplifting zones. These observations and our strain/stress analyses support a geodynamic model for the western Alps in which the current activity is mostly a result of gravitational 'body' forces. Earthquakes do not provide any direct evidence for ongoing convergence in the Alpine system, but a relationship with ongoing activity of complex block rotations of the Apulian microplate cannot be ruled out.

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

“…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 Delacou et al 2004). Note that the present discussion only concerns the Alpine part of the study area.…”

“…Our interpolation assumes a continuous velocity field and does not consider the effects of possible fault-related velocity discontinuities. However, active faults are sparse in the Western Alps, with very low displacements to be considered in a large scale analysis (Delacou et al 2004;Sue et al 2007a). Despite these limitations, the interpolated velocity field appears to well correspond to the measured field (Fig.…”

“…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).…”

Erosion-driven uplift of the modern Central Alps

“…Note also that the pattern of rock uplift rate is spatially disconnected to the location of the rotation pole, and rock uplift also occurs in the Western Alps (Stocchi et al, 2005), where largescale extension was clearly documented (Sue et al, 2000;Calais et al, 2002;Delacou et al, 2004;Champagnac et al, 2006). Therefore, the large discrepancy between the rate of shortening derived from the reversed model and the rate invoked from kinematic data either imply that the GPS data do not reflect ongoing (and hypothetical) long-term shortening, or that horizontal plate kinematics cannot be the main agent of rock uplift in the Central and Western Alps.…”