The recent tectonic regime of the central Alps was examined with a combination of structural, morphotectonic, and geochronological methods. The study area was the Lepontine dome / Central Alps, where an extensional stress regime seems to prevail today as documented by earthquake focal mechanisms.
Field work focussed on the latest, mostly brittle deformations. Analysis of faults patterns on the large scale was carried out using satellite imagery, digital elevation models and aerial photographs. Individual fault families were characterised by detailed field observations of fault morphology, petrography of fault rocks, cross-cutting relationships and displacement markers. Fault / striae measurements and their inversion provided a quantitative assessment of palaeostresses. We discovered tectonic pseudotachylytes associated with late faults. Laser ablation Ar-Ar analysis of these rocks allowed us to date the latest phases of Alpine faulting and exhumation.
Our results show that the fault system of the Lepontine Dome is a dense network of 4 fault-families working normal, normal-oblique and dextral at the regional scale. From map relations and kinematic data, the entire fault system is considered geometrically and kinematically coherent, at local outcrop scale as well as at the entire dome scale. In particular, the fault system is coherent with the major discontinuities bounding the dome to the West (Simplon fault) and to the South (Insubric line).
The paleostress field determined in the Lepontine Dome is quite homogeneous from a geographic viewpoint. It is largely dominated by the classical NE-SW brittle extension, described in the whole northwestern Alps, overprinted by two minor signals (N-S and WNW-ESE directions of extension). The last stage recorded under brittle conditions within the Lepontine dome is a NNE-SSW extension on ENE-WSW faults.
Dating was performed on four pseudotachylyte samples from the Lepontine Dome (Leventina nappe and Maggia nappe). All the Ar-Ar analyses are consistent internally and yield total gas ages values at: 11.2 ± 1.0 Ma, 12.1 ± 1.8 Ma, 10.3 ± 0.8 Ma for the Leventina samples and 9.4 ± 1.3 Ma for the Maggia sample.
We propose a kinematic model for the Central Alps in which the emplacement of the Lepontine Dome is driven by a pull-apart-like process controlled by the counterclockwise rotation of the Apulian indenter. Associated to these processes, lateral escape linked to the Apulia indentation would have enhanced to the global extensional scheme of the Lepontine Dome. The end of his exhumation is recorded under brittle conditions and associated to the 10 Ma old orogen-parallel extension we documented.