Mechanics of thick-skinned Variscan overprinting of Cadomian basement (Iberian Variscides)

Abstract

Remnants of the Cadomian basement can be found in the Iberian Variscides (IBVA) in several key sectors of its autochthonous units (composed of Neoproterozoic to Lower Palaeozoic metasedimentary sequences) and within the Continental Allochthonous Terrane (CAT). Comprehensive characterization of these critical exposures shows that the prevailing features are related to major geological events dated within the age range of 620-540 Ma. Indeed, near the Cambrian-Ordovician boundary, the IBVA Internal Zones experienced pervasive basement thinning and cover thickening, reflecting diffusive displacement of intracratonic rifting that continued until Lower Devonian times. In the thick-skinned Internal Zones, Helvetic/Penninic style nappes were generated, whereas flower upright axial structures developed along transpressive, intraplate shear zones. These features contrast with those preserved in the thin-skinned IBVA External Zones, dominated by décollements above (un-)deformed Palaeozoic and Cadomian basement. The inferred attenuation of rheological contrast between Cadomian basement and Palaeozoic cover can be explained by inherited fabrics due to thermal softening operated during the Cambrian-Lower Devonian extensional regime. Deeper décollements (and subsequent strain partitioning) are also expected to develop at the upper-lower crust (and at the Moho?) transition, as imaged by the available seismic profiling and MT surveys. The whole data implies a significant discontinuity between Cadomian and Variscan Cycles that should have constrained subsequent lithospheric evolution. To cite this article: A.

The Central Iberian arc, an orocline centered in the Iberian Massif and some implications for the Variscan belt

“…It has been assumed that the Badajoz-Córdoba shear zone is the older wrench structure Ribeiro et al 2007Ribeiro et al , 2009, because supposedly associated fabrics have been dated as Devonian and Early Carboniferous. But evidences of younger motion also exist.…”

Emplacement of the Lavadores granite (NW Portugal): U/Pb and AMS results

“…The Lavadores granite emplacement was controlled by one of the most important structure crossing the Variscan basement, the Porto-Tomar Shear Zone (PTSZ). The PTSZ is a north-south dextral transform fault that was active throughout much of the Devonian as well as the Variscan (Dias and Ribeiro, 1995;Ribeiro et al, 2009). Other authors propose that the PTSZ is a major fault only structured since the beginning of the Variscan orogeny (Pereira et al, 2010).…”

Petrogenesis of a late-Variscan rhyodacite at the Ossa Morena-Central Iberian zones boundary, Iberian Massif, Central Portugal: Evidence for the involvement of lithospheric mantle and meta-igneous lower crust

“…Sánchez-García et al, 2008 and references therein). A complex tectonometamorphic evolution related to the Cadomian and Variscan orogenies has been documented by several authors for the Ossa Morena Zone (Eguíluz et al, 2000;Henriques et al, 2015;Quesada and Munhá, 1990;Ribeiro et al, 2009). The tectonostratigraphic sequence identified in the area comprises the following units from bottom to top: the Série Negra Unit, the Sardoal Complex and the Mouriscas Complex ( Fig.…”

“…This contact is a key locality for the understanding of the late-Variscan magmatism, because it represents an important Neoproterozoic suture reactivated during the Variscan orogeny (e.g. Eguíluz et al, 2000;Henriques et al, 2015;Ribeiro et al, 2009). In this paper we present the first study of the rhyodacite, which includes the description of field relationships, petrography, mineral chemistry, U-Pb geochronology, whole rock geochemistry and isotopic data (Sr,␦ 18 O).…”