Articles | Volume 7, issue 2
Solid Earth, 7, 579–598, 2016

Special issue: From orogenesis to geoscience in the service of society: the...

Solid Earth, 7, 579–598, 2016

Research article 13 Apr 2016

Research article | 13 Apr 2016

Insights on high-grade deformation in quartzo-feldspathic gneisses during the early Variscan exhumation of the Cabo Ortegal nappe, NW Iberia

Francisco José Fernández1, Sergio Llana-Fúnez1, Pablo Valverde-Vaquero2, Alberto Marcos1, and Pedro Castiñeiras3 Francisco José Fernández et al.
  • 1Departamento de Geología, Universidad de Oviedo, Jesús Arias de Velasco s/n, 33005 Oviedo, Spain
  • 2Área de Laboratorios, Instituto Geológico y Minero de España, La Calera 1, 28760 Tres Cantos, Spain
  • 3Departamento de Petrología y Geoquímica, Universidad Complutense de Madrid, José Antonio Novais 12, 28040 Madrid, Spain

Abstract. High-grade, highly deformed gneisses crop out continuously along the Masanteo peninsula and constitute the upper part of the lower crustal section in the Cabo Ortegal nappe (NW Spain). The rock sequence formed by migmatitic quartzo-feldspathic (qz-fsp) gneisses and mafic rocks records the early Ordovician (ca. 480–488 Ma) injection of felsic dioritic/granodioritic dykes at the base of the qz-fsp gneisses, and Devonian eclogitization (ca. 390.4 ± 1.2 Ma), prior to its exhumation. A SE-vergent ductile thrust constitutes the base of quartzo-feldspathic gneissic unit, incorporating mafic eclogite blocks within migmatitic gneisses. A NW-vergent detachment displaced metasedimentary qz-fsp gneisses over the migmatites. A difference in metamorphic pressure of ca. 0.5 GPa is estimated between both gneissic units. The tectono-metamorphic relationships of the basal ductile thrust and the normal detachment bounding the top of the migmatites indicate that both discrete mechanical contacts were active before the recumbent folding affecting the sequence of gneisses during their final emplacement. The progressive tectonic exhumation from eclogite to greenschist facies conditions occurred over ca. 10 Ma and involved bulk thinning of the high-grade rock sequence in the high pressure and high temperature (HP–HT) Cabo Ortegal nappe. The necessary strain was accommodated by the development of a widespread main foliation, dominated by flattening, that subsequently localized to a network of anastomosing shear bands that evolved to planar shear zones. Qz-fsp gneisses and neighbouring mafic granulites were exhumed at > 3 mm yr−1, and the exhumation path involved a cooling of  ∼  20 °C/100 MPa, These figures are comparable to currently active subduction zones, although exhumation P–T trajectory and ascent rates are at the hotter and slower end in comparison with currently active similar settings, suggesting an extremely ductile deformation environment during the exhumation of qz-fsp gneisses within a coherent Cabo Ortegal nappe.

Short summary
High-grade gneisses record two partial melting events: early Ordovician and early Variscan. Primary sedimentary layering is well preserved locally at the top of the sequence. First stage of the exhumation process occurred in ∼ 10 Ma. Strain was progressively localized along the boundaries of the migmatitic qz-fsp gneisses. Deformation reduced substantially the thickness of the gneissic rock sequence. "Internal" extrusion of the migmatites is documented.