The Variscan basement in the western shoulder of the Lusitanian Basin (West Iberian Margin): insights from detrital-zircon geochronology of Jurassic strata

Abstract

There is no consensus about the geological nature of the westernmost portion 15 of the Iberian Massif. In the present research, the detrital zircon U-Pb signatures of 16 Jurassic strata of the Lusitanian Basin, known to be west-sourced, are combined with 17 published U-Pb data for the Precambrian-Palaeozoic basement and the Lusitanian Basin 18 units to better understand this poorly exposed portion of the Iberian Massif. Cryogenian to 19Ediacaran ages prevail in a northern Upper Jurassic unit, while Lower and Upper Jurassic 20 rocks in southern locations yield mostly Carboniferous to upper Permian zircons. These 21 age results, coupled with their respective U/Th ratios, suggest that the basin covers two 22 distinct terranes of the Iberian Massif. Another noteworthy feature of west-derived 23 deposits is the abundance of <310 Ma ages. It is proposed that a combination of crustal 24 thinning in the West Iberian Margin with regional eastward basement tilt, favoured the 25 enrichment of relatively young zircon in the western shoulder of the basin relative to its 26 eastern margin. The detrital zircon age signatures also reveal a middle to late Permian 27 thermal event in restricted areas, which is probably associated with the oldest stages of 28 Alpine extension in West Iberia.

Networks of geometrically coherent faults accommodate Alpine tectonic inversion offshore SW Iberia

“…Some of these fragments are derived from both easterly and westerly sediment sources, show evidence of having been recycled from eroded sediment. They reflect the presence of basement lithologies that are different from the Variscan units now outcropping in Portugal and Spain (Dinis et al, 2021).…”

“…Tectonic and geophysical data suggest the deep-offshore basins to be underlain by a tectonic terrain not identified onshore (Ribeiro et al, 2013), while other authors suggest these onshore Variscan terrains to extend oceanwards, under offshore sedimentary basins, complying with the general NW to ESE strikes of main faults and depocentres recognised in, and around, the Lusitanian Basin (Capdevilla and Mougenot, 1988). Recent Apatite fission-track analyses have shown sediment in the Lusitanian Basin to comprise a mix of lithological fragments of late Cryogenian to Ediacaran ages (Pan-African and/or Cadomian with peaks at 608-554 Ma) and Carboniferous to Permian ages (Variscan and post-Variscan, with peaks at 315-292 Ma) (Dinis et al, 2021). Some of these fragments are derived from both easterly and westerly sediment sources, show evidence of having been recycled from eroded sediment.…”

Detrital zircon age signatures of the Mesozoic in the Lusitanian Basin and implications for the evolution of Iberia–Newfoundland conjugate margins

“…High abundance of this age population in LB units formed at different latitudes indicates an association with thermal events affecting different terranes. The exhumation of their primary sources must have been conditioned by major fault structures that deeply cut the Variscan Orogen, such as those responsible for the western borders of the LB (Dinis et al, 2021) and those that control its eastern margin, accounting for the abundance of ~305-280 Ma ages in Triassic to Upper Cretaceous strata (Figure 2). These rocks necessarily have high zircon fertility to conceal the age signal of older thermal events.…”

“… Note : (1) Dinis et al (2016); (2) Dinis et al (2021); (3) Pereira et al (2016); (4) Dinis et al (2018); Pereira et al (2017b); (6) Lowe et al (2011); (7) Beranek et al (2022); (8) Hutter and Beranek (2020); (9) Piper et al (2012). …”