The arc of the western Alps in the light of geophysical data on deep crustal structure

Recently, two international deep seismic campaigns in the western and centr al Alps (Etude Continentale ct Oceanique par Reflexion et Refraction Sismiq ue - Progetto Strategico Crosta Profonda, ECORS-CROP [Roure et al. 1996b]; and National Research Program 20, NRP-20 [Pfiffner er al. 1997]) have been completed. Here we present a synoptic interpretation of the wealth of geoph ysical data about deep crustal structure of the Alps collected during the p ast 40 years, including the two above mentioned geophysical projects. The t ectonic reinterpretation of the ECORS-CROP high-resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology. Combined with previously published interpretations regarding the central Alps [Schmid er al., 1996], this stu dy reveals substantial differences in geometry and kinematics between trans ects across the western and central Alps, respectively. At depth the transi tion between the central and western Alps is marked by the western limits o f an Adriatic lower crustal wedge-shaped structure and the northeastern lim its of a similar structure made of European lower crustal material. At the surface it coincides with a corridor of dextral strike-slip along the Tonal e and Simplon lines. In addition, the thickness of the seismogenic zone sho ws a remarkable variation from over 40 km beneath the Penninic realm of the western Alps to less than 20 km in the central Alps. The formation of the western Alpine are was initiated during convergence and collision before 35 Myr ago, when the Adriatic micro-plate, moving northward with respect to t he European foreland, caused sinistral transpression in the western Alps wh ile the central and eastern Alps underwent head-on convergence and collisio n. During the postcollisional stage, i.e., after 35 Myr ago, the arcuate sh ape of the western Alps was accentuated by WNW- directed movement and antic lockwise rotation of the Adriatic microplate, decoupled from the central an d eastern Alps along the Tonale-Simplon dextral shear zone. This led to wed ging of lower crustal slices both in the western Alps and in the central Al ps. The new tectonic interpretation of the ECORS-CROP transect allows a kin ematic model to be established for crustal shortening in the western Alps d uring the past 35 Myr. The Ivrea mantle plays the role of a backstop in our tectonic model. We distinguish three episodes of post-collisional crustal shortening along the ECORS-CROP transect: From 35 to 30 Ma the Brianconnais basement was back-thrusted over the Gran Paradise units, leading to 30 km of crustal shortening (first episode). In the early Miocene, movements conc entrated mainly along the Penninic frontal thrust and resulted in about 60 km shortening (second episode). Post-12 Ma shortening within the external m assifs is associated with folding in the Jura mountains when the crust was shortened by an additional 30 km (third episode).