Preprints
https://doi.org/10.5194/se-2020-173
https://doi.org/10.5194/se-2020-173

  15 Oct 2020

15 Oct 2020

Review status: a revised version of this preprint is currently under review for the journal SE.

A tectonic carpet of Variscan flysch at the base of an unrooted accretion prism in NW Iberia: U-Pb zircon age constrains from sediments and volcanic olistoliths

Emilio González Clavijo1, Ícaro Dias da Silva2,3, José R. Martínez Catalán4, Juan Gómez Barreiro4, Gabriel Gutíerrez Alonso4, Alejandro Díez Montes1, Mandy Hofmann5, Andreas Gärtner5, and Ulf Linnemann5 Emilio González Clavijo et al.
  • 1Instituto Geológico y Minero de España. Plaza de la Constitución 1, 3º, 37001 Salamanca, Spain
  • 2Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
  • 3Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, Lisboa, 1749-016, Portugal
  • 4Departamento de Geología, Universidad de Salamanca. Plaza de la Merced, s/n, 37008 Salamanca, Spain
  • 5Senckenberg Naturhistorische Sammlungen Dresden, Königsbrücker Landstr. 159, D-01109 Dresden, Germany

Abstract. The allochthonous complexes of Galicia – Trás-os-Montes Zone (NW Iberia) are part of the tectonic stack that unrooted the Variscan accretionary prism. They are formed by individual tectonic slices marked by specific tectono-metamorphic evolution, which was piled up in a piggy-back thrust complex onto its relative autochthon, the Central Iberian Zone (CIZ). Consequently, allochthony decreases towards lower, more external and younger thrust sheets. The lowermost unit of this pile of slivers is known as Schistose Domain or Parautochthon and bears low metamorphic grade, contrasting with the higher temperatures and pressures estimated for the upper allochthonous units, but sharing the stratigraphic sequence with the underlying autochthon. The Parautochthon is divided in two structural and stratigraphic sub-units: (i) the Lower (LPa) made of synorogenic flysch-type sediments with varied turbiditic units and olistostrome bodies, showing Upper Devonian-lower Carboniferous age on base of the youngest zircon populations and fossiliferous content; (ii) the Upper (UPa), composed of highly deformed pre-orogenic upper Cambrian-Silurian volcano-sedimentary sequence comparable with both the nearby autochthon and the HP-LT Lower Allochthon, laying structurally above. The UPa thrusted onto the LPa by the the Main-Trás-os-Montes Thrust; and the LPa detached from the CIZ relative autochthon by a regional structure (Basal Lower Parautochthon Detachment) which follows the favourable Silurian carbonaceous beds.

A review on the detrital zircon studies of the synorogenic LPa complemented by 17 new samples geochronology is here presented. The results support the extension of the LPa underneath the NW Iberia allochthonous complexes, from Cabo Ortegal, to Bragança and Morais Massifs. Its current exposure follows the lowermost tectonic boundary between the Galicia – Trás-os-Montes (allochthon) and Central Iberian (autochthon) Zones. Youngest zircon age populations point to a maximum sedimentation age for the LPa formations ranging from Famennian to Serpukhovian and endorse the piggy-back evolution inside this unit, mimicking the general structure of the Galicia – Trás-os-Montes Zone.

The zircon populations in the LPa allow constraining the sedimentary provenance areas, showing the intervention of nearby sources (mostly the UPa) and/or multiply recycled/long transport sediments with typically N-Central Gondwana age fingerprint, also found in the Lower Allochthon, UPa and Autochthon. Complementary geochronology of volcanic olistoliths trapped in the LPa sediments and of upper Cambrian to Upper Ordovician rhyolites from the UPa is also presented, showing a direct relation between the major block's source area (UPa) and the setting place (LPa). Old zircon age patterns show that the LPa sedimentary rocks were recycled from detrital rocks of the allochthon (advancing wedge) and the nearby autochthon (peripheral bulge).

Emilio González Clavijo et al.

 
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Emilio González Clavijo et al.

Emilio González Clavijo et al.

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Short summary
In NW Iberia a low grade unit separates the unrooted accretionary prism from the autochthon. It was divided in Upper (pre-orogenic) and Lower (syn-orogenic) around the Morais Complex. A review on the sedimentary sequences characteristics, supported by U-Pb ages in 17 new samples of syn-orogenic sediments and volcanic olistoliths, allows to propose this model is general for the Galicia – Trás-os-Montes Zone forming a tectonic carpet exposed in most of the Parautochthon-Autochthon limit.