The late Variscan HT/LP metamorphic event in the Iberian branch of the Variscides: Relationships with crustal thickening, nappe emplacement, orocline development, and crustal evolution José R. Martínez Catalán 1 *, Francisco J. Rubio Pascual 2 , Alejandro Díez Montes 3 , James E. Alcock 4 , Rubén Díez Fernández 1 , Juan Gómez Barreiro 1 , Icaro Dias da Silva 3 , Emilio González Clavijo 3 , Puy Ayarza 1 1 Departamento de Geología, Universidad de Salamanca, 37008 Salamanca, Spain 2 Instituto Geológico y Minero de España, Ríos Rosas 23, 28003 Madrid, Spain 3 Instituto Geológico y Minero de España, Azafranal, 48, 37001 Salamanca, Spain 4 Geosciences, Penn State, Abington College, Abington, US *e-mail: jrmc@usal.es Late Variscan metamorphism in the Iberian Massif is characterized by high-T and low-P associations overprinting a Barrovian zonation developed during –and partly following– crustal thickening related to the Variscan collision. The high-grade rocks, reaching sillimanite-K feldspar- muscovite out parageneses, crop out at the core of gneiss domes where partial melting, migmatite development and syn-kinematic granitoids are abundant. Gneiss domes are often bounded on top by ductile detachments that, like the domes, formed during an extensional phase of deformation reflecting thermal relaxation and the subsequent collapse of the thickened continental crust. Gneiss domes occur at three domains of the Iberian Massif, the West Asturian-Leonese (WALZ), Central Iberian (CIZ) and Ossa-Morena (OMZ) zones. In the CIZ and WALZ, the migmatitic areas are disposed along a broad thermal lineament where syn-kinematic Variscan granitoids are also particularly abundant. This lineament coincides with the hinge zone of an orocline delineated by early Variscan structures of the CIZ, with a curvature opposite to that of the Ibero-Armorican arc. The existence of an orocline in central Spain marked by the trends of Variscan folds was first mentioned by Staub (1926, 1927), who named it the Castilian bend. The arc was discussed by Lotze (1929), whose influential contribution on the division of the Variscides of the Iberian Meseta (Lotze, 1945) consigned it to oblivion. Ignored for decades, Aerden (2004) noticed the arc in the patterns delineated by the Variscan folds in the Central Iberian Zone, the reason why Martínez Catalán (2011) proposed for it the name of Central Iberian arc. Aerden also pointed out that magnetic anomalies in Spain (Ardizone et al., 1989) delineate the arc in the central part of the country. The map of magnetic anomalies of the whole Iberian Peninsula depicts an inner zone with tightly folded, relatively strong anomalies, and an outer zone of faint and more openly curved magnetic lineaments. At the core of the Central Iberian arc, in NW Iberia, a huge nappe stack forms the Galicia- Trás-os-Montes Zone (GTMZ), where five allochthonous complexes and the underlying parautochthon include units derived from the northern Gondwana platform and continental edge, ophiolites marking the suture of the Rheic Ocean, and pieces of a Cambro-Ordovician ensialic island arc (Martínez Catalán et al., 2009). Coherently with its position, the lowermost thrust sheet of the GTMZ, known as the parautochthon, is in places affected by extensive migmatization related to the thermal lineament at the axis of the Central Iberian arc. The GTMZ does not presently extends to the central part of the Iberian Massif, but migmatization affects the lower parts of the autochthonous metasedimentary sequences, ranging from Neoproterozoic to early Paleozoic in age. However, not only high- temperature rocks crop out along the lineament, as migmatites and syn- to postkinematic granitoids alternate with greenschists to amphibolite facies rocks along it. In this contribution, we discuss the high-T and low-P late-Variscan evolution and the distribution of gneiss domes in the GTMZ, CIZ, and WALZ. For the domes of Lugo and Sanabria, a series of thermal models developed by Alcock et al. (2009) show that their development resulted from thermal relaxation following crustal thickening, including thickening caused by thrusting of the NW Iberian allochthon. The models also explain the main recognized pulses of granite production. Recent thermobaric estimations on the metamorphic evolution of the Spanish Central System (Rubio Pascual et al., 2012) have shown that during the Barrovian event, the rocks reached pressures largely in excess to those than can be reasonable for the thickened sedimentary pile above. An additional