From topography to topography: Surface elevation, Moho geometry and African lithosphere dynamics

Abstract

Multi-seismic experiments acquired across the central and southern part of the Iberian Peninsula provide a new insight into the structure and nature of the lithosphere beneath these areas. Normal incidence and wide-angle seismic reflection data acquired in the area resolve the internal architecture and constrain the distribution of the physical properties along an almost 280 km long transect that samples the major tectonic domains of the Central Iberian Zone (CIZ) and the associated suture (e.g., the Central Unit, CU). A high quality image, ~230 km long, down to 45 km depth (~15 s TWTT) is provided by the normal incidence data set. Based on the reflectivity characteristics, the image can be divided into an upper and mid-lower crust, ~13 km and ~18 km thick, respectively. The wide-angle seismic transect extended the crustal section towards the north across the Madrid Basin. This, latter data set also sample the CIZ until the CU. This is ~280 km long profile which provides very strong constraints on the distribution of physical properties (P wave and S wave velocities, Poisson's ratio) of the upper lithosphere. The PiP and PmP seismic phases constrain two discontinuities: the brittle to ductile discontinuity at ~13-19 km and the Moho boundary at ~31-35.5 km. Currently both appear to act as decoupling surfaces and are interpreted to represent levels of lithological/rheological variations. The internal structure of the Moho is further discussed taking into account the characteristics of the PmP reflections. Furthermore, low fold wide-angle P and S wave stacks reveal a marked crust-mantle transition which is most probably 5-6 km thick and relatively complex structure. The geometrical relationships of this structure with the crustal fabrics of the normal incidence image suggest that the Moho is most probably a result of the re-equilibration and/or other lithospheric processes active after the Variscan collision.