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|Title: ||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|
|Authors: ||Pereira, M. Francisco|
Silva, J. Brandão
Lee, James K.W.
Williams, Ian S.
|Keywords: ||Variscan belt|
Timing of deformation
South Portuguese Zone
|Issue Date: ||2012|
|Abstract: ||New U–Pb and 40Ar–39Ar geochronology and structural data from high- tomedium grade metamorphic shear
zones of the Ossa-Morena Zone, and structural data from Early Carboniferous basins (Ossa-Morena Zone and
South-Portuguese Zone), place additional constraints on the Variscan tectonics in SW Iberia. A zircon U–Pb
age of 465±14Ma (Middle Ordovician) measured on migmatite from the Coimbra–Cordoba shear zone is
interpreted as the age of protolith crystallization. This age determination revises the information contained
in 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 the
Coimbra–Cordoba shear zone (transpressional), migmatites were rapidly exhumed from a depth of 42.5km
to 16.6km over a period of ca. 10Ma in the Viséan (ca. 340–330Ma), indicating oblique slip exhumation
rates of 8.5 to 10.6mm/yr (Campo Maior migmatites) and 3.2mm/yr (Ouguela gneisses) respectively. In
the Évora Massif, the gneisses of the Boa Fé shear zone (transtensional) were exhumed from 18.5 to 7.4km
depth in the period ca. 344–334Ma (Viséan), with exhumation oblique slip rates of 2.8 to 4.2mm/yr. At
the same time, the Early Carboniferous basins of SW Iberia were filled by turbidites and olistoliths, composed
mostly 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 wrench
movements were active in SW Iberia during the Early Carboniferous for a period of at least 35Ma.|
|Appears in Collections:||GEO - Publicações - Artigos em Revistas Internacionais Com Arbitragem Científica|
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