Shear wave modeling and Poisson's ratio in the Variscan Belt of SW Iberia

Abstract

In 2003 two wide-angle reflection/refraction seismic transects were acquired in the Variscan Belt of SW Iberia. The approximately 250 and 300 km long, dense trace spacing transects revealed clear S wave arrivals in the shot gathers recorded by vertical component sensors in both transects. First S wave arrivals (Sg) and Moho reflections (SmS) are the most prominent phases that can be correlated from shot to shot. Sg is observed up to relatively large offsets and constrains the upper and middle crust S wave velocities. The SmS is seen from offset 0 (18 s twtt) to 150 km offset, where it intercepts first S wave arrivals (Sg). The upper mantle refracted phase (Sn) is difficult to recognize, although PmS/SmP converted phases can be identified. Using a 2-D ray tracing approach, two S wave velocity models for the crust of SW Iberia were obtained. These S wave velocity models complement the previous P wave velocity models and provide us with relatively well resolved Poisson's ratio crustal sections for SW Iberia. The resulting Poisson's ratio models present differences between tectonic zones at upper and middle crustal depths, thus supporting the existence of different tectonic zones prior to the Variscan collision. The most noteworthy feature is the high Poisson's ratio value (over 0.28) coincident with high P wave velocity areas (over 6.8 km/s) at midcrustal depths. In order to constrain the possible crustal composition, P wave velocities and Poisson's ratios have been compared with published laboratory measurements on different crustal rock types. This comparison indicates that the high P wave velocity and Poisson's ratios are compatible with a mixture of mafic to ultramafic rock types alternating with felsic ones. This result is consistent with the existence of mafic layered bodies in the middle crust, in the same way that has been suggested by previous works in this area.