K–Ar dating of synkinematic clay gouges from Neoalpine faults of the Central, Western and Eastern Alps

With ongoing Alpine convergence and deformation during exhumation there is a general progression from ductile to brittle behaviour on the Periadriatic Fault and kinematically related faults farther north (e.g. the Simplon–Rhone Fault and a diffuse, discontinuous zone of generally dextral strike–slip movement on the southern boundary of the Aar massif). K–Ar dating of fine grained illite from clay fault gouges provides a reliable method for establishing the maximum age of this transition to brittle faulting and, for short-lived faults, the approximate time of faulting itself. The new results establish that brittle faulting on the northeastern segment of the Canavese Fault (i.e. the part of the Periadriatic Fault southwest of Lago Maggiore) occurred around 20 Ma, with south-side-up kinematics. A K–Ar age of ca. 17 Ma for the crosscutting Giudicarie Fault in the Eastern Alps is effectively identical with an already published pseudotachylyte age and places a lower limit on major and continuous strike slip movements of the Periadriatic Fault. The age of brittle faulting farther north, on the southern border of the Aar Massif, is from 13.6 ± 0.3 to 8.3 ± 1.1 Ma, consistent with the younger cooling and exhumation in this area. The dominantly dextral brittle faulting becoming younger to the north reflects the continued indentation and anticlockwise rotation of Adria as Alpine units become exhumed and progressively welded to the southern block. In contrast to the Eastern Alps, there is no field evidence for (westward) lateral extrusion of Alpine units relative to Adria and the European foreland.

► K–Ar illite dating of Neoalpine fault activity.
► Canavese Fault had a south-side-up kinematics in the Aquitanian.
► Brittle dextral faulting in the Aar Massif from ca. 13.5 Ma on.
► Lateral extrusion model cannot explain orogen-parallel faults in the Western Alps.