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, together with surface geology, to unravel their evolution. In this paper we present an almost complete transect 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 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 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 on the evolution of the orogen, i.e. of the effects and extent of the late Variscan gravitational collapse. Also, its position and the limited thickness of the lower crust in central and NW Iberia constraints the response of the Iberian microplate to Alpine shortening. This discontinuity is here observed as an orogeny-scale feature with characteristics compatible with those of the worldwide, Conrad discontinuity.

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

“…Based on normal incidence and WA seismic reflection data acquired along the Iberian Massif, one orogen‐scale conspicuous mid‐crustal feature has been identified and related to the Conrad discontinuity (Ayarza et al., 2021). This boundary might have often acted as a detachment level during the Variscan deformation but also during the Alpine contraction, given the contrasting reflectivity patterns existing between the upper and the lower crust.…”

“…The crust‐mantle boundary is essentially flat in the Iberian Massif except when there are remnants of Variscan imbrications or it is affected by the Alpine contraction (Ayarza et al., 2021; Martínez Poyatos et al., 2012; Simancas et al., 2013). A generally flat subhorizontal Moho is identified at depths of 31–33 km (Ehsan et al., 2014; Palomeras et al., 2009; Palomeras, Ehsan, et al., 2021) in the southern CIZ, Ossa‐Morena zone, and South Portuguese zone.…”

Geometry and deformation mechanisms of the Central Iberian Zone and the Iberian Central System

“…Furthermore, this boundary separates areas with different patterns of deformation suggesting that the former have might acted as a detachment. The seismic discontinuity is further discussed and defined as a generalized feature in Ayarza et al (2020). Estimates of shortening at upper-and lower crust levels suggest that this detachment might have accommodated most of the deformation (Martínez Poyatos et al, 2012;Simancas et al, 2013).…”

“…and have interpreted it as the tectonic imprint on the lower crust of the pre-Variscan and Variscan deformation (Ayarza et al, 2020).…”

“…However, ductile deformation is observed in upper crustal outcropping rocks along the IM, (e.g., in the Schist-Greywacke complex in the IM). More recently, Ayarza et al (2020) argue that this boundary represents a detachment that acts as a rheological boundary between an old and long deformed lower crust and the rocks on top. However, as suggested above, this discontinuity is erased below the ICS as crustal melting and granitoids development have assimilated it underneath the mountain range.…”