The lithospheric structures beneath the Pyrenees, which holds the key to settle long-standing controversies regarding the opening of the Bay of Biscay and the formation of the Pyrenees, are still poorly known. The temporary PYROPE and IBERARRAY experiments have recently filled a strong deficit of seismological stations in this part of western Europe, offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution. Here we report the results of the first tomographic study of the Pyrenees relying on this rich data set. The important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections. Beneath the Massif Central, the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere. Our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the Pyrenees. In contrast, they reveal the segmentation of lithospheric structures, mainly by two major lithospheric faults, the Toulouse fault in the central Pyrenees and the Pamplona fault in the western Pyrenees. These inherited Hercynian faults were reactivated during the Cretaceous rifting of the Aquitaine and Iberian margins and during the Cenozoic Alpine convergence. Therefore, the Pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the European plate.
[1] We locate the sources of double-frequency (or secondary) microseisms in western Europe by frequency slowness analysis of array data as well as polarization and amplitude analysis at individual stations. Array analysis uses data recorded by a temporary array of broadband stations that we deployed in the Quercy region (southwest of France) and those from the Gräfenberg array, from 2 December 2005 to 30 January 2006. We determine attenuation laws for microseisms generated in the Mediterranean Sea and in the Atlantic Ocean, which allow us to use noise amplitudes to estimate distances from the source. We then combine azimuth and amplitude measurements to obtain precise locations of microseisms and estimate their source dimensions. Most of the time, microseismic noise originates in coastal regions where the swell reaches steep rocky coasts with normal incidence, in good agreement with the Longuet-Higgins model for the generation of secondary microseisms. In addition, we find evidence of occasional pelagic sources, which are closely related to moving storms, suggesting that nonlinear interaction between wave components can also generate secondary microseisms.
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