Controlled source seismic surveying has long provided the best resolved images of the Earth's interior. The structure of complete orogens has been unraveled with the use of this technique, providing -continent and continent-continent collisions. In this regard, vertical incidence and refraction/wide angle seismic reflection studies have been extensively carried out in the Iberian Massif, the westernmost outcrop of the European Variscides, and in Alpine belts that exhumed this Massif (Cantabrian Mountains and Spanish Central System). These studies targeted the link between the well constrained surface geological knowledge of the orogen and its deep structure. Featuring an almost complete section of the Variscan Orogen, the Iberian Massif outcrop includes: i) the two continents involved in the collision (Laurussia/Avalonia represented by the external South Portuguese Zone, and Gondwana represented by the Central Iberian, Galicia Tras-os-Montes, West-Asturian Leone and Cantabrian zones), ii) some peri-Gondwana terranes (represented by the Ossa-Morena Zone), and iii) the sutures between them all. Four datasets (ESCIN, IBERSEIS, ALCUDIA and CIMDEF) have sampled the Iberian's Massif external and internal zones, and together they reveal four major discontinuities, three of which are observed across the entire orogen. These are the Conrad, Moho, and Hales discontinuities. The Conrad discontinuity appears as a detachment zone between the upper and the lower crust. It features a smooth geometry varying in depth from 4-5 s TWT in the external zones to 7-8 s TWT in the internal part of the Central Iberian Zone, where late Variscan extension remobilized the crust and redefined its depth. Although this discontinuity has been reactivated during the Alpine orogeny at least in the Cantabrian zone, its geometry is mostly late-Variscan. The Moho discontinuity varies from 9 s TWT in the internal zones, to 14-15 s in areas reactivated by the Alpine orogeny (Cantabrian Mountains and Spanish Central System). Also, remnants of the Variscan imbrication between the Ossa Morena and Central Iberian Zones are observed in the southern Iberian Massif, indicating that re-equilibration of the Moho was not complete at the end of the Paleozoic orogeny. The Hales discontinuity has been observed from the southernmost part of the Iberian Massif to its central part, i.e, in the mantle underlying Avalonia and Gondwana. It appears as a 2-3 s (TWT) reflective band, mostly observed at long offsets (>180 km) in wide-angle records and located at depths that vary from 60 to 80 km in models. Its image in the CIMDEF profile is probably the best example that controlled source data has provided of this discontinuity worldwide. Its geometry is smooth and does not seem to be affected by the Alpine orogeny. Finally, the lithosphere-astenosphere boundary (LAB) has also been locally imaged in the wide-angle dataset of the ALCUDIA profile. Although in this area it has been modelled at 90-100 km depth, seismic data does not allow to picture its geometry beyond this profile, and thus, no age can be deduced from its geometry. As of now, the "Big Four" lithospheric discontinuities have been observed in the Iberi three of them appear and are continuous across the Variscan suture between Laurussia and Gondwana, constraining a lower limit for its age. In addition, two of them have been affected by the Alpine orogeny redefining its geometry without modifying its character, as no thermal events have taken place since the Permian. The outstanding image that the Conrad and the Hales discontinuity have in the Iberian Massif allows to define their attributes in an effort to identify them worldwide.

Data acquisition was funded by projects GEO 90-0660 (CICYT), 1FD1997-2179/RYEN1 (CICYTFEDER), GL2007-63101/BTE and CGL2014-56548-P (MICINN). Research has also been suppported by EU grants 8711212 (EPOS-SP), 170522 (SERA), and EIT17024 (SIT4ME)