Publication Date:
2009-12-22
Description:
Following a Middle-Late Devonian (c. 390-360 Ma) phase of crustal shortening and mountain building, continental extension and onset of high-medium-grade metamorphic terrains occurred in the SW Iberian Massif during the Visean (c. 345-326 Ma). The Evora-Aracena-Lora del Rio metamorphic belt extends along the Ossa-Morena Zone southern margin from south Portugal through the south of Spain, a distance of 250 km. This major structural domain is characterized by local development of high-temperature-low-pressure metamorphism (c. 345-335 Ma) that reached high amphibolite to granulite facies. These high-medium-grade metamorphic terrains consist of strongly sheared Ediacaran and Cambrian-early Ordovician (c. 600-480 Ma) protoliths. The dominant structure is a widespread steeply-dipping foliation with a gently-plunging stretching lineation generally oriented parallel to the fold axes. Despite of the wrench nature of this collisional orogen, kinematic indicators of left-lateral shearing are locally compatible with an oblique component of extension. These extensional transcurrent movements associated with pervasive mylonitic foliation (c. 345-335 Ma) explain the exhumation of scarce occurrences of eclogites (c. 370 Ma). Mafic-intermediate plutonic and hypabyssal rocks (c. 355-320 Ma), mainly I-type high-K calc-alkaline diorites, tonalites, granodiorites, gabbros and peraluminous biotite granites, are associated with these metamorphic terrains. Volcanic rocks of the same chemical composition and age are preserved in Tournaisian-Visean (c. 350-335 Ma) marine basins dominated by detrital sequences with local development of syn-sedimentary gravitational collapse structures. This study, supported by new U-Pb zircon dating, demonstrates the importance of intra-orogenic transtension in the Gondwana margin during the Early Carboniferous when the Rheic ocean between Laurussia and Gondwana closed, forming the Appalachian and Variscan mountains.
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