Extension was the predominant mechanism of deformation in the Iberian Plate during the northern propagation of the opening of the Atlantic Ocean during Mesozoic times as well as in the rest of Western Europe. The Iberian Plate moved independently from Africa and Europe along the Azores-Gibraltar plate boundary in the S and the North Pyrenean Fault Zone in the N (e.g., Srivastava et al., 1990; Olivet, 1996). Rift systems and final continental break-up developed along the western margin of the Iberian Plate (offshore Galicia and Portugal; e.g., Malod and Mauffret, 1990), within the continental plate (Iberian and Catalan rifts; e.g., Salas et al., 2001), and along the two plate boundaries (Betics and Pyrenean rifts in the S and N, respectively), (Fig. 3).
Figure 3. Restored map
Restored map at the end of Late Cretaceous times to show the distribution of principal sedimentary basins along the Pyrenean rift system. This reconstruction combines the plate reconstruction by Olivet (1996) and results of crustal-scale transect reconstruction by Vergés and García Senz (2001). Abbreviations: Ca: Cameros Basin, Ma: Maestrat Basin, Col: Columbretes Basin, F-M: Figueres-Montgrí Basin. Position and extend of Columbretes Basin after (Roca, 1996a). Sardinia is shown in its restored position prior to Neogene opening of the Gulf of Lyons (Olivet, 1996). The Balearic Islands are shown in their restored position, prior to Neogene opening of the Valencia Trough (Vergés and Sàbat, 1999).
In this large-scale plate tectonics framework, the Pyrenean rift took place in an ESE-WNW trending branch and culminated with the continental separation between Iberia and Europe and the opening of the Bay of Biscay along its western segment (e.g., Le Pichon and Barbier, 1987; Pinet et al., 1987). The Pyrenean rift connected the Bay of Biscay with the Tethys Ocean to the E (Fig. 3).
The initiation of the Pyrenean rift took place synchronously to the opening of the North Atlantic starting in Triassic times at about 250 Ma (Ziegler, 1990). Two periods of rifting (late Jurassic-Early Cretaceous and late Barremian-Albian times) have thinned the crust between Iberia and Europe controlling the formation of most important Pyrenean extensional basins: (Fig. 4), (e.g., Vergés and García Senz, 2001).
Figure 4. Major extensional geodynamic events
Major extensional geodynamic events developed during the Early and Late Cretaceous. (from Vergés and García Senz, 2001). Time scale after Gradstein et al. (1995).
Several major concurrent geodynamic events took place from middle Albian to the end of Cenomanian along the Pyrenean rifted region (Fig. 4):
(a) end of extension in all sedimentary basins,
(b) opening of the Bay of Biscay (García-Mondéjar, 1996; Le Pichon and Barbier, 1987),
(c) anticlockwise 35¼ rotation of the Iberian Plate,
(d) development of pull-apart basins linked to the transtensional motion of the Iberian Plate boundary (the present-day North Pyrenean Fault Zone; Choukroune and Mattauer, 1978),
(e) magmatic events, metamorphism and emplacement of upper mantle slices into the pull-apart basins (Fabriès et al., 1998; Montigny et al., 1992).
Away from the transcurrent North Pyrenean Fault Zone, post-rift development was characterised by the thermal relaxation of the lithosphere widening the former rift basins with opposed fault systems. These syn-rift faults were later inverted during compression becoming the Pedraforca-South Central and the Basco-Cantabrian units to the S and the NW Pyrenean thrust system to the S of the Lacq-Mauleon basins.
Differential vertical motions across the boundaries of these Mesozoic basins are determined by subsidence analysis from the Lacq Basin (Brunet, 1984), and the Landes High and the Basco-Cantabrian Basin (Gómez et al., 2002), (Fig. 5). The late Barremian-Albian rift and post-rift tectonic subsidence curves of the Lacq and Basco-Cantabrian basins show a similar evolution. On the contrary, the Landes High experienced uplift during the same period. The Basco-Cantabrian Basin was affected by renewed subsidence during early stages of compression and then was uplifted above the thrust system, which produced flexural subsidence in the Landes High foreland basin to the N. The Lacq Basin experienced moderate subsidence in the early stages of basin inversion. The initial hydrocarbons generation started during Cenomanian times (e.g., Bourrouilh et al., 1995; Le Vot et al., 1996). However, the tectonic inversion and uplift of the Basco-Cantabrian Basin produced the total exhumation of the petroleum systems after Paleocene-Eocene times (Vergés and García Senz, 2001; Gómez et al., 2002), whereas they were preserved in the Lacq Basin.
Figure 5. Subsidence curves
Subsidence curves for the Lacq Basin (from Brunet, 1984), the Landes High and the Basco-Cantabrian Basin (from Gómez et al., 2002), showing differential tectonic evolutions, which controlled the petroleum generation and preservation.