The complexity of a mountain belt is often regarded as directly related to a complex paleogeography, when actually it is often due to a complex structural history. In the western Alps, after a century and a half of detailed structural work, the general nappe framework is now well understood and a common structural model accepted by most alpine geologists. However, its palaeogeography is still a matter of debate, and very often, complex schemes are presented, with multi-oceans plate tectonic models incorporating micro-continents corresponding to present day very tiny crustal blocks. We have shown that such models do not correspond to a certain simple plate tectonic reality, which can be deduced from large scale reconstructions using all available data, and well constrained by magnetic anomalies from the Atlantic (Stampfli et al., 2001a; Stampfli et al., 1998).

We want to sum up here the main geological constraints coming from the western Alps sector and their bearing on the present plate tectonic models of the western Tethyan region, representing a key stone in the global reconstructions we developed these past ten years (Stampfli and Pillevuit, 1993; Stampfli, 1996; 2000; Stampfli et al., 1991; 1998; 2001a). We are presenting here a geological account of major geodynamic events which can be recognised mainly on a western Swiss Alps transect and the discussion is enlarged to surrounding Alpine areas. We are concentrating on the Alpine cycle only, from the opening of the Alpine Tethys in Early Jurassic time to its closure during the Tertiary; the pre-Variscan (von Raumer et al., 2002), Variscan (Stampfli et al., 2002b) and Cimmerian (Stampfli et al., 2002a) cycles have been recently treated in separate papers.

Part of this material was presented in a geological guide book of the western Alps, during the second field meeting of the IGCP 453 project (collisional orogen) in September 2001 in Switzerland (Stampfli, 2001), and represent a largely revised version of ideas presented in Stampfli et al. (1998).