The Hercynian collision in the Armorican Massif; evidence of different lithospheric domains inferred from seismic tomography and anisotropy

The Hercynian belt is a continental collision orogen extending from south-west Iberia to the Bohemian Massif in Czech Republic. The successive stages of its formation are dated from 400 to 260 Ma. The Armorican Massif is a preserved segment of this orogen. It presents structures oriented NW-SE, parallel to the general Hercynian trend in this region. The massif is divided into three domains (North, Central, and South-Armorican domains) separated by two main shearzones, the North-and South-Armorican shear zones. As the Armorican Massif escaped from any important tectonic or thermal event since the end of Hercynian times, it is particularly suited for the study of an old collision orogen. Thus, in the framework of the GeoFrance3D-ARMOR2 project, two passive seismological experiments were conducted in 1997 and 1999 in the Armorican Massif. The main goals concerned the characterization of the deep geometry of both shear zones, the understanding of their geodynamic bearing on the long term evolution of the Hercynian belt, the study of the lithospheric deformation, and the 3D imaging of the Champtoceaux nappes. The data allow to model seismic anisotropy and to build a 3D P-wave velocity model beneath the Armorican Massif. Crustal images do not evidence any deep rooting of the Champtoceaux nappes in the lower crust. However, the upper mantle images show a clear signal interpreted as the relic of the northward subduction which lasted until Devonian ( approximately 350 Ma). The results also show that the North-Armorican Shear Zone is limited at depth to the crust and topmost mantle, while the South-Armorican Shear Zone can be traced over the whole lithosphere. The strong velocity contrasts are associated to probable relic thermal anomalies but are also significantly related to chemical anomalies.