Estudios sísmicos de la corteza varisca del suroeste de la Península Ibérica

Resumen

The crustal and upper mantle structure across the Variscan orogenlc belt of the SW-lberian Peninsula has been studied by a seismic lithospheric transect. This Variscan orogen consists of three major tectonic terranes: the South Portuguese Zone, the Ossa-Morena Zone and the Central Iberian Zone, which were accreted in Late Paleozoic times. This seismic study includes: a deep normal incidence seismic reflection profile (IBERSEIS) and, dense wide-angle seismic reflection records. The IBERSEIS is a 303 km long and 20 s deep seismic reflection profile designed to obtain a high resolution crustal scale image of the orogen. Two wide-angle seismic transects were acquired during fall 2003 across the SW Iberian Variscan orogen and parallel to the IBERSEIS (one is coincident with it). A total o f 690 digital seismic recording instruments (650 TEXANS and 40 Reftek 3 component units) from the IRIS-PASSCAL Instrument Pool have been used. The transects, consist of 3 and 6 shot points, respectively, with an, approximately, 60 km shot point interval. These recently acquired experiment was designed to provide velocity constraints on the lithosphere and complement the previously acquired normal incidence seismic profile (IBERSEIS). These terranes show a distinctive seism ic signature, as do the sutures separating them. Late strike-slip movements through crustal wedges are apparent in the seismic image and have strongly modified the geometry of sutures. The upper crust appears to be decoupled from the lower crust all along the seismic line, but some deformation has been accommodated at deeper levels. A sill-like structure is imaged in the middle crust as a 1-2 s thick and 175 km long high-amplitude conspicuous reflective band. The M oho can be identified along the entire transect and in both seismic reflection data sets, it is subhorizontal and located at 10 to 11 s, indicating a 30-35 km average crustal thickness. Its seismic signature changes laterally, being very reflective beneath the South Portuguese Zone and the Central Iberian Zone, but discontinuous and diffuse below the Ossa Morena Zone.