Early carboniferous wrenching, exhumation of high-grade metamorphic rocks and basin instability in SW Iberia: Constraints derived from structural geology and U–Pb and 40Ar–39Ar geochronology

摘要

New U–Pb and 40Ar–39Ar geochronology and structural data from high- tomedium grade metamorphic shearzones of the Ossa-Morena Zone, and structural data from Early Carboniferous basins (Ossa-Morena Zone andSouth-Portuguese Zone), place additional constraints on the Variscan tectonics in SW Iberia. A zircon U–Pbage of 465±14Ma (Middle Ordovician) measured on migmatite from the Coimbra–Cordoba shear zone isinterpreted as the age of protolith crystallization. This age determination revises the information containedin the geological map of Portugal, in which these rocks were considered to be Proterozoic in age.This paper describes the evolution of Variscan wrench tectonics related to the development of shear zones,exhumation of deep crustal rocks and emplacement of magma in the Ossa-Morena Zone basement. In theCoimbra–Cordoba shear zone (transpressional), migmatites were rapidly exhumed from a depth of 42.5kmto 16.6km over a period of ca. 10Ma in the Viséan (ca. 340–330Ma), indicating oblique slip exhumationrates of 8.5 to 10.6mm/yr (Campo Maior migmatites) and 3.2mm/yr (Ouguela gneisses) respectively. Inthe Évora Massif, the gneisses of the Boa Fé shear zone (transtensional) were exhumed from 18.5 to 7.4kmdepth in the period ca. 344–334Ma (Viséan), with exhumation oblique slip rates of 2.8 to 4.2mm/yr. Atthe same time, the Early Carboniferous basins of SW Iberia were filled by turbidites and olistoliths, composedmostly of Devonian rocks. The presence of olistoliths indicates significant tectonic instability during sedimen-tation with large-scale mass movement, probably in the form of gravity slides. Deformation and metamor-phism dated at 356±12Ma, 321±13Ma and 322±29Ma respectively suggests that Variscan wrenchmovements were active in SW Iberia during the Early Carboniferous for a period of at least 35Ma.