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

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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 and/or 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.