A new structural model for the SW Irish Variscides - The Variscan front ofthe NW European Rhenohercynian

The Variscan front of NW Europe has been drawn in the past from Belgium acr oss southern England and southern Wales towards southern Ireland but the te rm Variscan front is not well defined and means different things to differe nt people. A tectonic front should represent the outcrop of a frontal thrus t or the tip line of the basal detachment. In fact such a tectonic front ca nnot be traced along the so-called Variscan front in Ireland. Field-based s tudies of the western part of the Irish Variscides show that the fold-and-t hrust belt continues north of the so-called Variscan front but deformation decreases gradually. Therefore we suggest, supported by seismic data, a det achment which ends as a blind thrust far to the north. The tip line of this basal detachment represents the true tectonic front of the Variscides in S W Ireland. Shear indicators show locally a dextral shear component near maj or faults and 3D-microstructural strain analyses show varying X-axes and ob late deformation indicating transpression during the tectonic evolution of the western Irish Variscides. Therefore a thin-skinned model associated wit h dextral shearing and transpression in the hanging wall of the basal detac hment is suggested. The evolution of the Variscan fold-and-thrust belt in N W Europe is related to the West European indentation model but also to pre- Variscan basement structures as the Leinster-Wales and London-Brabant massi fs. Finally, an attempt is made to outline the Variscan front along the ent ire NW European Rhenohercynian fold-and-thrust belt (FTB). (C) 1999 Elsevie r Science B.V. All rights reserved.