From the Surface Topography to the Upper Mantle Beneath Central-Iberian-Zone. the Alcudia Seismic Experiments.

Abstract:

Normal incidence and wide-angle seismic reflection data acquired in the Central and southern parts of the Iberia Peninsula resolve the internal architecture and constrain the distribution of the physical properties along an almost 350 km long transect that samples the major tectonic domains of the Iberian Massif, including the Central Iberian Zone (CIZ) and the associated sutures. The internal architecture down to almost 70 km depth (~15 s TWTT) is resolved by the normal incidence data set. It images a number of elements that characterize the tectonics of the study area, which is one of the best exposed fragment of the Variscan orogenic Belt. A well marked brittle-to-ductile (B2D) transition separates the crust in two, the upper and mid-lower parts, approximately, 13 km and 18 km thick, respectively. The upper crust appears to be decoupled from the mid-lower crust and responded differently to shortening. The Mohorovicic discontinuity is located at ~10.5 s (TWTT) , it is relatively thick, and highly reflective beneath the CIZ. The wide-angle seismic transect extended the lithospheric section towards the north across the Madrid Basin. This profile provides very strong constraints on the distribution of physical properties (P- and S- wave velocities, Poisson's ratio) of the upper lithosphere as well as a high resolution image of the base of the crust beneath the area. This data is one of the first datasets to present solid evidence of a relatively significant crustal thickening beneath the Madrid Basin. The crustal thickness varies from ~31 km beneath the CIZ to ~35.5 km beneath the Madrid Basin. This data set also reveals two major discontinuity levels, the B2D and the Moho, both represent levels of lithological/rheological variations. The characteristics of the the PmP and SmS seismic phases suggest further details on the internal structure of the Moho. Furthermore, low fold wide-angle P and S wave stacks reveal a marked crust-mantle transition which is most probably a 5-6 km in thick and relatively complex structure.