ESTUDIO-SISMICO-DE-LA-CORTEZA-IBERICA-NORTE-3.3 - A SEISMIC IMAGE OF THE VARISCAN CRUST IN THE HINTERLAND OF THE NW IBERIAN MASSIF

An offshore vertical incidence reflection seismic study with simultane ous on-land wide-angle recording has been conducted, as part of the Es tudio Sismico de la Corteza Iberica Norte (ESCIN) Project, in the NW I berian Variscan Belt, an area deeply affected by Variscan and Alpine d eformation episodes. Line ESCIN-3.3 was recorded across strike of the main Variscan structures, transecting two of the zones in which the NW Iberian Massif has been subdivided. This study intends to correlate t he seismic features observed in the near-vertical reflection profile w ith the velocity model deduced from the wide-angle data, with the well -known geological structure of the Variscan upper crust, and with some other geophysical features described on the basis of offshore and ons hore experiments carried out in the area. The main results can be summ arized as follows: (1) identification of two important sedimentary bas ins in the shallow part of the marine seismic section, related with th e Mesozoic extension that gave rise to the opening of the Bay of Bisca y, (2) correlation of a number of subhorizontal and west dipping inter mediate depth reflections with Variscan compressional structures, (3) interpretation of a local set of strong reflections at 6 s (two-way tr avel time) as lower crust emplaced at an anomalous shallow level (thou ght to image the northward prolongation of a body with high P wave vel ocity and high magnetic susceptibility, described on land underneath a late Variscan antiformal structure, the Lugo Dome), and (4) recogniti on of two controversial highly reflective subhorizontal bands, located at 7-9 and 11-12 s, respectively. The shallowest of them appears to h ave a lower crustal P wave velocity and to be separated from the deepe st one, probably with similar characteristics, by mantle P wave veloci ty material. On this basis, a duplication of the lower continental cru st is inferred. The deepest of the bands could either represent the re mnants of a late Paleozoic crustal root or an Alpine underthrusting of the lower continental crust.