Phase 6: The early Alpine subduction

This time interval comprises a single stage between anomaly M0 (120.4 Ma, Barremian-Aptian boundary) and 110.0 Ma (Lower Albian). The reconstruction of the velocity and acceleration fields at 110.0 Ma is illustrated in Figures 15-16. As shown in Figure 16, the younger boundary of this stage is not associated to important variations of the relative velocity of Africa with respect to Eurasia. However, the reconstruction shows some important features of the Alpine tectonics during the Cretaceous Quiet Zone.

Figure 15. Reconstruction at 110.0 Ma (Lower Albian)

Reconstruction at 110.0 Ma (Lower Albian)

Vectors represent direction and magnitude of the relative velocity field between conjugate pairs of plates. 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. The location of the Euler pole of relative convergence between Northeast Africa and Eurasia is marked by a small empty circle.

A first important feature of this time period is represented by the onset of seafloor spreading in the North and South Atlantic, and by the opening of the Biscay Bay [Srivastava et al., 1990]. In particular, spreading rates were high in the South Atlantic (~43 mm/yr at the Romanche Fracture Zone and ~63 mm/yr at the Agulhas-Falkland Shear Zone) and the Central Atlantic (55 mm/yr in the southernmost region). Simultaneously, the African plate experienced internal deformation with the onset of right-lateral motion along the CASZ and rifting episodes in the Benue Trough and Termit Basins [Wilson and Guiraud, 1992 ; Genik, 1992 ; Guiraud and Maurin, 1992].

Figure 16. Acceleration field of Africa withrespect to Eurasia at 110.0 Ma (Lower Albian).

Acceleration field of Africa withrespect to Eurasia at 110.0 Ma (Lower Albian).

Acceleration field of Africa withrespect to Eurasia at 110.0 Ma (Lower Albian).

Regarding the Tethyan realm, we note the establishment of two different trench systems, which were presumably separated by a transform fault (Fig. 15). In the Western Mediterranean, old oceanic crust of the Pennine Ocean (Eurasian plate) was subducting southwards under the active margin of Adria. In the Eastern regions the old Jurassic trench of Sanandaj-Armenia-East Pontides extended further west to reach the West Pontides and Rodophe, where Jurassic oceanic crust was subducting northwards. Convergence rates and styles of subduction indicate a strong differentiation between these two trench systems. In general, we observe an eastward increase of the subduction rate, starting from the ~5.2 mm/yr at the highly oblique trench of the Valais Trough through the ~18.0 mm/yr of the Eastern Alpine trench. To the East, North-dipping oblique subduction under the Rodophe-West Pontides zone occurred at rates ranging between ~18.3 mm/yr and ~31 mm/yr. Finally, the highest magnitudes were reached in the easternmost regions, where pre-Jurassic oceanic crust was subducted at a rate of ~60 mm/yr.

From the point of view of the kinematic model presented in the previous sections, the overall result of the plate boundary reorganization at chron M0 (120.4 Ma) was the formation of a system of converging plates characterized with a new stage pole. From the Middle Jurassic through the Early Cretaceous, a large oceanic plate subducted under Eurasia at a constant rate of 0.698°/Myr about a stage pole located at (55.21N,31.76E). This stage pole was calculated assuming that the kinematic pattern established during the initial stage of seafloor spreading in the Central Atlantic, between 175.0 Ma and 170.0 Ma, constrained the rate of convergence in the Eastern Tethys during the subsequent stages. Then, finite rotations of the Vardar plate with respect to Northeast Africa (Table 2) were calculated using Equation 3 and the assumption that small variations of the relative velocity between Africa and Eurasia were compensated by variations of the spreading rate at the ridge that separated Vardar from Northern Gondwana. As discussed in the previous section, the events that occurred at anomaly M0 caused the end of this mechanism of compensation, and a new system of subduction zones established that were characterized by a different rate and pole of convergence. As for the previous phases, we could assume that the stage pole for the motion of Africa with respect to Eurasia during this initial stage (between 120.4 Ma and 110.0 Ma) constrained the rate of convergence of all the subsequent stages. However, the field associated to variations of the relative velocity between Africa and Eurasia illustrated in Figure 16 shows a weak component of acceleration towards the trench. Moreover, we will show that a state of trench compression characterized the Alpine and Tethyan subduction systems until the Lower Campanian. Hence, a state of equilibrium in the mechanism of convergence between the major plates was not established until the Late Cretaceous. This means that mechanisms for the conservation of the equilibrium were not activated until that time.