Phase 8: The Early Pyrenean Orogeny

This phase encompasses two stages, the first of which being represented by a short time interval between anomaly 34 (83.5 Ma, Santonian-Campanian boundary) and 80.2 Ma (Middle Campanian), whereas the second one lasted at 74.3 Ma (C33n, Upper Campanian). A uniform velocity field (Fig. 19) characterized the western regions, from the northern margin of Iberia to Western Pontides, because no relative motion occurred between the Mediterranean microplates (including Iberia) and Africa during this phase. To the East, subduction of the Inner Tauride Ocean beneath the Sakarya-Kirsehir block continued to be constrained by the ENE-directed field of velocity of Africa relative to this block. An interesting feature of the reconstruction of Figure 19 is represented by the westward prolongation of the Alpine trench, in the region presently occupied by the Pyrenean orogen. The observation of recent seismic reflection profiles interpretations [Teixell, 1998 ; Beaumont et al., 2000] suggests that this early stage of compression was accommodated by thrusting of continental crust of Iberia onto Eurasia along the North Pyrenean Frontal Thrust (NPFT). If this scenario is correct, the south-dipping Alpine subduction zone can be traced further West without inversion of polarity, as indicated in Figure 19. However, independently from these structural details, we observe that the onset of the Pyrenean orogeny was not related to the opening of the Biscay Bay and the rotation of Iberia, but occurred after the completion of this process.

Figure 19. Reconstruction at 74.3 Ma (Upper Campanian)

Reconstruction at 74.3 Ma (Upper Campanian)

Vectors represent direction and magnitude of the relative velocity fieldof Africa relative to Eurasia. Blue lines represent the modeled 172.0 Ma, 170.0 Ma, M25, M21, M16 and M10 isochrons. The 130.0 Ma, M4 and M0 isochrons are indicated in green. Dashed red lines represent areas of incipient extension.

The acceleration field at the Upper Campanian boundary is shown in Figure 20. We note the absence of variations of relative velocity at the convergent boundaries of the Northern Mediterranean region. This indicates that the main trench systems reached a state of equilibrium at the end of the phase. Conversely, the Inner Tauride Trench was still in a highly rotational compression field, as illustrated in Figure 20. The principal feature of this map is the weak northwest-directed variation of velocity of Eurasia and Iberia with respect to Africa, which implies either the onset of an extensional stress field in the Southern Mediterranean region (if Adria and Southern Turkey are kept fixed to Iberia) or extension at the western margin of Adria. Geologic data from the Eastern Mediterranean (Antalya Complex and Kyrenia Range) seems to indicate spreading episodes during the Late Cretaceous [Robertson, 1998], whereas no evidence of extensional phenomena comes from the Apenninic domain. Hence, we will assume that Upper Campanian rifting and possibly spreading affected the Southern Mediterranean region, immediately prior of the onset of subduction in the easternmost areas.

Figure 20. Acceleration field of Iberia, Adria and Southern Turkey with respect to Africa at 74.3 Ma (Upper Campanian)

Acceleration field of Iberia, Adria and Southern Turkey with respect to Africa at 74.3 Ma (Upper Campanian)

Arrows South of Sakatya and Kirsehir indicate acceleration of subduction rate of the Inner Tauride Ocean. Dashed lines indicate zones of extension.