The lithospheric structure of the Central Iberian Zone: The ALCUDIA deep seismic reflection profile

The 250 km long, vertical incidence Vibroseis seismic reflection profile ALCUDIA was acquired in spring 2007. It samples the Variscan Central Iberian Zone from Toledo to Fuenteovejuna. Its main goal was to continue the structure obtained for the crust and mantle from the IBERSEIS transect towards the N and NE. The ALCUDIA transect crosses some important structures, such as the Toledo fault, Santa Elena Fault, Alcudia anticline, Almadén syncline, and some major magnetic anomalies. The acquisition parameters, 35 m station spacing, 70 m VP spacing resulted in a 60-90 fold high resolution seismic reflection image. A 20 s long Vibroseis sweep was recorded by a 400 station recording cable. After preliminary processing, the upper crust shows a moderate reflectivity that can be easily correlated with identifiable surface geologic features, mainly folds. The section can be divided in 4 domains of reflectivity that can be key to stablish different domains of crustal evolution. A highly reflective mid-to-lower crust with a few mantle reflectors and well defined features in the upper crust with the indication of detachments zones that might link to the mid-crustal reflective zone.
Upright folds mapped on surface are well imaged in the upper crust and appear to be detached from the middle crust, being evidenced by the flat upper limit of the reflectivity and some individual subhorizontal reflections. The middle and lower part of the crust is strongly reflective for 4,5 s to 10 s (twtt). This highly reflective band thickens towards both ends, most probably due to later deformation. The high reflectivity is indicative of strong lamination, crosscating, that has been afected by deformation, although to a different extents. The southern part of the profile images a sudden change in the seismic fabric, which is marked on surface by the existence of the Pedroches batholith. South of this batholith, a doming structure within the mid-crust marks a sharp change in the seismic signature. The same is observed in upper crust which differs notably from that of the rest of the transect. This changes jointly with other geological constraints provide evidence to interpret this zone as a boundary, most probably an acreationary contact between two crustal domains of different nature.