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

Abstract

[EN]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.