This region encompasses the center-west area of the Iberian Peninsula. It extends in the E-W direction along 400 km approximately, from the central part of the Guadarrama Sierra that belongs to the Central System Mountain Range, to the Portuguese Atlantic coast. The Iberian Massif governs this area, and its main relief, the Central System Mountain Range, crosses this region from east to west. Moreover, there are two remarkable sedimentary basins, to the west in Portugal, next to the Atlantic coast, the Lusitanian Basin, and in the northeast portion, the Duero River basin in the Salamanca, Avila, Zamora, Segovia and Valladolid provinces. The Tajo River basin is the southern limit of the region.

Geologically speaking, this region mainly corresponds to the central portion of the Iberian Massif, which is the group of precambrian and paleozoic rocks that were mostly deformed and metamorphized during the Hercynian or Variscan orogeny. This orogeny was the origin of a mountain belt, currently quite eroded and partially buried, that in Europe extends along over 3,000 km from the Guadalquivir River to Central Europe and the Balkans. The Iberian Massif is the southern portion of this mountain belt, and it is the best-known record of the Hercynian orogeny effects.

The Iberian Massif is divided into five different zones. From east to west and from north to south these zones are, the Cantabrian zone, the Western-Asturian-Leonese zone, the Center-Iberian zone, where it is included the Galicia-Trás-Os-Montes subregion, the Ossa-Morena zone and the Subportuguese zone.

The Center-Iberian zone governs this region. It is the axial part of the Iberian Massif, and therefore, where the intensity of the metamorphism and the presence of granite intrusions are the highest. Also, it includes the Central System Mountain Range, this landform was part of a relevant relief beyond the Paleozoic that always emerged over the sea level.

The Iberian Massif dominates the rock outcrops in the western part of the Iberian Peninsula, and it is buried below the Duero River basin and the Tajo River basin where it acts as the basement. The sediment infill of these two Cenozoic basins is mainly continental and clastic, and its generation and deposition are linked to the Alpine orogeny.

The Iberian Massif is as well the basement in the Lusitanian Basin in Portugal, but this basin is Mesozoic, and its sediment infill shows the evolution from a continental to a marine environment along the Triassic and the Jurassic periods.