Implications of the seismic structure for the orogenic evolution of the Pyrenean Range

Abstract

The dynamic evolution of the Pyrenees is discussed in the light of geophysical data. Recent deep seismic sounding have revealed the crustal structure of the Pyrenees which is used to test the different evolutionary models proposed until now. The crustal thickness of the Paleozoic Axial Zone (PAZ) and the North Pyrenean Zone (NPZ) differ by more than 10 km, ranging from about 30 km in the NPZ to 40–50 km in the PAZ. The transition from PAZ to the NPZ, identified at the surface as the North Pyrenean Fault (NPF), is sharp at depth and marked by a vertical step, at least in the eastern half of the range. The NPZ is characterized by additional throws and dips of the Moho in the east whereas in the west a heterogeneous middle to lower crust is encountered, with high velocity anomalies. The seismic results suggest that the PAZ and the NPZ belong to different plates, the NPF being the plate boundary. These results are inconsistent with evolutionary models involving lithospheric subduction or crustal doubling and intracratonic rifting with the main tectonic lineations following NNE-SSW directions. They rather suggest that after a period of extension, two main orogenic events took place: a phase involving shearing and thinning which affected mainly the present-day NPZ and a later compressive phase which explains the building up of the chain, the thickening of the crust and the enhancement of a pre-existing difference in crustal thickness between the European and Iberian plates.