Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley

Abstract

An integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE-SW to NW-SE trending Rhone-Simplon fault zone and the gently S-dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone-Simplon fault zone, strike-slip earthquakes on steep-dipping faults within the Helvetic nappes are a consequence of regional NW-SE compression and NE-SW extension. To the south, vertical maximum stress and N-S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes andlor continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission-track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene. Terra Nova, 9,[91][92][93][94] 1997

Extensional neotectonics around the bend of the Western/Central Alps: an overview

“…5b. Our results are consistent with previous regional studies (e.g., Maurer et al 1997;Baroux et al 2001;Kastrup et al 2004). …”

“…The first seismotectonic studies showed a radial fan for the P-axes around the Western Alpine Arc (Ahorner et al 1972;Fréchet 1978;Pavoni 1980a, b;Ménard 1988). By the end of the 1990s, an extensional regime was recognized in the internal zones of the belt, but locally also in some areas situated in the external zones (Maurer et al 1997;Baroux et al 2001;Kastrup et al 2004) (Fig. 2).…”

Erosion-driven uplift of the modern Central Alps

“…The average of these three rotation poles is [8.9E, 45.7N], located at the boundary between the Po Basin and the southern foothills of the Central Alps. This location of Adria's Euler pole is consistent with the aseismicity of the Lepontine Dome , the extensional and dextral seismotectonic regime in the Western Alps and in the Valais (Maurer et al, 1997, Deichmann et al, 2002, and with a compressional regime observed in seismotectonics and paleoseismology in the Eastern Alps, east of longitude 9.5°E (e.g. WORKING GROUP CPTI, 2004).…”

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

“…In contrast, the central Valais area, some 10 to 50 km further to the west, is one of the most seismically active zones of Switzerland in particular and the Western Alps in general (Deichmann et al 1998). Two distinct present-day stress regimes have been identified on either side of the Rhone valley from the inversion of fault plane solutions (Maurer 1993, Maurer et al 1997. A dextral strike-slip regime with a slight component of extension prevails in the Rawil saddle, north of the Rhone valley, but mostly concentrated along a subvertical WSW-ENE lineament running below the Rawil saddle.…”

“…Using well established techniques of fault/striae measurements and their inversion (Angelier 1994), we mapped paleo-stress directions and regimes in the larger Simplon area. Given the late, clearly post-mylonitic age of these brittle faults, their analysis allows to bridge the gap between the well studied kinematics of the SFZ mylonites (Mancktelow 1990) and the present day stress regime as known from the inversion of earthquake data (Maurer et al 1997, Kastrup 2002. Faults and systematic joint sets are ubiquitous and well exposed in rocky outcrops of the high alpine area around the Simplonpass and the Gondo gorge.…”