Three-dimensional shear velocity structure of the Mauleon and Arzacq Basins (Western Pyrenees)

We present a 3-D shear wave velocity model of the Mauleon and Arzacq Basins from the surface down to 10 km depth, inverted from phase velocity maps at periods between 2 and 9 s. These phase velocity maps were obtained by analyzing coherent surface wave fronts extracted from ambient seismic noise recorded by the large-N Maupasacq seismic array with a matched filtering approach. This new model is in good agreement with a local earthquake tomography study performed on the same acquisition dataset. Our passive imaging models reveal the upper crustal architecture of the Mauleon and Arzacq Basins, with new details on the basement and its relationship with the overlying sedimentary cover. Combining these new tomographic images with surface and subsurface geological information allows us to trace major orogenic structures from the surface down to the basement. In the basin, the models image the first-order basin architecture with a kilometric resolution. At depth, high velocity anomalies suggest the presence of dense deep crustal and mantle rocks in the hanging wall of north-vergent Pyrenean Thrusts. These high velocity anomalies spatially coincide with a positive gravity anomaly in the western Mauleon Basin. In addition, our models reveal major changes from the Chainons Bearnais to the western Mauleon Basin across a set of orogen-perpendicular structures, the Saison and the Barlanes transfer zones. These changes reflect the along-strike variation of the orogenic evolution that led to the preservation of the former rifted domain and its underlying mantle in the orogenic wedge of the Western Pyrenees. We discuss the implications of these results for the 3-D architecture of the Mauleon Basin and its underlying basement.