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. The allochthonous complexes of Galicia–Tras-os-Montes Zone(NW Iberia) are part of a rootless tectonic stack which preserves part of aVariscan accretionary prism. They are formed by individual tectonic slicesmarked by specific tectonometamorphic evolutions, which were piled up in apiggy-back mode onto its relative autochthon, the Central Iberian Zone(CIZ). Allochthony decreases from the structurally upper thrust sheetstowards the lower ones. The lowermost unit of the stack is known as theParautochthon or Schistose Domain. It is characterized by a low metamorphicgrade in contrast with higher temperatures and/or pressures estimated forthe overlying allochthonous units and shares the stratigraphic sequencewith the underlying autochthon. The Parautochthon is divided in twostructural and stratigraphic sub-units: (i) the Lower Parautochthon (LPa) is made ofsynorogenic flysch-type sediments with varied turbiditic units andolistostrome bodies, showing Upper Devonian–lower Carboniferous ageaccording to the youngest zircon populations and fossiliferous content; (ii) the Upper Parautochthon (UPa) is composed of highly deformed preorogenic upperCambrian–Silurian volcano-sedimentary sequence comparable with the nearbyautochthon and to some extent, also with the high-P and low-T LowerAllochthon laying structurally above. The UPa was emplaced onto the LPaalong the Main-Tras-os-Montes Thrust, and the LPa became detached fromthe CIZ relative autochthon by a regional-scale structure, the Basal LowerParautochthon Detachment, which follows a weak horizon of Siluriancarbonaceous slates. A review on the detrital zircon studies on the synorogenic LPa complementedby zircon dating of 17 new samples is presented here. The results supportthe extension of the LPa underneath the NW Iberian allochthonous complexes,from Cabo Ortegal, to Braganca and Morais massifs. Its current exposurefollows the lowermost tectonic boundary between the Galicia–Tras-os-Montes (allochthon) and Central Iberian (autochthon) zones. Theyoungest zircon age populations point to a maximum sedimentation age forthe LPa formations ranging from Famennian to Serpukhovian and supports thepiggy-back mode of emplacement of the Galicia–Tras-os-Montes Zone, ofwhich it represents the latest imbricate. The zircon age populations in the LPa allow the sedimentaryprovenance areas to be constrained, showing the intervention of nearby sources (mostly theUPa) and/or multiply recycled and long-transport sediments with a typicallynorth-central Gondwana age fingerprint, also found in the Lower Allochthon, UPaand Autochthon. Complementary geochronology of volcanic olistoliths trappedin the LPa sediments and of late Cambrian to Upper Ordovician rhyolites fromthe UPa is also presented. It shows a direct relationship between the majorblocks source area (UPa) and the setting place (LPa). Old zircon agepatterns show that the LPa sedimentary rocks were recycled from detritalrocks of the allochthon (advancing wedge) and the nearby autochthon(peripheral bulge).

solid earth

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