ELSEVIER Tectonophysics 267 (1996) 143-175 TECTONOPHYSICS Dynamics and seismotectonics of the West-Alpine arc G. Giglia, G. Capponi, L. Crispini, M. Piazza Dipartimento di Scienze della Terra, Universith di Genova, Corso Europa 26, 16132 Genoea. Italy Received 15 May 1995; accepted 29 April 1996 Abstract In this paper we present a comparative review of structural, seismic and focal mechanism data from the West-Alpine arc. In the Western Alps, seismic activity is concentrated along an external belt, corresponding to the Penninic front, and an internal belt, corresponding to the Austro-Alpine front and its southern extension. These seismic belts are connected in the NE by a seismic lineament, corresponding to the Simplon and Centovalli Line and in the south by the E-W-trending seismic Stura "couloir", located between the Argentera and Dora Maira massifs. The Simplon-Centovalli-Tonale system and the Stura "couloir" are dextral and sinistral strike-slip systems respec- tively; this implies a westward translation of the West-Alpine arc and the Po Plain. Based on these observations, a seismotectonic model is proposed in which the frontal Penninic thrust and the basal surface of the accretionary wedge corresponding to the Penninic sole thrust are reactivated. Activity along the frontal thrust increases the arcuate shape of the Western Alps and disengages them from the Central and the Ligurian Alps along the tear faults of the Simplon-Centovalli-Tonale system and the Stura "couloir" respectively. In the wider framework of northern Italy, the sector of the eastern Alps north of the Gailtal Line, is moving in an orogen-parallel direction towards the east. Areas with lateral escape in opposite directions, towards the west for the Po Plain and towards the east for the Eastern Alps, north of the Gailtal Line, are separated by the South Alpine Atesine indenter. A model postulating a double lateral escape and a central indenter matches most of the features of the models of "poin~onnement". The seismic features of northern Italy agree with the seismicity of the stable sector of northern Europe up to the North Sea. This area is fragmented into blocks bounded by seismic bands, some of which probably reactivate pre-existing structures. The overall stress field of Cratonic Europe is dominated by N-NW-directed trajectories of maximum horizontal compression. The seismicity of the European Alpine foreland is interpreted as the response of a stable block to the same stress field of northern Italy, which is induced by the on-going NW-SE Africa-Europe convergence. We suggest that the westward translation of the West-Alpine arc continues at present and is the driving mechanism of the observed seismicity. Keywords: seismotectonics; West-Alpine arc; NW Italy; strike-slip systems; Simplon-Centovalli-Tonale system; Stura "couloir" 1. Introduction The Alpine belt, from the Central to the Western and Ligurian Alps, has a horse-shoe shape. The development of the West-Alpine arc is one of the most debated arguments in the Alpine geologic liter- ature and a number of papers have been dedicated to its development, focusing on paleogeography, tec- 0040-1951/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0040-1951(96)00093-5