Thickening vs. extension in the Variscan belt: P–T modelling in the Central Iberian autochthon

• Metamorphism in Variscan Central Iberia is studied through pseudosection modelling.
• A minimum tectonic overburden of 3 kbar (9 km) is estimated.
• Data point to the orogen-parallel emplacement of a nappe from NW to Central Iberia.
• Orogen-parallel tectonics can be traced from the Bohemian Massif to Central Iberia.
• Central parts of the belt acted as a W-wards spreading, constrained orogenic plateau.

The Variscan tectonothermal structure of the NW and Central Iberian Massif shows an important thickening during the Early Carboniferous, prior to its thermal weakening and gravitational collapse in the Middle Carboniferous, triggering the uplift of its orogenic roots. Pseudosections modelling of selected samples from Somosierra, in the Barrovian metamorphic area of Central Iberia, yield P–T conditions of at least 6 kbar and 500–540 °C for a micaschist of the garnet zone, 5.0–9.7 kbar and 580–620 °C for a psammitic schist of the staurolite zone, above 9.0 kbar and 680 °C for a kyanite micaschist of the sillimanite zone, and 4.0–9.0 kbar and 750–800 °C for a migmatitic paragneiss structurally beneath the Barrovian sequence. These data, together with previous metamorphic and structural data, imply the near-parallel emplacement of a > 9 km-thick allochthonous nappe over the Central Iberian foreland during the Early Carboniferous. Emplacement was probably related to the lateral extension of a mid-European orogenic plateau in the form of a channel between opposite-verging collisional wedges. This orogen-parallel extension flow can be traced from the Bohemian Massif through the French Massif Central, South Domain of the Armorican Massif and NW of the Iberian Massif, to the Central Iberian area. The highly oblique emplacement of the thick allochthonous nappe explains the orogen-parallel stretching lineations, the development of Barrovian and subsequent low-pressure metamorphisms, and it could also explain the massive granitic magmatism located in the NW and central areas of the Central Iberian Zone.