Pyrenean Orogeny and Plate Kinematics

The development of the Pyrenees, a range of mountain which lies between Iberia and Eurasia, has remained a subject of much debate between geologists and geophysicists for a long time. The debate concerns about the large amount of compressive motion which geophysicists estimate took place across the Pyrenees based on the plate kinematics of the North Atlantic during the opening of the Bay of Biscay with which geologists differ based on their interpretions of the observations made across the Pyrenees. To find a satisfactory answer to this question we did three things. One, we have derived a mean pole of rotation for the entire opening of the Bay of Bisacy whose position is constrained not only by the magnetic anomalies identified in the Bay of Biscay and the North Atlantic but also by the geometry of a triple junction which had remained at the mouth of the Bay of Biscay during this period. The location of such a pole of rotation implies that about 400 km of compression or subduction took place between the Iberian and Eurasian plates in the Pyrenean domain during the opening of the Bay of Biscay from chrons M0 to 33o time (118 to 80 Ma). Two, we have re-examined the deep seismic profile (ECORS) shot across the Pyrenees to substantiate evidence of such a subduction during this time. Thirdly, we constructed tomographic sections across the Pyrenees using teleseismic data with an irregular spatial gridding function of the number of available rays. Two high velocity features dipping northwards beneath the Pyrenees and the Aquitaine basin have been identified. Their upper boundaries correspond to two major decollement zones identified on the deep seismic ECORS profile. We interpret these features as two distinct slabs. The southern slab is linked to the subduction of the western tip of the Tethys ocean simultaneously with the formation of backarc basins elongated along the future Pyrenean domain. It was active at least from 150 Ma (late Jurassic) to 107/90 Ma (lower Albian-Cenomanian). The northern slab, active since 107/90 Ma, is linked to the subduction of the lower continental crust located between the Tethys suture and the Pyrenean domain. In the upper crust, normal faults as well as the North Pyrenean fault became reverse faults and former backarc basins were inverted, giving rise to the uplift of the Pyrenees as a double asymmetrical wedge.