Cadomian tectonics in northern Brittany: a contribution of 3-D crustal-scale modelling

Both available and newly acquired geological and geophysical data processed in the framework of the ARMOR project have been used to construct a 3-D ge ometric and geological model of the Cadomian domain of northern Brittany. T he major results of the data processing are presented and discussed relativ e to a composite profile transecting the studied domain and constructed fro m the surface geology, reflection seismics, magnetotelluric soundings and g ravimetric data. They are then extrapolated at regional scale through a ser ies of cross-sections compiled from the surface geology and gravimetric dat a and calibrated onto the composite profile. The 3-D crustal-scale models w ere computed according to two procedures. The first, using a STRIM Matra Da tavision modeler, defines various interfaces through correlation between ad jacent cross-sections and is well adapted to the 3-D representation of geol ogical structures. The second, based on Voronoi diagrams, leads to a volume tric model of the geological units and is a powerful tool for calculating 3 -D geophysical responses of the model when the physical properties of the r ock units are known. The tectonic implications of the computed 3-D crustal- scale geometry of the Cadomian domain are then discussed in terms of volcan ic-are thrusting, with special emphasis being placed on the obliquity of th e shortening direction by reference to the thrust fault trend. Finally, the data are used to demonstrate the reactivation of some Cadomian faults duri ng the deposition of Ordovician sandstones and following the emplacement of the Paleozoic dolerite dyke swarm of northern Brittany. (C) 2001 Elsevier Science B.V, All rights reserved.