Pre-critical wide-angle reflections from the Baltic shield: evidence for a 1.8 Ga subduction complex

Abstract

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 and three-dimensional wide-angle arrivals from 14–40 km offset were recorded on a linear geophone array placed off-end to marine reflection line 2. The multifold data from this station have been CDP-sorted and stacked, and also displayed as a single fold, high-resolution time-distance record section with an average trace spacing of 4 m. Prominent wide-angle reflections observed on both wide-angle stacks and receiver gathers can be correlated with steep northward dipping reflectors observed in the lower crust on coincident near-normal incidence reflection (marine) profiles. High-resolution ray tracing through a three-dimensional model shows that the observed reflections likely occur in a reflective zone, 8 km thick, composed of individual reflectors 100–300 m thick. The necessary impedance contrasts can be produced by alternating high and low velocities or densities. These reflectors parallel the trend of the Skellefteå mineral district and its associated conductivity anomaly, and are here interpreted as metamorphosed remnants of subducted oceanic-type crust and its overlying sediments.