Collisional tectonics of the Baltic Shield in the northern Gulf of Bothnia from seismic data of the BABEL project

Abstract

Listen

Translate article

SUMMARY Densely spaced wide-angle data from stations recording airgun shots along BABEL profile 2 in the northern Gulf of Bothnia were analysed, modelled, and compared with the normal-incidence recordings. Profile 2 extends from the metasedimentary and metavolcanic Proterozoic Central Svecofennian Province into the volcanic Northern Svecofennian Province of magmatic arc affinity and crosses the Proterozoic-Archaean boundary of the northern Baltic Shield as defined from isotope studies. We used a 2-D traveltime inversion to derive P- and S-wave velocity-depth models and a distribution of Poisson’s ratios along the profile. A series of north-dipping wide-angle reflectors characterize the middle and lower crust in the southern part of the model, coinciding with similar events in previous multichannel wide-angle data and the CMP recordings. Several subhorizontal upper to mid-crustal wide-angle reflectors and a well-resolved Moho boundary at 42 to 45 km depth coincide with reflective zones or changes in reflectivity patterns of the CMP data. P-wave velocities increase from 5.7 km s-’ at the near-surface to 6.4 and 6.6 km s-l in the mid-crust and up to 7.6 km s-l in the lowermost crust. P, velocities reach 8.0 to 8.2 km s-’ below the Moho in the northern part of the model but are less than 8.0 km s-’ in the southern part. Poisson’s ratios, calculated from the P- and S-wave models, range from 0.21 to 0.25 in the uppermost crust to about 0.30 in the lower crust and upper mantle. Our velocity and Poisson’s ratio models indicate that the upper crust contains predominantly felsic, quartz-rich rocks. The increase of P-wave velocity and Poisson’s ratio in the mid- to lower crust is explained by increasingly mafic composition. We propose a geological model across the northern Gulf of Bothnia that combines our velocity-depth model with results from the seismic normal-incidence experiment, previous geophysical information, and geological surface observations. The model consists of a possibly multiple Proterozoic north-dipping subduction zone with remnants of oceanic crust and zones of imbricated metasediments and mafics on top. There is evidence for a metasedimentary fore-arc basin in the middle crust and a layer of mafic to ultramafic cumulates at the base or below parts of the lower crust. South-dipping mid- and lower crustal zones of high reflectivity in the northern part of profile 2 indicate that Archaean crust underlies the Northern Svecofennian Province. The major features of this Proterozoic collision zone, such as the suggested subduction slab, zones of crustal reflectors dipping parallel to the slab, a metasedimentary basin, and mafic to ultramafic cumulates between the subduction slab and overlaying Moho, are well-preserved analogues of recent oceaniccontinental collision and subduction zones.