Ambient noise tomography of the Pyrenees and the surrounding regions: inversion for a 3-D Vs model in the presence of a very heterogeneous crust

Abstract

The lithospheric architecture of the Pyrenees is still uncertain and highly debated. Here, we provide new constraints from a high-resolution 3-D S-wave velocity model of the Pyrenees and the adjacent foreland basins. This model is obtained from ambient noise tomography on records of temporary and permanent seismic arrays installed in southwestern France and northern Spain. We first computed group velocity maps for Rayleigh waves in the 5 to 55 s period range using noise correlation stacks at 1500-8500 station pairs. As the crust is very heterogeneous, poor results were obtained using a single starting model in a linearized inversion of group velocity dispersion curves for the shear wave structure. We therefore built a starting model for each grid node by full exploration of the model space. The resulting 3-D shear wave velocity model is compared to data from previous geophysical studies as a validation test. Despite the poor sensitivity of surface waves to seismic discontinuities, the geometry of the top of the basement and the Moho depth are retrieved well, except along the Cantabrian coast. Major reflectors of the ECORS deep seismic sounding profiles in the central and western Pyrenees coincide with sharp velocity gradients in our velocity model. We retrieve the difference between the thicker Iberian crust and the thinner European crust, the presence of low-velocity material of the Iberian crust underthrust beneath the European crust in the central Pyrenees, and the structural dissymmetry between the South Pyrenean Zone and the North Pyrenean Zone at the shallow crustal level. In the Labourd-Mauléon-Arzacq region (western Pyrenees), there is a high S-wave velocity anomaly at 20-30 km in depth, which might explain the positive Bouguer anomaly of the Labourd Massif. This high-velocity lower crust, which is also detected beneath the Parentis area, might be an imprint of the Albian-Aptian rifting phase. The southeastern part of the Massif Central has an unusual velocity structure, with a very shallow Moho (21-25 km) above an uppermost mantle with anomalously low shear wave velocity.

Ambient noise tomography of the southern sector of the Cantabrian Mountains, NW Spain

“…The velocity patterns around the Ventaniella fault are interpreted as the large-scale transition between the Variscan massif of the peninsula and the more recent Alpine crust to the east. In terms of S-wave velocities, our results are consistent with those found by Macquet et al (2014) and Palomeras et al (2017). At 5 km depth, they have obtained velocities in our study area that range between 2.5 and 3.5 km s −1 .…”

“…To date, in the Iberian Peninsula, few regional ambient noise tomography studies have been done inland (Villaseñor et al 2007;Silveira et al 2013;Palomeras et al 2017) or in the continental margins (Corela et al 2017). At a larger scale, Macquet et al (2014) have obtained tomographic maps of the north of Spain as part of a broader study of the Pyrenees.…”

The Pyrenean architecture as revealed by teleseismic P-to-S converted waves recorded along two dense transects

“…The last striking feature in the migrated western section is the weakly expressed Iberian Moho at the southern end of the profile (distances between 0 and 50 km), which suggests a weak or gradual velocity gradient, in good agreement with the results of the ambient noise tomography by Macquet et al (2014). Interestingly, the regional tomographic model by Chevrot et al (2014) shows low P velocities in the Iberian lithosphere in this region.…”

Crust and mantle lithospheric structure of the Iberian Peninsula deduced from potential field modeling and thermal analysis

“…The overlap of these positive anomalies caused by high-density bodies at depth with thick sedimentary basins could indicate a linked genetic origin as suggested in Macquet et al (2014) and attributed to the Aptian-Albian rifting event. The importance of such extensional events related to large exposures of mantle rocks flooring the sedimentary basins in the Central and Western Pyrenees has been shown in recent works (e.g., Lagabrielle et al, 2010;Jammes et al, 2009).…”

“…The Saint-Gaudens anomaly is attributed to the presence of a high density block about 60 km long, 25 km wide and extending from 7 to 15 km (Vacher and Souriau, 2001). Recently, Macquet et al (2014) using a high-resolution 3D S-wave velocity model for the Pyrenees and adjacent areas concluded that the positive gravity anomaly located at the Labourd-Mauléon-Arzacq region can also be correlated with a high S-wave velocity anomaly at 20-30 km depth.…”