Thickening and exhumation of the Variscan roots in the Iberian Central System: Tectonothermal processes and 40Ar/39Ar ages

Abstract

40Ar/39Ar dating of white mica inmetapelites, thermobarometric calculations and new structural and geological mapping have been carried out in the Somosierra region (Iberian Central System, ICS) to investigate the Variscan thickening–exhumation processes and the evolution of the paleo-thermal structure in a key region of the Central Iberian Zone. The rocks underwent an intermediate P/T event (M1) during a D1 stage of crustal thickening initiated between 354 and 347 Ma. This eventwas probably related to a first S to SE-directed emplacement of theNW Iberian allochthon, and during the initial development of amid-crustal subhorizontal shear zone accommodating the ductile flow of thermally weakened crust parallel to the main orogenic trend. Syn-convergent extension of the upper levels occurred by 335–327 Ma, when tectonic unroofing led to D2 domal uplift of the orogenic roots and to recrystallization of rocks under intermediate to low P/T conditions (M2). Individualized gneiss domes developed along the orogenic trend, whereas some post-M2 thrusts dated at c. 319 Ma reflect lateral flow related to limited gravitational spreading of the domes. Younger upright to SW verging folding of the ICS dome during D3 progressive renewal of contractive deformation took place after 316 Ma. Final D4 gravitational collapse occurred under low P/T conditions (M3) at c. 300 Ma, accompanied by the intrusion of large volumes of anatectic granitoids.