Abstract

The Pyrenean range is bordered to the north by the North Pyrenean fault, which represents the boundary between the European and the Iberian plates. Reflection profiles have shown that the Moho is vertically offset by 15 km at the latitude of the fault. In order to determine the structure beneath the crust, we have recorded teleseismic events on seismic stations deployed on both sides of the fault. Large P-wave traveltime residual variations are observed over a very short distance. The residuals given by the rays coming from the south are close to those predicted from the known Moho offset (southern stations are 0.5 s later). They rule out a stacking of the Iberian and European crusts underneath the North Pyrenean zone, as postulated in some models. In contrast, rays coming from the north give residual variations opposite to those expected from the known crustal structure (southern stations are as much as 0.9 s earlier). The simplest model fitting the data introduces an abrupt change in lithospheric thickness about 30 km north of the North Pyrenean fault; i.e., the southern lithosphere is about 60-70 km thicker than the northern lithosphere. The P- wave residual variations as a function of incidence angle suggest the existence of an anisotropy; a low-velocity vertical axis is in the vicinity of the fault. This simple structure can be explained by the collision of a thick Iberian plate with a thinned Aquitaine plate margin, a collision involving the imbrication of the crusts and a small decoupling of the lithosphere at the base of the Iberian Moho.

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