Reconciling competing models for the tectono-stratigraphic zonation of the Variscan orogen in Western Europe

• Three models have emerged to explain the distribution of tectonostratigraphic zones:
• Indentation tectonics due to continent collision between Laurussia and a promontory of Gondwana during the Devonian;
• Development of the Cantabrian orocline at ca. 295 Ma;
• Orogen-scale modification due to regional strike-slip shear zones.
• The indentor and orocline models (1 and 2) have been viewed as incompatible.
• They can be reconciled if Gondwana’s leading edge was irregular, but the zones were linear within the inner indentor

The Late Paleozoic Variscan orogen in Europe is the result of convergence and collision between Laurussia and Gondwana during closure of the Rheic Ocean. The orogen is divided into tectonostratigraphic zones that have a distinct curvature (Ibero-Armorican Arc, IAA) and record the Late Cambrian–Early Ordovician opening of the Rheic Ocean, the migration of terranes from the Gondwanan margin towards Laurussia, as well as the closure of that ocean and development of the IAA. Three models have emerged to explain the distribution of tectonostratigraphic zones:(1)indentation tectonics due to collision with Laurussia by a (Ibero-Aquitanian) promontory of Gondwana during the Devonian;(2)development of the Cantabrian orocline at ca. 295 Ma in the inner core of the IAA, an interpretation supported by abundant paleomagnetic, structural and geochronological data; and(3)orogen-scale modification due to large-scale strike-slip shear zones.The indentor and orocline models (1 and 2) have been viewed as incompatible because the former requires curvature along the Gondwanan margin prior to Devonian collision with Laurussia, whereas the latter requires that the tectonostratigraphic zones were much more linear prior to bending at ca. 295 Ma.Recent data have rekindled the hypothesis that the Cantabrian Orocline is connected to a second orocline to the south (Central Iberian Orocline), highlighting the possibility that inner (Gondwanan) and outer (Laurussian) zones of the Ibero-Armorican arc may be structurally discordant with respect to each other, implying that the geographic limits of orocline formation are presently unclear.The two models can be reconciled if the geography of Gondwana's leading edge was irregular, but the tectono-stratigraphic zones remained approximately linear within the inner zones of the putative Ibero-Aquitanian indentor and (ii) deformation associated with initial collision was largely accommodated by sinistral (SW Iberia) and dextral (Armorican Massif) motion along shear zones on either side of the promontory.