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 simultaneous on-land wide-angle recording has been conducted, as part of the Estudio Sismico de la Corteza Iberica Norte (ESCIN) Project, in the NW Iberian Variscan Belt, an area deeply affected by Variscan and Alpine deformation 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 the seismic features observed in the near-vertical reflection profile with 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 onshore experiments carried out in the area. The main results can be summarized as follows: (1) identification of two important sedimentary basins in the shallow part of the marine seismic section, related with the Mesozoic extension that gave rise to the opening of the Bay of Biscay, (2) correlation of a number of subhorizontal and west dipping intermediate depth reflections with Variscan compressional structures, (3) interpretation of a local set of strong reflections at 6 s (two-way travel time) as lower crust emplaced at an anomalous shallow level (thought to image the northward prolongation of a body with high P wave velocity and high magnetic susceptibility, described on land underneath a late Variscan antiformal structure, the Lugo Dome), and (4) recognition of two controversial highly reflective subhorizontal bands, located at 7-9 and 11-12 s, respectively. The shallowest of them appears to have a lower crustal P wave velocity and to be separated from the deepest one, probably with similar characteristics, by mantle P wave velocity material. On this basis, a duplication of the lower continental crust is inferred. The deepest of the bands could either represent the remnants of a late Paleozoic crustal root or an Alpine underthrusting of the lower continental crust.