Why is the Ligurian Basin (Mediterranean Sea) seismogenic? Thermomechanical modeling of a reactivated passive margin

Abstract

Tectonics, v. 27, p. TC5011, 2008. http://dx.doi.org/10.1029/2007TC002232International audienceThe seismic activity of the Ligurian Basin, the northeastern termination of the western Mediterranean basin, is larger than in surrounding regions, even though recent geodetic studies attest that this area is subject to very low levels of deformation. This basin is an example of a type of passive margins that cannot be considered solely as inert sites of sedimentation and of progressive subsidence and that are reactivated in a compressive pattern; other examples include the Kwanza basin (Angola) and the Brazil margin. We investigated, by means of 2-D thermomechanical modeling, the structural and rheological heterogeneities that can lead to concentration of strain in this marginal basin. We deduced that the deformation of the basin is due to its particular geometric features, narrow and with a thick surrounding continental crust, related to its position at the southern termination of the Alps. This sharp transition, in terms of both geometry and rheological contrast, is a main factor in explaining the weakness of the margin. We discuss the importance of buoyancy forces versus tectonic forces, as well as thermal effects, on the observed reactivation. Influence of contrast in rheology between an oceanic-type crust and continental crust is also studied. Geodynamical implications are proposed for the region. The good agreement between the predicted localized deformation and the observed seismicity distribution should help improve seismic hazard assessment in the region

Structure and evolution of a passive margin in a compressive environment: Example of the south-western Alps–Ligurian basin junction during the Cenozoic

“…Pulses in the deformation occurred in the Argentera massif at w11 Ma, and then at w6 and 3.5 Ma (Bigot-Cormier et al, 2000;Foeken et al, 2003), that may have propagated down to the deep margin during the Pliocene (Bigot-Cormier et al, 2004;Foeken et al, 2003). Finally, the current regional seismicity reveals a present-day strike-slip state of stress in the Nice subalpine chain (Baroux et al, 2001;Courboulex et al, 2001Courboulex et al, , 2007 and a compressional state of stress in the deep Ligurian basin (Augliera et al, 1994;Béthoux et al, 1992Béthoux et al, , 2008Courboulex et al, 1995;Larroque et al, 2009). The causes of this recent deformation are still a matter of debate and may be related to Alpine compression and/or to gravitational spreading of the Alps toward the Ligurian basin (e.g.…”

Reappraisal of the 1887 Ligurian earthquake (western Mediterranean) from macroseismicity, active tectonics and tsunami modelling

“…13). The emergence of this major thrust plane at the foot of the Ligurian margin may be controlled by changes in the rheology in the ocean–continent transition zone (Béthoux et al 2008). Therefore, we favour the idea the 1887 rupture might have been propagated from the Ligurian thrust through the LFS and might have reached the oceanic floor (Fig.…”

“…This approach provides objective, reproducible estimates of location uncertainties and can be applied to all historical earthquakes with more than a few intensity assignments. Because the location uncertainty in the Bakun & Scotti (2006) intensity prediction models also depends on depth, two depths, 10 and 20 km, were tested, covering the range of instrumental depths estimated in the source region of the 1887 event (Bossolasco et al 1972; Déverchère et al 1991; Augliera et al 1994; Eva et al 2001; Béthoux et al 2008).…”

The GROSMarin experiment: three dimensional crustal structure of the North Ligurian margin from refraction tomography and preliminary analysis of microseismic measurements

“…Geodesy shows some shortening of less than 1 mm/yr between Corsica and the Ligurian coast and also between this coastal area and the axial zone of the southwestern Alps . Two candidates have been proposed to explain this deformation: a local cause is the strong altimetric gradient between the Argentera massif and the deep basin, which would result into downhill gravity collapse whereby strain would focus on structures between the axial Alpine zone and the toe of the North Ligurian margin [Béthoux et al, 2008]; the alternative explanation implies the broader scale convergence motion between Africa and Eurasia, some residual part of which would be accommodated on the margin and lead to its compressive reactivation , possibly through the differential motion of the Apulian block [Béthoux et al, 1992;Calais et al, 2002].…”

“…The structural heterogeneity of the Ligurian domain and its complex geological history have led to propose that structural discontinuities would be natural candidates for the accommodation of deformation [Béthoux et al, 2008]. Hence, finding active zones and characterising the extent of the main geological domains in the area are likely intricate problems.…”