A tomographic study of the lithosphere beneath the Pyrenees from local and teleseismic data

Abstract

A tomographic image of the Pyrenean lithosphere from the surface down to 200 km is deduced from the propagation of the seismic waves recorded at the regional Pyrenean seismograph network. Crustal and subcrustal models are derived independently from the inversion of two different data sets. P and S velocity models for the crust are obtained from the propagation of local earthquakes. The upper crust appears very heterogeneous and exhibits sharp velocity contrasts. In the uppermost 7 km, an east‐west trending zone with velocities up to 6% higher than the surrounding material is observed all along the Pyrenees. This zone coincides in most places with the exposure of the Paleozoic rocks of the Axial Zone. Low velocities are observed on both sides of this high‐velocity zone. At the surface, they coincide with well‐known sedimentary basins but dip down to about 11 km depth toward the axis of the range. In the central and eastern Pyrenees, two large, high‐velocity bodies stand out in the North Pyrenean Zone and extend down to the base of the upper crust. They coincide with large positive gravity anomalies and are interpreted as lower crust or upper mantle blocks uplifted into the upper crust. The lower crust is poorly resolved but appears to be less heterogeneous than the upper crust. Moreover, the lower crustal structure is almost uncorrelated with the upper crustal structure. The major feature of the lower crust is a low‐velocity anomaly which stretches along the North Pyrenean Fault in the western Pyrenees and beneath the Axial Zone in the central Pyrenees; its location nearly coincides with the distribution of the seismicity. In the upper mantle, P models are derived from the travel times of the remote events recorded at the Pyrenean stations. The most important feature is the presence, in the eastern and central Pyrenees, of a vertical, low‐velocity heterogeneity south of the North Pyrenean Fault down to 80–100 km. This structure exhibits a 2% velocity contrast with respect to the surrounding structures and is interpreted as lower crustal material subducted during the Pyrenean convergence. This heterogeneity is not observed in the western Pyrenees, suggesting that the crustal shortening has been completely accommodated by several thrusts in the thinned Aquitaine crust.