The collapse of the Variscan belt: a Variscan lateral extrusion thin-skinned structure in NW Iberia

摘要

The Iberian Massif of the Variscan Belt is a big area of exposed Palaeozoic rocks holding an S-shaped regional bend. Profuse data and genetic models support the existence of the northern arch (Ibero-Armorican Orocline: IAO) created by Late-Variscan buckling. The southern arch (Central-Iberian Orocline: CIO) was proposed near a century ago but it remains controversial. Tectono-metamorphic studies in the core of the CIO constrain its geometry and kinematics and provide crucial data to unravel the geological evolution of the region. The CIO is here envisaged as produced by the lateral thin-skinned extrusion of a fan-like wedge (with fragments of the NW Iberia allochthonous complexes) onto the Iberian autochthon, using the parautochthon as the lower tectonic sheet. The progression of a roughly E-W linear orogen towards the W (today's NW Iberia) controlled by the obliquity of the continental collision of Gondwana and Laurussia created an escape way for the collapsing materials coming from the accretionary prism. The extrusion wedge expanded westwards along the orogenic extensional vector and progressively curved areas to the west, dragging the previous structures and the stratigraphy while incorporating the Variscan foreland basins at the base. We propose the French Massif Central (FMC) as a provenance area for the extrusion. In this sector, evidences of Eo-Variscan (410-360Ma) subduction-obduction stages match to those described in the NW Iberia allochthonous complexes. The synorogenic extension towards NW and gneiss-dome formation in the FMC at 370-355 Ma fits with the known ages and kinematics (after restoring the IAO) of the first compressive stages in the parautochthon/autochthon of NW Iberia (360-340Ma) evidencing the relationship between regions. Tightening of the CIO occurred in the late-Variscan compressive stage, due to the formation of the IAO that wrapped the entire CIO thus rotating the extrusion fan more than 90ocounter-clockwise in the latest Palaeozoic.