A tectonic carpet of Variscan flysch at the base of a rootless accretionary prism in northwestern Iberia: U–Pb zircon age constrains from sediments and volcanic olistoliths
Authors: González Clavijo Emilio , Dias da Silva Ícaro , Martínez Catalán José R. , Gómez Barreiro Juan , Gutiérrez-Alonso Gabriel , Díez Montes Alejandro , Hofmann Mandy , Gärtner Andreas , Linnemann Ulf 
Published: April 2021

Abstract. The allochthonous complexes of Galicia–Trás-os-Montes Zone
(NW Iberia) are part of a rootless tectonic stack which preserves part of a
Variscan accretionary prism. They are formed by individual tectonic slices
marked by specific tectonometamorphic evolutions, which were piled up in a
piggy-back mode onto its relative autochthon, the Central Iberian Zone
(CIZ). Allochthony decreases from the structurally upper thrust sheets
towards the lower ones. The lowermost unit of the stack is known as the
Parautochthon or Schistose Domain. It is characterized by a low metamorphic
grade in contrast with higher temperatures and/or pressures estimated for
the overlying allochthonous units and shares the stratigraphic sequence
with the underlying autochthon. The Parautochthon is divided in two
structural and stratigraphic sub-units: (i) the Lower Parautochthon (LPa) is made of
synorogenic flysch-type sediments with varied turbiditic units and
olistostrome bodies, showing Upper Devonian–lower Carboniferous age
according to the youngest zircon populations and fossiliferous content; (ii) the Upper Parautochthon (UPa) is composed of highly deformed preorogenic upper
Cambrian–Silurian volcano-sedimentary sequence comparable with the nearby
autochthon and to some extent, also with the high-P and low-T Lower
Allochthon laying structurally above. The UPa was emplaced onto the LPa
along the Main-Trás-os-Montes Thrust, and the LPa became detached from
the CIZ relative autochthon by a regional-scale structure, the Basal Lower
Parautochthon Detachment, which follows a weak horizon of Silurian
carbonaceous slates. A review on the detrital zircon studies on the synorogenic LPa complemented
by zircon dating of 17 new samples is presented here. The results support
the extension of the LPa underneath the NW Iberian 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. The
youngest zircon age populations point to a maximum sedimentation age for
the LPa formations ranging from Famennian to Serpukhovian and supports the
piggy-back mode of emplacement of the Galicia–Trás-os-Montes Zone, of
which it represents the latest imbricate. The zircon age populations in the LPa allow the sedimentary
provenance areas to be constrained, showing the intervention of nearby sources (mostly the
UPa) and/or multiply recycled and long-transport sediments with a typically
north-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 late Cambrian to Upper Ordovician rhyolites from
the UPa is also presented. It shows a direct relationship between the major
blocks 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).