Evolution of the Iberian Massif as deduced from its crustal thickness and geometry of a mid-crustal (Conrad) discontinuity

Abstract

Abstract. Normal incidence seismic data provide the best images of the crust and lithosphere. When properly designed and continuous, these sections greatly contribute to understanding the geometry of orogens and, along with surface geology, unraveling their evolution. In this paper we present the most complete transect, to date, of the Iberian Massif, the westernmost exposure of the European Variscides. Despite the heterogeneity of the dataset, acquired during the last 30 years, the images resulting from reprocessing the data with a homogeneous workflow allow us to clearly define the crustal thickness and its internal architecture. The Iberian Massif crust, formed by the amalgamation of continental pieces belonging to Gondwana and Laurussia (Avalonian margin), is well structured in the upper and lower crust. A conspicuous mid-crustal discontinuity is clearly defined by the top of the reflective lower crust and by the asymptotic geometry of reflections that merge into it, suggesting that it has often acted as a detachment. The geometry and position of this discontinuity can give us insights into the evolution of the orogen (i.e., of the magnitude of compression and the effects and extent of later-Variscan gravitational collapse). Moreover, the limited thickness of the lower crust below, in central and northwestern Iberia, might have constrained the response of the Iberian microplate to Alpine shortening. Here, this discontinuity, featuring a Vp (P-wave velocity) increase, is observed as an orogen-scale boundary with characteristics compatible with those of the globally debated Conrad discontinuity.

Four decades of geophysical research on Iberia and adjacent margins

“…The sinking of part of the upper will then explain why the Tagus Basin has lower topography than the Duero Basin. Ayarza et al (2021a) have interpreted the mid-crustal discontinuity identified throughout the Iberian Massif as the rheological boundary between the upper and lower crust pointing that its geometry, position and extent are consistent with the characteristics of the Conrad discontinuity.…”

“…The Moho appears as a near horizontal discontinuity at approximately 30-33 km depth. The compressional tectonics in the Central Iberian System, has been imaged using a dense wide-angle seismic reflection data profile (Ayarza et al, 2021a), global phase interferometry (Andrés et al, 2019) and ambient seismic noise (Andrés et al, 2020). These data constrain a normal velocity model with velocities of 6.0-6.2 km/s, increasing to 7.0 km/s in the lower crust and a limited root under the core of the Central Iberian system reaching locally a depth of 38 km.…”

Crustal Imbrication in an Alpine Intraplate Mountain Range: A Wide‐Angle Cross‐Section Across the Spanish‐Portuguese Central System

“…This pattern is not observed in other ranges formed in the borders of the Iberian microplate, that is, the Cantabrian Mountains, where Teixell et al (2018) proposed the preservation of the mid-crustal detachment allowed for underthrusting of the lower crust. Accordingly, we suggest that the crustal structure and deformation mechanisms affecting Gredos during the Alpine contraction are significantly controlled by the inferred volume of granitoids at depth, playing a role in the nucleation of the SPCS (de Vicente et al, 2018), and enhancing coupled Alpine deformation between the upper and lower crust (Ayarza et al, 2021).…”

“…The structure of the SPCS has attracted significant attention mainly due to its high topography in the interior of a tectonic plate. So far, research efforts have focused on finding models to explain the observed intraplate deformation (Cloetingh et al, 2002;De Vicente et al, 1996;de Vicente & Vegas, 2009;Vegas & Suriñach, 1987), the different tectonic and stratigraphic evolution of its foreland basins (de Vicente & Muñoz-Martín, 2013;Fernández-Lozano et al, 2011;Silva et al, 2017), its lithospheric structure (Andrés et al, 2019(Andrés et al, , 2020Ayarza et al, 2021), and the tectonic origin of its topography (Casas-Sainz & de Vicente, 2009).…”

“…The second model proposed the formation of a crustal-scale south-vergent thrust system that exhumed the SPCS. This model implies the disruption of a pre-existing mid-crustal detachment and allows coupled deformation between the upper and lower crust (Andrés et al, 2019(Andrés et al, , 2020Ayarza et al, 2021).…”