Seismic structure and composition of the southern central Iberian crust: The ALCUDIA wide angle seismic reflection transect

Abstract

The nature of the crust beneath central Iberia was estimated by a wide-angle seismic reflection/refraction transect, ALCUDIA-WA, which sampled the southern half of the Variscan Central Iberian Zone, covered in the north by the Cenozoic Tajo Basin. The shot gathers recorded by vertical component sensors revealed well defined P- and S-wave phases. These arrivals were modeled by an iterative forward approach providing 2D crustal models showing variations in the velocity distribution with upper crustal P- and S-wave velocities increasing northwards. The lower crust P-wave velocities are homogeneous along the profile while the S-wave velocities slightly increase northwards. The Moho is placed at 32 km depth in the southern edge of the profile, deepening northward down to 35 km beneath the Tajo Basin. The Poisson's ratio, calculated from P- and S-wave velocities, varies along the profile at upper crustal depths. The highest values are located below the Mora and Pedroches batholiths. These resulting physical properties can serve to constrain the crustal composition by comparing them with laboratory measurements on rock samples. Our results suggest that the upper crust in the southern and central segments of the ALCUDIA profile is made up of low-grade metasedimentary rocks, while the northern segment is dominated by igneous rocks, in agreement with the surface geology. Separated by a sharp boundary located between 12 km (south) and 18 km (north) depth, the lower crust is more homogeneous and shows low Poisson’ ratios compatible with a rather felsic composition. However, outstanding lamination described in coincident vertical incidence data indicates some degree of intercalation with mafic components.