PRECRITICAL WIDE-ANGLE REFLECTIONS FROM THE BALTIC SHIELD - EVIDENCE FOR A 1.8-GA SUBDUCTION COMPLEX

During the collaborative BABEL experiment in 1989, over 30,000 airgun shots were fired along reflection profiles in the Gulf of Bothnia and Baltic Sea and recorded at 62 onshore seismic stations, giving densely sampled wide-angle data of many forms. At one such station, in-line a nd three-dimensional wide-angle arrivals from 14-40 km offset were rec orded on a linear geophone array placed off-end to marine reflection l ine 2. The multifold data from this station have been CDP-sorted and s tacked, and also displayed as a single fold, high-resolution time-dist ance record section with an average trace spacing of 4 m. Prominent wi de-angle reflections observed on both wide-angle stacks and receiver g athers can be correlated with steep northward dipping reflectors obser ved in the lower crust on coincident near-normal incidence reflection (marine) profiles. High-resolution ray tracing through a three-dimensi onal model shows that the observed reflections likely occur in a refle ctive zone, 8 km thick, composed of individual reflectors 100-300 m th ick. The necessary impedance contrasts can be produced by alternating high and low velocities or densities. These reflectors parallel the tr end of the Skelleftea mineral district and its associated conductivity anomaly, and are here interpreted as metamorphosed remnants of subduc ted oceanic-type crust and its overlying sediments.